TOWN OF WOODSIDE PlON~ER HOTEL BUILDING. VVOODS!DE ROAD AT \iVH!SKEY Hill/ BOX 4005. VVOODSIDE. c;\.LIFOHN!A 94062 / {415) fJ51- 77f34

July 15, 1974

Mr. Earl W. Hart Division of Mines and Geology Ferry Building San Francisco, Ca.

Dear Mr. Hart:

94111

I am enclosing a copy of a report entitled "Reconnaissance of Active Traces of San Andreas Fault in Woodside" and together with a map of the traces. This report was written by Dr. William Dickinson in August of 1973,

A copy of this report was mailed to the Division of Nines and Geology in Sacramento last fall.

/'~ truly yours,

\ I A-/ ( ',(,.I . £-,"--z-c- ',,___::/-"~"'-~-.,__, (loan Olson Deputy Town Clerk

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Reconnaissance o_f Active Traces of

San Andreas Fault in Woodside

William R. Dickinson

Registered Geologist No. 1374

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Purpose and Methods

Types of Seismic Hazards

San Andreas Fault System

San Andreas Fault Movements

San Andreas Fault Patterns

Active Fault Trace Criteria

Ground Rupture Belt Width

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CONTENTS

Fault Traces in Woodside

Woodside Branching Master Trace

Trances and Vineyard Traces

Searsville and Canada Traces

West Union Creek Traces

.Appendix: Glossary

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PURPOSE AND METHODS

This study of rift features along the San Andreas fault zone within the town

of Woodside was undertaken at the request of Town Geologist Jon C. Cummings acting

on behalf of the Town Council. This report is intended solely for the information

of town officials as an aid in planning for the town, and as a guide for seeking

detailed studies of specific parts of the rift belt where additional information

may be needed.

The conclusions about active fault traces within Woodside were reached on the

basis of a two-stage investigation: (1) office examination of airphoto stereo­

pairs at several scales to detect lin~aments that. might represent active fault

traces, and (2) ground examination of all suspected active fault traces to seek

confirmatory indications of rifting not visible on airphoto images, and conversely

to discover evidence to eliminate from consideration as fault traces such spurious

lineaments. as fences, excavations, and artificial plantings. Subsurface investiga­

tions by trenching were not undertaken, but in current practice trenching is typi­

cally the next stage of investigation, beyond airphoto and ground reconnaissance,

for either of two purposes: (1) if the location. of certain active traces must be

specified with more precision, or (2) if the nature of certain active traces must

be established with more confidence •

. The scope of the study was· limited to ten days of geologic work. Within this

framework, time was divided roughly as follows: 25% .for airphoto interpretation,

.50% for field investigations, and 25% for trace-plotting and report-writing.

TYPES OF SEISMIC HAZARDS

There are three main kinds of hazards associated with earthquakes: (1) ground

tremor from seismic ··shaking, (2) ancillary landslides and ground lurch, and

'(J) ground rupture from fault movement. Only the third type of hazard can be ascer­

tained from a study of active fault traces like ·those which were the object of this

study.

Tremor. The violent shaking that can affect broad regions around the epicenter

·Of a major earthquake is the most generally hazardous effect, but is by no means <

limited to the vicinity of active fault trace~. The degree of damage to buildings

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and other structures from tremor is controlled both by the type of construction

and by the type of foundation ground upon which structures are sited.

Landslides. Landsliding and irregular lurching of ground may be triggered

by severe tremor as secondary effects of earthquakes. Conditions of slope insta­

bility or unsuitable foundation materials may be accentuated by tremor, but

generally will not be induced where the potential for failure did not.exist pre­

viously.

Rupture. Where fault surfaces intersect the ground surface, actual breakage

of the ground, and shift of the blocks of ground on different sides of the fault,

will occur during fault movements. This rift hazard is restricted to rift belts

where active fault surfaces intersect the ground surface along lineaments called

fault traces. Any buildings or other structures coupled ·firmly to the earth

blocks on both sides of an active fault trace will sustain direct damage if fault

movements occur. Utility lines and transport arteries that cross active fault

traces may also be subject to interruption when fault movements occur. Knowledge

of the locations of active fault traces can be used to predict the positions for'

p.otential damage from ground rupture.

SAN ANDREAS FAULT SYSTEM

The San Andreas fault extends on land from Point Arena near Fort Bragg on the

north to beyond the Salton Sea on the south. The fault forms one segment of the

complex boundary between the Pacific and North American plates or blocks of litho­

sphere which are in contact from Alaska to the mouth of the Gulf of California.

Along the San Andreas segment of the boundary, the two vast pieces of the earth's

·'rigid outer rind are evidently sliding horizontally past one another, and the . • ..

fault itself is apparently the complex surf ace of shear between them. Such a ·

shearing boundary where two masses of lithosphere are sliding horizontally with

respect to ·one another is called a transform. The San Andreas transform sy~tem is

one of the major structural features of the earth's surface. It has been in essen-

. tially. continuous operation for at least 5 million years, and perhaps for roughly

10 million years. Minor earth~uakes and ground movements along a number of the

faults' in the. system during the past few decades show that activity is continuing.

It will doubtless continue to function as a major zone of movement far into the

future.

The long discontinuity in the earth's crust called loosely the San Andreas ' fault is•not a simple break in geometric terms. The complex rupture incorporates

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branches or strands that weave in and out from common junctions, splays that angle

off and die out, and parallel faults that follow near the main rupture for varying

distanFes. The whole geographic array of movement surfaces forming an intricate

hierarchy of master and subsidiary faults is called the San Andreas fault system.

Some segments of the fault system include several major active fault zones, and in

the Bay Area, the Hayward and Calaveras faults of the East Bay are parts of the

overall system, as well as the San Andreas fault proper on the Peninsula. Each

major fault zone of the system includes its own complex array of active fault

strands, and the surface trace of each active strand has the potential for ground

rupture. The active fault traces together define the rift belt, which for•the San

Andreas fault zone traverses the town of Woodside from its northwestern extremity

to its.southeastern extremity. The rift belt of active traces is marked by topo­

graphic discontinuities and other landscape scars formed by past dislocation·of

the ground surface during episodes of ground rupture by fault movements too recent

for effects to have been erased by processes of weathering and erosion.

