intensity of weathering as a relative exposure age indicator of mass

Intensity of weathering as a relative exposure age indicator of mass movements at Machu Picchu

heather viles, vít vilímek, richard přikryl, jiří zvelebil

Abstract

�e Machu Picchu archaeological site is thought to have been affected by landsliding activity in the past, and may be at risk from mass movements in the future. In order to assess this risk it is necessary to establish a clear chronology of past landsliding activity, using information such as the degree of weathering to constrain surface exposures. Microscopy, including petrological and microfractographical observations, on a range of weathered samples, shows the presence of different weathering intensities around the site. Heavily weathered samples are found to have been affected by biological weathering (lichens), exfoliation micro-cracking and chemi-cal alteration of feldspars. A preliminary ranking of weathering intensity and thus surface exposure age has been produced. Our findings lend support to recent engineering geological-geomorphological interpretations of the site history, but further research on weathering here is necessary before more confident inferences can be made over the dating of past landslides.

key words: weathering of granites, recent morphological processes, landslides, Machu Pic-chu, Peru

1. Introduction and aims

To aid conservation and management of the UNESCO World Heritage site of the Incaic Machu Picchu Sanctuary in Peru it is necessary to be aware of the major geological and geomorphological processes which may pose a hazard to the site. !e geomorphological setting of this Andean area is rather complex and there are many gaps in our knowledge, however aerial photo interpretation, field mapping and literature review all suggest that landsliding activity and rock falls have been an important influence on recent landscape evolution here (Vilimek et al. 2005). Some authors have proposed that there have been very large-scale landslides at Machu Picchu (e.g. Carreño, Bonnard 1997) and that there is serious threat of future land-slides which could damage the site in future (e.g. Sassa 2001). In order to evaluate the threat of landsliding at the site today, we need to be able to constrain the ages of the various assumed past landslides more accurately.

128 heather viles — vít vilímek — richard přikryl — jiří zvelebil

!e aim of this study is to examine the contribution that observations of the weathering status of the granitic rocks around Machu Picchu can make to under-standing the recent history of landsliding at the site. !e degree of weathering on a surface can be measured in many different ways and can provide (under suitable conditions) an estimate of the exposure age of the surface. !e weathering status of a sample of surfaces from different locations at and around the Machu Picchu archaeological site has been assessed using a series of simple techniques based on as-sessment of weathering rinds in order to help constrain the relative ages of a number of hypothesized old landslide events. Our study aids attempts to throw further light on the chronology of landsliding activity in the recent past.

2. Granite weathering and relative surface age dating

Granite weathering processes and products have been well studied in many areas around the world. Granites are prone to damage from salt, frost and thermal weath-ering under suitable climatic conditions (e.g. French, Guglielmin 2002, Birot 1954, Migoń 2006) with the production of microfracturing leading to rapid disintegration. Granites are also prone to chemical attack through hydrolysis of the more suscep-tible minerals (e.g. biotite and feldspars) especially under wet and hot conditions (e.g. Ng et al. 2001). Finally, granites are also prone to biochemical and biophysical weathering processes, especially produced by lichens and microorganic biofilms (see Romao, Rattazzi 1996, Silva et al. 1999 for examples). Granites weather both when exposed at the surface and when buried within soils, and there has been much debate over whether (and how) rates vary between surface and buried positions. Birkeland (1999) illustrates, from studies of weathering of granites within tills in the Sierra Nevada, California, that over periods of up to a few 10s of thousands of years surface clasts weather more rapidly than subsurface ones that show virtually no weathering. However, in tills of around 140,000 years in age subsurface granite clasts are found to be more weathered than those on the surface.

!ese different weathering processes create a range of resultant products and characteristics which can be used for relative age dating, such as weathering rinds, differential surface relief, and alterations in rock hardness. Weathering rind thick-ness has been used as a relative age indicator in many previous studies (e.g. Gellatly 1984, Mills, Allison 1995). On suitable rocks the progress of weathering leads to the development of a pronounced discoloured and altered layer which can easily be measured. Granite has been found to be particularly conducive to weathering rind formation, with Birkeland (1999, table 7.3), for example, finding thicker weather-ing rinds developed on granites than on other rocks from a compilation of data from clasts in the western US. Rates of weathering rind development on granite clasts calculated from these results range from 0.02 to 4.5 μm a�¹.