SAN·ANDREAS FAULT MOVEMENTS

The dominant sense of movement along the principal active faults of the San

Andreas system is dextral strike-slip. This term means that the main relative move­

ments of the earth blocks on opposite .. sides of the fault are horizontal with respect

·to one another, and that the ground on the opposite side qf the fault from the obser­

ver moves to the right, relative to the position of the observer, during episodes of

fault movement. Subordinate vertical displacements of irregular sense may accompany "'. the horizontal displacements. A total cumulative displacement, or relative''.horizon-

tal shift, of at least 175-200 miles is well documented from detailed geologic

studies of rock units that were cut by the fault, moved sideways, and are now

exposed that far apart on opposite sides of the fault.

Ground rupture and shift along active faults of the San Andreas system can take

two forms: (1) semicontinuous slow slip, or creep, unaccompanied by major earth­

·quakes; and (2) episodic fast slip or sudden offset accompanied by major earthquakes.

Either style of slip can dislocat·e structures built across an active fault trace,

although the former disrupts gradually whereas the latter causes essentially instan­

. taneous disruption. 'J1ie. s_egment of the fault zone on the Peninsula apparently has

not undergone any creep since·the major offset associated with the earthquake of

L906, when fault movements of 5 to 10 feet were noted in the vicinity of Woodside. ~ • c The largest observed historical offset on the San Andreas fault was about 15 feet

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near Olema in Marin County in 1906, and the largest inferred historical offset on

the San Andreas fault was about 30 feet on the Carrizo Plain in 1857. Creep rates

are known, to r'lll:ge locally up to 2 inches per. year, but measured rates of less than

one inch per year are more common.

The factors that control the style of slip., and lead to behavior characterized

by creep or by sudden offset, are not fully known. Patterns of creep and sudden

offset vary in space along the trend of the fault, and likely vary in time as well.

Creep and sudden off set are probably not mutually exclusive aspects of displacement

along any given fault segment. Creep sometimes occurs in tiny discrete jumps, and .

sudden offsets are sometimes preceded or followed by periods of creep.

Slip presumably occurs when the stress buildup across the fault exceeds the

strength of the rocks in the fault zone. The buildup occurs as the plates or blocks

·of lithosphere beside the fault drift past one another at a semi-steady rate. When

·the temporarily locked fault gives way, creep or sudden offset occurs. If the off­

set is sudden, elastic rebound of the rock masses adjacent to the fault generates

vibrations that cause

red~e the likelihood

earthquakes. Creep may thus function as a safety valve to

of catastrophic failure and severe earthquakes. Alternatively,

creep as an indicator of instability may serve as a precursor of severe earthquakes.

The relationship of creep events to sudden offsets is still uncertain. The signifi­

cance of fault creep may well differ along different fault segments as geologic con­

ditions vary and possibly also at different times along the same fault segment.

Various estimates of the long-term rate of dextral strike-slip between Pacific

and North American lithosphere are of the order of 1 to 2 inches per year. Such an

approximate value can be inferred from independent lines of evidence including the

progressively greater offset of successively older geologic formations, the spacing

of magnetic anomalies on the sea-floor, the changing geodetic positions of triangu­

lation stations resurveyed several times during this century, and maximum creep rates

on fault segments south of the Bay Area in the central Coast Ranges during the past

·decade .. In the absence of relief by slow creep, the potential for sudden offset in

excess of 5 feet is thus seemingly reached in about half a century -following a major ·~

offset accompanied by a major earthquake along any given segment of the fault.

Although no one can yet predict just when another fault movement will occur along the

San Andreas in Woodside, the length of time since the last known.offset in 1906 is J sufficient to warn the prudent that an additional movement in some form could be

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SAN ANDREAS FAULT PATTERNS

Detailed geologic mapping of various segments of the San Andreas rift belt I .

during the past two decades has shown that the web of active fault traces displays

several characteristic patterns. A general understanding of these typical patterns

in space, and their relations to inferred sequences of movements in time, is an aid

to interpreting a trace map for any given segment, like that in Woodside. Before

describing some key patterns, a.review of nomenclature is useful to insure full

comprehension:

(1) A fault, in geologic terms, is a surface of shear which breaks through"

rock masses and along which the dislocated ·rock masses are displaced by sliding move­

ments; a fault trace is the linear trend marking the intersection of a fault surface

with the ground surface.

(2) The San Andreas fault system refers to the whole complex network of active

strike-slip faults, and other related faults, which together form the complex trans­

form boundary between Pacific and North American lithosphere in coastal California.

(3) A fault zone refers to a·band of closely related faults whose traces form

a discrete bundle or belt separate from those of other fault zones. Individual

fault surfaces within a given fault zone can be called fault strands. In a fault

zone with a long history of movement, as is the case for the San Andreas fault zone

on the Peninsula, some fault strands may be active and others inactive. In this con-

. text, active means capable of accomodating renewed fault movements by serving as a

surface of slip, whereas inactive means dormant or extinct, hence no longer serving

as a surfac.e of potential slip. Similarly, active fault traces are those of active

fault strands where ground rupture occurs during faul~ movements.

(4). The rift belt along a fault zone is the span of ground laced by active

fault traces displaying !:.!!!_ scars formed by disturbance of the ground surf ace by

fault movements which occurred recently enough to 'leave scars on the present land­

scape. The most typical rift scars are elongate features aligned along the trend of

active fault traces; these include: (a) topographic steps called fault scarplets,

·which may be only a part of larger compound slopes called fault scarps; (b) depres­

sions or small troughs called fault furrows, (c) humps or welts called pressure

ridges, and (d) benches called fault terraces on otherwise smooth slopes. More

localized rift scars in~lude small undrained depressions filled'by lakes or marshes

called ~ ponds, and streambeds or ridgecrests whose trend is offset in a dextral

dogleg where ~ctive traces transect their courses. The rift belt of active fault

traces may lie wholly within or along one side of the fault zone, which as· a whole

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is commonly broader. Inactive fault strands may interlace the bedrock, hidden be­

neath surficial deposits and soils, across a wider band of ground than that affecte

by the active fault strands which break through the surficial deposits and soils to I

form rift scars. Owing to the presence of easily eroded zones of crushed rock that

occur along fault strands, or to contrasts in rock type across fault strands, ero­

sion may etch even inactive fault traces into some form of topographic relief. Sue!

fault-line relief is not as sharp or geologically raw as the rift scars formed by

actual disruption of a previously formed ground surface.