129intensity of weathering as a relative exposure age indicator

3. Geodynamic and geological setting of the study area

!e archaeological site of Machu Picchu is located in the Cordillera Oriental of the Andean mountain system. !e ancient city and its agricultural terraces are located between Machu Picchu Mt. (3,051 m a.s.l.) and Huayna Picchu Mt. (2,700 m a.s.l.). Intensive erosion by the Urubamba River has created a canyon with nearly vertical walls in some places. Accumulations of sediment at the bottom of the valley are rare and consist of some alluvial fans, recent fluvial terrace material or sediment from various types of slope movements (rock slides, rock falls) which occur on the valley-side slopes.

!e climate on the Machu Picchu Ridge (where the archaeological site is situ-ated) is very humid, mountainous, and subtropical in nature. !e mean annual tem-perature varies from 12 to 15 °C and at elevations between 2,450 and 2,500 m a.s.l. the climate is frost-free. Diurnal temperature ranges here are greater than seasonal variations in temperature. !e annual average precipitation is 1,950 mm with rather strong seasonality (the wettest period is between December and March). Only the bottom of the Urubamba Valley has a more warm subtropical type of climate (mean annual temperature between 15 to 18 °C).

!e bedrock around Machu Picchu is composed of deformed leucogranites and tonalites of the Macchu Picchu pluton of Permian age which has been dated at 240 ± 10 Ma based on Rb-Sr method (Ponce et al. 1999). !ese rocks have been deformed in a ductile to brittle regime into various types of fine to ultra fine rocks, ranging from cataclastic granites with a slightly developed foliation system to well foliated ultramylonitic rocks. In most samples, plagioclase is more common than microcline, and muscovite and biotite are quite scarce. Quartz content is variable, but K-feldspar and plagioclase are usually more dominant.

Tectonic zones orientated in NW–SE and SW–NE directions have heavily dis-turbed the bedrock. !e active role of tectonics in recent landscape evolution is not well known as yet. !ere is evidence that the tectonic settings have had a major influ-ence on fluvial geomorphology. !e plan of the erosional, deeply incised valley of the Urubamba River coincides with the general structural plan of the area. !e river meander follows both main tectonic lines (Machupicchu and Huaynapicchu faults, which run NE–SW and the Urubamba fault, which runs NW–SE). Furthermore, there is at least a passive role of tectonics in deforming and shattering rocks within the slopes, thus predisposing them to mass wasting processes, including large-scale slope movements.

!e existence and superposition of slope movement generated landscape features of different spatial orders bear witness to a multi-stage development involving the oc-currence of several generations of landslides with different spatial magnitudes (Sassa 2001, Vilímek et al. 2005). Vilímek et al. (2005) hypothesize that there is evidence for the occurrence of a huge, very old and definitely pre-Incaic rock-slide. !is event is thought to have destroyed the mountain crest of the central part of the Urubamba River meander resulting in the relatively broad “saddle form” of the future archaeo-logical site and chaotic accumulation of rock blocks especially in the “Quarry” area.


Relicts of the original ridge (i.e. Machu Picchu and Huayna Picchu Mts.) bound that newly formed saddle. !e relatively flat saddle area provided a seemingly ideal construction site for the Incaic Sanctuary in this heavily dissected mountainous re-gion in which there are few areas of flat land. We suggest that primary predisposition by tectonic shattering together with further disturbance to the granites by this hy-pothesised huge prehistoric slope movement, would have led to the archaeological site being prone to landsliding activity during both Incaic and post-Incaic times. Accepted archaeological wisdom, however, ascribes the ‘quarry’ site to quarrying activity in Inca times which would imply that block surfaces here have been much more recently exposed than hypothesized by Vilímek et al. (2005).

4. Methods

4.1 Conceptual approach

Because of the climatic and tectonic history and present conditions at Machu Pic-chu we hypothesize that, once exposed by landslides (or by any other destructive or transportation process, including quarrying), granite rock faces and boulders are affected by a combination of mechanical, chemical and biochemical weathering processes caused by temperature fluctuations, water and lichens. Aspect-induced microclimatic differences might influence the relative intensity of these processes, but, for the purposes of this study, we do not expect there to be any major dif-ference in overall weathering rates. Where soil covers a rock face or boulder we hypothesize that chemical weathering will dominate. Following the research of Birkeland (1999) discussed in section 2 above we propose that during the past 10,000 years weathering below the soil surface has been minimal, and subaerial weathering over the same time period will have had the net result of producing a weathering rind. Although we cannot establish whether weathering proceeds in a linear or in a nonlinear fashion (e.g. following some sort of exponential decay) over time without the use of dated surfaces to create an exact chronology, we hy-pothesize that older exposure surfaces will be characterized by thicker weathering rinds, with increased alteration of susceptible minerals and more common micro-fracturing.