Within the rift belt along typical segments of the San· Andreas fault zone,.six

characteristic geometric patterns of active fault traces can be recognized, singly

or in combination across the rift belt or in succession along the rift belt. The

six can be considered the basic elements of which the web or network of active fault

traces is built:

(1) a continuous master· trace, along which essentially all movement is concen-

trated as coherently linked slip (single-strand mode).

(2) a continuous master trace, along which most movement is concentrated as

coherently linked slip, but with subordinate slip along discontinuous subparallel

·subsidiary traces nearby (master-subsidiary mode); .some distributed ground warp

must occur beside the master trace beyond the terminal ends of the subsidiary traces

(3) a continuous master trace, along which most movement is concentrated as

coherently linked slip, but with subordinate slip along diverging splays that.branch

locally from the master trace and terminate without rejoining it (master-splay mode)

some distributed ground warp must occur off the end of or beside each splay trace.

(4) multiple coordinate branching traces, either co-equal or with one as masteI

.and most commonly as pairs, in a continuous braided network of traces formed by

strands"that split and. rejoin (braided-strand mode); the distribution of overall move

ment as different slip on the braided branches is v.ariable, and geometric gradations

·with all other modes are common; some distributed ground warp must occur near the

junctions of branching traces, and in general must increase as the angle of diver-

gence between traces increases •

. (5) - (6) discontinuous coordinate en echelon traces ch the main movement

can 'step progressively across the rift

right (dextral-echelon mode) or to the

, ' belt .as slip >'·traces stepping 'to

the left (sinistral-eche·i~ ~ode); some distri : .·,._,~--·

buted ground warp must occur between or off the ends of the· ·traces. (As viewed in

plan· from the side, dextral-echelon traces by this convention step "down" to the

·right, and &inistral-echelon traces si:ep "down" to the left).

As noted for several of the trace modes, the complex geometry of trace patterns

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implies some ground warp, or deformation of the ground surface, in areas of the rift

belt away from active fault traces. The· active fault strands and their traces can '

· be viewed as the surfaces and lines along which strain rates and magnitudes are great

enough to cause relative slip of surf icial blocks and consequent rupture of the

ground surface. The warp of the remaining ground within the rift belt, but away from

the traces, is too slight to cause ground ruptures. The amount of off-trace ground

warp required in several of the modes is difficult to assess. Its nature, extent,

and magnitude depend upon (a) the distribution of movement ·among the various traces,

{b) the orientation of the traces to the controlling slip direction, and (c) ~he ease

or difficulty with which rock masses adjacent to the traces can deform. In the general

case of multiple braided traces, even the form of ground warp to be expected in

association with each trace junction is indeterminate.

The general sense of the ground warp, whether contractior.al or extensional in

the direction parallel to the controlling slip direction, can be anticipated with

moderate confidence within simple segments of the rift belt having the following

types of trace patterns: (a) a master trace plus one subordinate trace, (b) a pair

of branch traces that merge at both ends into a single master trace, (c) a pair of

en echelon traces. If the master trace, or the overall trend of the en echelon

traces, is assumed to define the controlling direction of relative slip, the following

rules can be stated for dextral strike-slip (see fig. 1):

(1) Where a subsidiary trace lies subparallel to the master tr~ce in the master­

subsidiary mode, ground warps beside the master trace near the ends of the nearby

subsidiary trace should be, as viewed from across the master trace, extensional near

the right or dextral end and contractional near the left or sinistral end.

(2) where a splay trace, as viewed from across the master trace, splays dex­

trally off the master trace in-the master-splay mode, the ground warp should be con­

tractional near the trace junction and extensional in the gap between the master

--trace and the end of the splay trace; the reverse should apply for sinistral splay

traces. in the master-splay mode.

(3) Where a master trace diverges into two branches, the simplest form of the

braided-strand mode, the ground warps near the two junctions s~ould be analogous to.

warps near the junctions of splay traces with master traces, and should be governed

by.the orientation of each branch trace to the controlling slip direction.

(4) In the overlap between dextral echelon traces, ground warp should be exten­

sional; :l,n the overlap between sinistral echelon traces, ground warp sliould be con­

tractional.

These tentative rules for anticipated ground warp near dextral strike-slip

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faults can be generalized to imply extensional_ ground warping associated with most

dextral stepovers of fault traces, and contractional ground warping associated wit

mos~ sinistral stepovers of fault traces. Ground warping adjacent to segments of

curvilinear master traces not everywhere· parallel to the controlling slip directio

can also be ant.icipated. If the segments out of proper alignment are treated also

as dextral or sinistral stepovers, extensional and contractional warping, respec­

tively, should occur.

ACTIVE FAULT TRACE CRITERIA

The positions of future ground ruptures cannot be predicted unless potentiall)

active fault traces can be identified~ Past experience has shown that rupture of

the grou~d surface is not equally likely throughout the span of the fault zone, but

rather is recurrent within the rift belt, and occurs there along previously estab­

lished active fault traces marked by rift scars on the landscape. There is no doub

_that wholly new surfaces of ·slip are initiated from time to time within and near th

rift belt. Otherwise we would not observe the phenomenon of a broad fault zone wit

numerous old fault strands, most of them inactive. The restriction of recent slip

to a comparatively few fault strands with active fault traces displaying elongate

rift scars implies that each strand has a·time of initiation, a time of activity,

·and a time of abandonment as a slip surface. For immediat_e purposes, however, the

identification of recently active fault traces allows prediction of the most likely

positions of future ground rupture from fault off set with a high degree of accuracy

and reliability. Along active fault traces, ground rupture at the time of the next

fault ~ffset is almost a certainty. Elsewhere within the rift belt, the rift hazarc

cannot be regarded as nil, but is low by comparison and dependent on the initiation

of new fault strands or the reactivation of dormant ones. Within the fault zone out

side the rift belt, the rift hazard is lower still and approaches nil.

An explanation for the recurrence of slip along established active fault traces

·is provided by the general observation that the active fault traces with rift scars

·along a given segment of a strike-slip fault zone are the ·straighter of the recogni­

zable fault traces. As such, they are the surface expressions of the fault strands

that can most easily accomodate horizontal ground shift parallel to the controlling

.slip direction. Fault strands_ that are dormant or extinct display curvilinear

traces whose sinuosity is apparently roughly proportional to the length of time sine

they were abandoned. Ground warp within the rift belt evidently flexes fault traces

progressively with the_ passage of time, and the flexed fault strands are eventually

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abandoned when newly initiated fault strands appear as straight traces that s

circuit the bends in bowed fault traces. Newer straight strands and their tr

, thus amputate flexed segments of older strands and traces, thereby serving to

straighten the surface of slippage. Relative sinuosity thus appearn to be a '

criterion to guide estimates of the relative potential for future activity amc

different fault traces marked by rift scars along the same segment of the rift

Straightness is probably a better indicator of relative activity than prominen

rift scars, so long as some scars are present. In addition,. segments of the r

belt along which all recently active fault traces are sinuous probably have th

likelihood of developing wholly new fault strands with new, straighter traces

future. As the sinuosity increases, the potential for locking permanently prol

increases as well. Unfortunately, we presently lack any means to quantify the'

various likelihoods related to. trace sinuosity. Moreover, all fault traces di•

playing fresh rift scars on the present landscape must be regarded as active.

fidence that the active fault traces fully delimit the positions for future gro

rupture is diminished by some measure in proportion to the complexity of the ob

ved trace pattern; in segments of the· rift belt characterized by braided curvil

traces, the possibility that a new straight master trace may at some time in th

future supplant the complex web of sinuous traces seems real.

The best initial way to locate active fault traces, within the overall con,

of the rift belt, is the examination of airphoto stereopairs to detect lineament

marked by rift scars. Each individual rift scar may be so subtle as to pass alrr

unnoticed during random ground traverses. It is their peculiar alignment, in co

_junction each with the others, that first arouses geologic suspicion. On airpho

images, the trends of fault traces may be marked not only by small topographic i

larities, but also by (a) lineaments marking vegetation changes, (b) lineaments

marked by tone changes on the images, and (c) lineaments of dark tones on the illk

These three effects are related to differences in. groundwater conditions induced

by the presence of a barrier of impermeable crushed rock called fault gouge alon~

the surface of a fault strand. Lineaments marked by contrasts in vegetation or b

a trend of unique vegetation can be called vegetal linears. Lineaments marked by

contrasts in ground tone or by a trend of dark moist ground can be called gouge

shadows. In wooded areas, a special form of airphoto lineament expressed by a

narrow trend marked by reduced tree height can be called forest scars.

Topographic rift scars also visible on airphoto images include fault scarple

·fault. furrows, pressure ridges, fault terraces, sag ponds, and dextral· doglegs of

ridgelines or streambeds. Different typ~s of topographic rift scars may succeed

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one another along contiguous segments of active fault traces. Particularly indi­

cative of a fault-induced rift origin by strike slip are crossover scarplets where

small topographic steps facing opposite direcdons are in line and joined end to

end.

The delineation of suspected active fault traces by airphoto interpretation

requires skill, but is a standard technique practiced routinely by geologists.

Results are enhanced when airphotos are available at several scales taken from

several altitudes. For this study, three scales were used. The rift b·elt as a

whole is clearest at small scale, the continuity of individual active fault traces

is clearest at intermediate scale, and the character and position of individual

rift scars is clearest at large scale. No single one of the airphoto sets at any

one scale would provide as much information as the three together.

Suspected active fault traces must also be examined with care on the ground

to establish whether the suspected rift scars are truly anomalies on the present

landscape. Where rocks of different character are juxtaposed across a fault strand,

weathering and erosion may produce a topographic expression along an inactive fault

strand. Gouge zones of inactive fault strands may also be exhumed by erosion to

serve as a groundwater barrier.

Confirmation of suspected active fault traces by ground visits to rift scars

requires skill, but also is a standard technique practiced routinely by geologists.

Its success requires understanding of the normal processes and results ·of landscape

development. Fault-induced features then appear to the observer as otherwise inex­

plicable facets of the landscape.

Corroborating evidence for the existence of active fault traces can be gained

from the presence of features caused by ground warp in expected locations. This

.technique.has not yet been applied widely, but has good promise as an auxiliary tech­

.nique of investigation.

In places where rift scars are faint, quickly erased by other natural processes,

or obliterated by activities of man, the positions of active fault traces are uncer­

tain. The trends of the traces must be extrapolated across the unknown ground from 0

the nearest clear evidence. of surface breakage. Even where rift scars are clear,

there are uncertainties inherent in the combined technique of airphoto interpreta­

tion and ground reconnaissance. Precision of location has inherent limits and the

judgment of activity of mapped traces. is based upon observations of the ground sur­

face alone.

The quality of the information about supposedly active fault traces can be

enhanced, if the need exists, by subsurface examination of fault strands exposed in

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trenches that traverse suspected traces at right angles. Such trenches must be

made with large back-hoes or bulldozers, for they must be at least 20 feet deep

and more than 100 feet long to achieve satisfactory results. Experienced obser­

vers can idetect in the walls of the trenches _the tiny shear surfaces formed by

fault slip. These mark the fault strands, and the most active ones can be recog­

nized by ·the fact that they break through layers of sediment and soil all the way

up to the ground surface. Inactive fault strands correspondingly do not break

through to the surface, but are capped off by overlying layers of surficial

materials. Valid observations of this kind must be made by skilled obser,vers, but

trenching for this purpose is now a standard technique of engineering geology.

The potential utility of trenching for locating active fault traces requlres

comment. In my opinion, trenching cannot improve significantly the ability to

recognize the presence of an active fault trace where rift scars are present,

although it might remove the unlikely possibility that spurious rift scars were

improperly mapped in error. In ground without rift scars, where active fault

traces are mapped by extrapolation only, trenching should permit a clear decision

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:h:::·a::~v~nf:~~: :::::st:::~:i:: -~ located. Even where rift scars are present, trenching might improve the precision \

of located traces. Rift scars are commonly about 25 to 50 feet wide and there is I about that much inherent uncertainty in the location of traces from surface obser).

vations alone without trenching.

GROUND RUPTURE BELT WIDTH

_The most difficult factor to estimate in evaluating the rift hazard from •.

·active fault t~aces is the width of the ground rupture belt to be expected in case

of renewed movement. The styl~ of ground rupture can take two general forms.

Especially where offset is great, a'ragged gash with irregular feathered margins

commonly extends continuously along an act.ive trace, but locally may expand into

a· belt of rumpled ground broken by multiple parallel fractures with tilted or . .

buckled stretches of ground between the.fractures. Where offset is slight, and

the cohesion of surficial earth is not fully disrupted, an active trace may be

marked by a band of gaping ~ echelon fractures oriented at a clockwise acute

angle to the trend of the trace. In either case, the wid_th of disturbed ground

along a ruptured trace may locally be as narrow as 5 to 10 feet, is commonly only

10 to 25 feet, appears gener,ally to be no more than 50 feet, and probably does not

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exceed 75 to 100 feet where a single clearcut trace is discernible. Where multi­

ple traces are mapped in some complex trace pattern, each trace must be viewed as

having the potential for ground rupture across a similar span.

The width of ground occupied by a rift scar may be a rough guide to the

characteristic width of ground rupture along a particular trace, but only crudely

so, for estimates of the transverse dimensions of many subtle rift scars are little

more than guesses within the general range of 25 to 50 feet. Probably the only

reliable way to ascertain characteristic widths of ground rupture at particular

places along particular traces is by trenching. To acquire such information is

both tedious and costly, for results obtained at one place along a given'trace

cannot be extrapolated with confidence.even to other places along the same trace,

and certainly not to other apparently similar traces nearby.

Without trenching, only some general limits can be set on expected spans of

ground rupture b~side mapped traces. At the scale of the town map, a heavy inked

line representing the estimated centerline of a mapped trace has a width that

scales roughly 10 feet, and error in transferring data from airphotos to base map

may imply an effective width or uncertainty of up to 25 feet. As the rift scars

mapped have a ground width of roughly 25-50 feet, the potential error in the

plotted location of the inferred centerline of a well located trace is.about 50 ~ feet. If the belt of ground rupture is generally less than about 50 feet wide,_)

it should be confined within a band about 100 feet wide centered on the mapped

. trace. Where the position of a trace must be interpolated locally, an additional\

error of perhaps 25-50 feet may enter into plotting the position of a trace. \ I

Severe ground deformation may well be confined in most places to the central half J of the rift hazard belt. On the other hand, rare gashes that veer off the main~

trace at pr:esently unpredictable sites may locally affect other ground within a·

band perhaps 250 feet wide.

These estimates of the width of disturbed ground to be expected along active

fault traces are tentative, for there is no general agreement among geologists as

to the appropriate figure for the width of the rift hazard belt. Nor do they

. include the indeterminate span of warped ground outside the belts of trace rupture.

Severe jostling during a major earthquake may also cause ground dislocation from

lurch of unsteady materials at greater distances from active fault traces.

FAULT TRACES IN WOODSIDE

The rift belt of the San Andreas fault zone through Woodside (see fig. Z) is

. •

I dominated by a continuous master trace with rift scars along its whole length •

This active fault trace, here named the Woodside trace, is apparently characteri­

zed by recurrent slip, and most slip within the San Andreas fault zone in Woodside

is probably concentrated along it during.fault movements. The Woodside master

trace brahches locally in the braided-strand mode, but mostly only into double or

paired traces. Where the fault branches into more than a pair of braided strands,

all but one or_ two have high trace sinuosity suggesting inactivity. The Woodside

trace lies generally along the northeast side of the prominent rift valley, or

fault-controlled elongate lowland, that extends from Portola Valley northwest past

Adobe Corner.

Entering Woodside from the southeast, and also located near the northeast

edge of the rift valley, is the Trances trace, which in Portola Valley forms with

the Woodside trace a typical pattern of the sinistral-echelon mode. In Woodside,

the northwest extremity of the Trances trace appears as a subsidiary trace para!-

. lel to the Woodside trace. The Trancos trace apparently connects, near the site_

of old Searsville, with the.southeast extremity of the Vineyard trace, which also

lies near but northeast of the Woodside trace as a subsidiary parallel or dextral

splay trace in relation to the master trace. Some segments of the Ttancos and

Vineyard traces that are straight and.parallel to the Woodside trace display rift

scars indicative of recurrent slip, though probably of' a lesser characteristic

magnitude than activity along the Woodside trace itself. Other segments of the

Trances and Vineyard traces with comparatively high trace sinuosity lack clear rift

·scars and may be inactive.

Well northeast of the Woodside trace are two subsidiary parallel traces that

are roughly aligned and probably connect near central Woodside. To the northwest,

the Canada trace lies west of Canada Road, but within the same valley, and to the

_southeast, t;he Searsville trace passes beneath Searsville Lake and apparently ter­

minates in Hidden Valley. The Canada trace displays only ~he single-strand mode,

but the Searsville trace locally displays braided~strand and master-splay modes as

well, to the south and to the north of the lake, respectively. Subdued rift scars

along both the Searsville and Canada traces indicate that straight segments of

·eat:h, with lciw trace sinuosity, experience some level of recurrent activity •.

Along West Union Creek, and extending into the rift valley to the southeast,

a parallel cluster _of branching traces apparently form a compound sinistral splay

trace in relation to the Woodside master trace. Straight segments of three dis­

crete traces among the cluster are parallel to the Woodside trace and display rift 0

scars locally. Among the three, the one closest to the Woodside trace has ~he most

.··

I /

.·

-14-

prominent rift scars, and the one farthest from the Woodside trace has the most

subdued rift scars. This pattern of evidence probably reflects correctly the

relative importance of recurrent slip on.the West Union traces, all of which I

branch complexly to the southeast and apparently feather out on the floor of the

rift valley.

The compl~xity of trace patterns within the rift belt in Woodside dictates

that different kinds of traces be shown by different symbols on the trace map

(see fig. 3). The following differentiation is adopted for the purposes of this

report:

~ A traces shown by solid line are accurately located segments of actjve

traces marked by clearcut rift scars. By inference, Type A traces are well loca­

ted segments of traces characterized by recurrent activity. Those segments along

the Woodside master trace apparently experience most of the slip locally during

·fault movements, and those along other traces commonly exhibit auxiliary slip.

~~traces shown by dashed line are drawn by interpolation to connect

Type A segments, and indicate segments of active traces along which clearcut rift

scars are locally masked by heavy vegetation or construction. Except for being

less accurately located, Type B traces by inference represent trace segments

fully as active as Type A traces with the same incidence of recurrent slip.

~.9. traces shown by dot-dash line.are trace segments marked by topogra­

phic relief which is not of clearcut rift origin, but may be of erosional origin

·along inactive fault traces.

~.!>.traces shown by triple-dot-dash line are trace segments 'marked only

by gouge shadows, vegetal linears, or forest scars that may indicate the position

of an inactive trace rather than an active one •

. ~ ! traces .. shown by queried dashed lines are inferred extensions or co11-

nections of well marked traces- across ground where evidence for the existence of

a fault.is.weak or lacking.

Traces made only of type A and B segments are characterized by prominent rift

s~ars at close intervals, have low overall trace sinuosity, and are inferred to

·display recurrent activity.along their whole length. Traces including type A seg­

ment~ locally, but also made in part of type C and D segments, are characterized

by more subdued or more widely spaced rift scars, and are inferred to display re­

current activity only at intervals along their length where comparatively straight

segments occur. Traces made only of type C and D segments lack clearcut rift scars,

' have high cverail trace sinuosity, .and are inferred to be either recently abandoned

or recently exhumed traces that are probably dormant or extinct. Type E segments

.

-15-

must be regarded as only approximate locations of possible traces whose actual

existence is in doubt.

WOODSIDE BRANCHING MASTER TRACE

As it crosses Portola Road in Portola Valley, the Woodside trace is marked

by a small scarplet beside a small frame house. Photographs taken in 1906 show

that· the ground rupture associated with the San Francisco earthquake did.in fact

occur along this scarplet. Northwest of this point, the Woodside trace generally

follows the course of Sausal Creek to the marshes near old Searsville. Near

Wyndham Circle, the Woodside trace is marked along the northeasterly edge of the

Sausal Creek bottoms by small scarplets concealed by thick bushes, and just nicks

the creek at the most easterly points of its channel curves. Downstream in the

Family Farm area, the combination of heavy tree cover and extensive landscaping

construction mask the course of the Woodside trace in detail as far as the southern

Family Farm Road, but the landscape scar along the trace continues unbroken in con­

tinuity through the obscured area. From the southern Family Farm Road to the

northern Family Farm Road near the latter's junction with Portola Road, the Wood­

side master trace is marked cleariy by a prominent vegetal linear formed by the

contrast between tall redwoods on the northeast and wetland deciduous trees of

lower height on the southwest. In this area, a slightly bowed branch trace,

probably inactive but possibly active, is marked tO the west of the master trace

by a forest scar that diverges from the master trace just north of the southern

Family Farm Road and rejoins the master trace just south of the northern Family

Farm Road.

The Woodside trace again crosses Portola Road in the marshes above Searsville

Lake wh~re it is marked by vegetal linears which also suggest a split of the trace

into two co-equal branches for a sho'rt distance just northwest of Portola Road.

Along lower Alambique Creek, a single trace is well marked. by a prominent vegetal

linear which connects with a small scarplet beside the creek. The trace crosses

Portola Road once again a few feet east of the bridge over the creek, and passes

northwest into a deep redwood grove where the course of the trace is masked in

detail for a short distance. The strongly bowed branch trace lying east of the ')

Woodside0

master trace in the marshes above Searsville Lake is marked by a subdued

scarp beside Alambique Creek and by vegetal linears in the marshes. An abrupt slope

near the northwest junction of this branch trace with the master trace and low-lying

marshy ground near the southeast junction of this branch trace with the master trace ..

. ,

I -16-

;, .. are comp.atible. with the contractionaL.and extensional ground warp expected respec-

tively near those trace junctions.

From Alambique Creek to Bear Gulch Creek, the Woodside trace lies along the

' foot of the prominent fault scarp that bounds· the rift valley on the northeast.

This segment of the master trace is well marked continuously by a sharp break in

slope between the sloping face of the scarp, which faces southwest, and the flat

floor of the rift valley,(whose edge is marked by a large sag pond lake and several

pockets of marshy ground.') For a short distance near the lake, which has an island~

a branching.or subsidiary trace, which is marked by a fault terrace on the scarp,

lies parallel to and northeast of the master trace. From it"s straightness and· its

parallelism and proximity to the master trace, this branching"or subsidiary trace

must be regarded as potentially active over most of its length. Linkages to the

master trace are shown on the map as extensions of the straight part of the trace

along the fault terrace, but are marked only by vegetal linears, and the small

terrace trace may well be a parallel subsidiary trace rather than a branching trace

in the braided-strand mode as shown. A steep slope on the lower face of the main

fault scarp near the northwest end of this terrace trace, and a gentle_ lower face

near the southeast e"nd, are compatible with the'contractional and extensional

ground warp expected near those respective ends.

Along Bear Gulch Creek, the Woodside trace is well marked by several fault

furrows on stream terraces. In the Bear _Gulch Creek bottoms just south of Woodside

Road, the Woodside trace splits into two co-equal branches which together cross

Woodside Road and extend along West Union Creek to Kings Mountain Road, thence be­

side Josselyn Lane before rejoining in the uplands northeast of West Union Creek •

., _, Southeast of Woodside Road, the southwesterly of the two traces is well marked by

a fault· furrow, but the northeasterly of the two traces is extended to ~heir common.

junction near Bear Gulch Creek on tenuous topographic evid~nce. Just northwest of

Woodside Road, however, the northeasterly of the two traces is marked by a prominent

fault scarplet, whereas the southwesterly of the two traces is extended from the

road to the edge of· a sag pond on tenuous topographic evidence. Both of the branch

·_ tJ'aces are then extended to the northwest mainly by interpolation, with some help

from vegetal linears, across the flats along West Union Creek, across Kings Moun­

tain Road, and up the slopes to their common junction at a topographic saddle in

the uplands northwest of Josselyn Lane. The interpolation is controlled, however,

by a small hut sharp fault scarplet on each of the traces ·-just southeast of Kings

Mountain Road, and by a subdued fault terrace on each of the traces crcssiu~ slopes

in the Josselyn Lane.area.

..

I \

" '-17-

Across undeveloped.ground between the Josselyn Lane area and the cul-de-sac

at the end of Raymundo Drive, the Woodside trace is well marked at intervals by

fault scarplets, fault furrows, fault terraces, crossover scarplets, vegetal linears,

and gouge shadows. The northeasterly branch trace near Raymundo Drive is marked

only by a subdued scarp and a forest scar, hence may be inactive, but both branches

that diverge to the northwest from Raymundo Drive are clearly marked by rift scars,

hence are both active traces.

TRANCOS AND VINEYARD TRACES

The Trancos trace extends northwest·from the intersection of Farm Road with

Portola Road along a straight break in ·slope between Farm Road and Sausal Creek,

then along a small fault scarplet and fault furrow defining a slightly curved

trend on the northeast flank of the small ridge southwest of Farm Road. Though

not as prominent as those along the nearby Woodside master trace, the rift scars

along these comparatively straight segments of the Trancos trace nearly parallel

to the Woodside trace suggest that both traces are potentially active. The southern

continuation of the Trancos trace, named from Los Trancos Woods to the south, is

known from photographs taken in 1906 to have suffered ground rupture at Alpine

Road and beyond during the San Francisco .earthquake.

Just northwest of the point where Farm Road bends away, the Trancos trace

becomes more sinuous; curves toward the Woodside trace, and is marked only by vege­

tal linears and gouge shadows except for subdued topographic expression. This

sinuous and probably nearly inactive segment of the Trancos trace approaches close

to the Woodside trace but apparently veers away along a vegetal linear to connect

with the Vineyard trace.

The link apparently connecting the Trancos t~ace.of Portola Valley with the

Vineyard trace near Vineyard Hill is a trace segment that trends across the northern

'Family Farm Road and the marshes above Searsville Lake near old Searsville. Just

west of Family' Farm Road, this trace segment is marked by a small fault scarplet

and fault furrow linked end to end, but continuations of the trend defined by these

rift scars are shown only by a curving vegetal linear to the southeast toward the

Trancos trace and a curving marsh scar, analogous to a forest scar,. to the north­

west toward the Vineyard trace.

The Vineyard trace is best marked west of Vineyard Hill where its locally

straight course is defined by fault furrows, fault terraces, gouge shadows, and a

large sag pond. Unlike the town's other sag ponds, which are asymmetric with an

-18-

active fault trace along one side only, this sag pond is a symmetric lake, lying

along a prominent fault furrow and bisected by an active fault trac~ The junction

of the Vineyard trace on the northwest with the Woodside master trace is inferred ' from a subdued rault terrace and vegetal linears, but the connection may be an inac-

tive trace segment. If so, the Vineyard trace is now a subsidiary parallel trace

rather than a qextral splay trace in relation to the master trace, but in either

case is an apparently active fault trace with rift scars where straight and paral­

lel to the Woodside master trace.

From the vicinity of Mountain Home Road southeast to near the junction of

Sand Hill Road with Portola Road, the Vineyard trace is sinuous and marked only

by subdued topographic expression and gouge shadows. This trace segment, which is

probably inactive, projects to connect with the postulated Vineyard-Trancos trace

link marked by vegetal linears and rift scars near old Searsville.

SEARSVILLE AND CANADA TRACES

The Searsville trace, named for Searsville Lake, trends northwest from the

floor of Hidden Valley across Searsville Lake to the hill slope across Bear Gulch

Creek from Whiskey Hill Road. The trace pattern reflects a braided-strand mode

southeast of Searsville Lake and a master-splay ~ode northwest of Searsville Lake.

Southeast of Searsville Lake, the most apparent rift scar is a prominent

double fault furrow on the undeveloped floor of Hidden Valley at its northwest end

on the northeast side of Corte Madera Creek. Along trend from this rift scar, the

trace segment to the southeast is marked only by gouge shadows and vegetal linears

along Corte Madera Creek near the northeastern edge of Hidden Valley. Farther

south, the trace apparently curves to an alignment oblique to the Woodside master

trace and crosses the floor of Hidden Valley near its southeast end. The oblique (

trace segment is interpreted to be a linear depression, probably formed by the '

extensional ground warp expected for this trace orientation. As the depression

has been occupied in the past by a now abandoned stream course, it may be an entirely

erosional feature. However, from.its apparent linkage by vegetal linears to the

fault furrows farther northwest, and from its uniqueness on the floor of the valley,

the linear depression is interpreted here as a rift scar occupied as a stream chan­

nel and modified somewhat by bank erosion.

Near the southwestern edge of Hidden Valley, a second.trace is marked by a

fault terrace.on the slope west of Trail J.ane. The trend of a gouge shadow near

the intersection with Hidden Valley Lane suggests that· this southwestern trace·

I -19-

extends along the latter roadway toward a junction with the northeastern trace near

the southeast end of the linear depression that crosses the floor of Hidden Valley.

A subdued,fault scarplet occurs at the southwestern edge of Hidden Valley just

beyond the trace junction, but otherwise no southeastern continuation of either

trace was found in a search as far as Alpine Roa<l. The linear depression on the

floor of Hidden Valley thus appears to be the result of extensional deformation

associated with the termination of the Searsville trace on the southeast.

Northwest of the floor of Hidden Valley, the southwestern and northeastern

branches of the s·earsville trace are .marked by vegetal linears and forest scars in

the swampy woodlands between Hidden Valley and Searsville Lake, but no topographic

rift scars could be located by ground traverses, perhaps in part because of the

heavy cover. The two branch traces apparently converge steadily and join near

Searsville Lake. A steep shore slope at the southeastern end of the lake may be

a fault scarplet or merely an erosional expression of the trace.

Between Searsville Lake and Sand Hill Road, the Searsville trace is marked

only faintly by gouge shadows and apparently erosional topographic expression

suggestive of an inactive fault trace. A sinistral splay trace, however, imparts

a slight dextral dogleg to a ridgeline in one place.. Northwest of Sand Hill Road,

the splay trace is marked only by gouge shadows and subdued topographic relief

that could be erosional, and cannot be detected beyond a location near the inter­

section of Winding Way and Manzanita Way. On the same hillside northwest of Sand

Hill Road, the main Searsville trace is marked, however, by a fault scarplet, fault

terrace, and fault furrow in line near the base of the slope just southwest of the

lower course of Manzanita Way. After crossing Manzanita Way, the Searsville trace

is marked by faint gouge shadows until it reaches Bear Gulch Creek. No clear evi­

dence of it~ presence can be detected along the inferred trace extending along the

stream valley from that point t'o thed.ntersection of Mountain Home, Woodside, and

Canada Roads in central Woodside. This inferred trace segment is suspected, however,

as a connecting linkage between the Searsville and Canada traces. The only

re.asonable course for this connecting trace is quite sinuous, and it is presumably

inactive if present.

The Canada trace is marked by a straight break in slope interpreted as a fault

scarplet near central Woodside, by a subdued crossover scarplet and a subdued fault

furrow southeast.and northwest, respectively, of Olive Hill Road, and by a fault

· furrow near Raymundo Drive. Elsewhere, the trace is interpolated across featureless

ground guided by gouge shadows. The straightenss of the trace and its parallelism

to the Woodside master trace suggest, however, that it has the potential to absorb

.minor slip along its whole course.

/ I .. II.

. -20-

WEST UNION CREEK TRACES

In the valley of_ West Union Creek near the southeastern corner of Huddart Park,

three fault traces, each parallel to the nearby Woodside master trace, display rift

scars. The three are here termed the proximal, medial, and distal West Union Creek

traces with respect to distance from the Woodside trace. They are apparently

branches of a sinistral splay trace off the master trace.

The proximal West Union Creek trace is marked best by a prominent asymmetric

sag pond with a combined fault furrow and terrace adjacent on the southeast. North~

west of the sag pond, this trace is apparently marked by vegetal linears and forest

-scars that trend across heavily wooded slopes toward a dextral dogleg in a ridgeline

near the junction of this proximal splay trace with the master trace. Southeast of

the sag pond, the trace is apparently marked by a straight reach of West Union Creek

and by combined gouge shadows and vegetal linears that trend toward a dextral dogleg

of a ridgeline in the angle between West Union Creek and Kings Mountain Road across

from the Old Woodside Store. Southeast of Kings Mountain Road, the trace is marked

by a fault furrow which apparently trends toward a pair of faint lineaments marked

by vegetal linears and gouge shadows without associated topographic rift scars.

This succession of trace features suggest that the proximal splay strand dies out

as a slip surf ace beneath the rift valley.

The medial and distal West Union Creek traces are best marked only locally by

a subdued fault furrow and a subdued pressure ridge, respectively, where the two

traces lie on opposite sides of West Union Creek. To the northwest, the two traces

are marked only by sinuous vegetal linears and forest scars which seemingly merge

together and then curve to join the proximal West Union Creek trace near the latter's

junction with the Woodside master trace. These northwesterly extensions of the ~

medial and distal traces are based on tenuous evidence, may be incorrect, and in

any case are likely inactive. To th'e southeast, the two traces apparently merge

and follow a break in slope just northeast of and roughly parallel to Greer Road.

_This topographic feature trends toward a ~¥bdued scarplet on the floor of the rift o.£on_~ a fau.Q.t f£ll°n'W

valley, and this rift scar apparently trendsAtoward a pair of faint lineaments ·

marked by vegetal linears and gouge shadows without associated.topographic rift scars.

This succession of rift features again suggests that the merged medial-distal splay

strand dies out as a slip surface beneath the rift valley._

Far to the southeast, within the rift valley, sinuous and apparently inactive

traces ·that may be related to the West Union Creek splay traces can be detecte.d by

breaks in slope and gouge shadows that occur both northwest and southeast of the

marshes near old Searsville.

-21-

APPENDIX: GLOSSARY

The following terms are defined where underscored on the pages indicated:.

'

braided-strand mode

branching traces

crossover scarplets

dextral dogleg

dextral-echelon mode

dextral splay trace

6

6

10

5

6

7

dextral strike-slip fault 3

en echelon traces 6

f=lt 5

fault furrow 5

fault gouge 9

fault .scarp . 5

fault terrace 5

fault scarplet 5

fault strand 5

fault system

fault trace

fault zone

forest scar

gouge shadow

ground lurch

ground rupture

ground warp

5

5

5

9.

9

2

2

7

·master trace 6

master-splay mode 6

master-subsidiary mode 6

pressure ridge

rift belt

rift hazard

rift scar.

rift valley

sag pond

single-strand mode

sinistral-echelon mode

5

5

2

5

13

5

6

6 ~-........... _'-"_

sinistral splay trace

splay trace

subsidiary parallel trace

trace sinuosity

transform

vegetal linear

·.

7

6

6

9''

2

9.

j L

..

•

e ____ c c-----£

£

c

bra.Ide d-strand mode

(dou..6/e- bra.Y1ch masfer)

. dex.tra..f- echelon mode-

y.:------- ......,.g :r1n5le-sir-and wiode

w1fh cu..rvi/i,.,ea.r- ma.sfer

Fi3ure 1.

SKETCH. oF TYP!cl9L TRflcE PRTTERNS ·

( de~traf_ 'Strife -sf? . fau.fl wif,1 con fro//;~

sf,., d1r0

ecf1~n Aor/2onl-af' OY/ 1aye) . Exfecfed yrou'!d waJ>. /'/ear tmce ends ancl

trace j£Lnc..f1ons ·and frace 6en,ds denoted

/,./ E {exfe11i1011afl) and C (confracf1-;;,nJ)

·.I .!

.1

' '

tJo.

F!Gtl RE. .2. -5/(ETCf/ /}'//IP To S-ilot..V

T/?/!CE ·PllTTER/V IN WoDOS!DE

8rcp1c/ii'J · Shown

Wood-s-,·de. /l?as ter Trace

b/. sold ~ea7 /tnet;s) _

SU,bordt'rlafe. s;!a; and .ruind,~7 Iara/le/ traces

. :fh<JtVI? _ £1 da.s-hed .4ea7 ,!tnes) doltecl Where.

Conn ecf ions- tt.ncerfain (Canada..- S"eC\r:;-ville j \/1~e7ar-/-Trancos-)