4.2 Field sampling

Two sets of samples were collected in the field for two different types of weathering analyses. Samples A–K (Tab. 1, Fig. 1) were sampled in 2002 and were examined in Oxford using simple microscopy to investigate the degree and nature of weathering on rock surfaces. An additional set of samples (MP1–MP8) was taken in 2004 for petrological and microfractographical observations (Tab. 2) undertaken in Prague. In both field seasons the main strategy for sampling was to cover a wide range of natural rock surfaces within the archaeological site which have been hypothesised


to have been created by different landslide events or by other natural or human-induced processes. Additional samples were taken from a fresh rock fall (2002) from a lateral wall of Putucusi Mt., which is about 3 km from Machu Picchu on the op-posite site of the Urubamba River in order to provide a ‘control’ sample of a freshly exposed rock surface.

4.3 Microscopic analysis techniques

Samples were prepared by (a) using cold chisel and hammer to create fracture sur-faces from the top surface into the core of the rock and (b) using a diamond-tipped,

Fig. 1 Photo of the Machu Picchu archaeological site showing the sampling locations. (Photo V. Vi-límek)


water cooled benchtop saw to cut samples perpendicular to the top surface. Both methods produced a view from the top surface into the centre of the sample, and thus allowed an assessment of weathering rind presence and characteristics to be made. Cut samples were then observed under a dissecting microscope, using low to medium power magnification lenses in order to enable measurement of the thick-ness of weathering rinds.

Table 1 First set of samples A–K (2002). Southern Hemisphere – sunshine from the North.

Sample ID Location/ nature Orientation

A Outcrop in open air, next to Roca Sagrada N facing

B Rock face under Intiwa-tana Hill E facing

C Nr top of Wayna Picchu, old fault zone SE facing

D Top of Wayna Picchu in open air Horizontal

E On rock face in open air at Rodadero NE facing

F Weathered fault zone, freshly excavated, N end of Plaza S facing

G Outcrop at N end of Plaza E facing

H Outcrop in narrow gap next to Roca Sagrada SE facing

I Boulder in ‘quarry’ N facing

J Boulder in ‘quarry’ S facing

K Fresh rockfall from S slope of Putucusi hill SE facing

Table 2 Second set of samples MP1–MP8 (2004). Southern Hemisphere – sunshine from the North.

Sample ID Location/ nature Orientation

MP1 Huaynapichu Mt. – rock wall; fault zone with striations

S facing

MP2 Top of a separate rock block, close to the summit of Huaynapichu Mt. – frequent cracks in the block

Most of the day on the sunshine

MP3 Area of so called “quarry” 10 cm underground

MP4 Top surface from one of huge blocks in the Indus-trial sector

Partly on sunshine, partly covered by vegetation.

MP5 Top surface from one of huge blocks in the Indus-trial sector

5–10 cm underground

MP6 Fault zone (NE–SW) in the northern sector of Machu Picchu area; iso-lated boulder in the crashed zone

130 cm underground

MP7 Area of so called “quarry” Edge of the surface part

MP8 Recent rock fall (beginning of 2002) from Putu-cusi Mt.

One of the blocks for-merly located 1–3 m inside the massif


4.4 Microfractography and petrographical analysis techniques

Irregular pieces of rock specimens were diamond sawed perpendicularly to the exposed surface and carefully polished by diamond paste. !e polished surfaces were washed with clean water. Additional cleaning in ultrasonic equipment ensured removal of particles from micro cracks loosened during polishing and/or pores. !e slabs were then thoroughly dried to a constant weight at temperatures not exceeding 45 °C. !e dried samples were stored in a mould and placed in a vacuum chamber and vacuumed for about 20 minutes. A mixture of epoxy resin CHS EPOXY 513 (Spolchemie, Czech Republic) and fluorescent dye (EPODYE, Struers, Denmark) was introduced to the mould a6er vacuuming of dry samples. !e vacuum was held until air bubbles ceased to rise. !e samples were removed from the mould when the resin started to polymerise. A6er 24 hours of hardening, the excess resin on the polished rock surface was carefully polished away. Uncovered thin sections were prepared using standard procedures as recommended by the Czech Geologi-cal Survey. !e thin sections were observed using a polarising microscope (LEICA DMLP, Laboratory of Optical Microscopy, Institute of Geochemistry, Mineralogy and Mineral Resources, faculty of Science, Charles University in Prague) equipped with a source of ultraviolet light. !e photomicrographs were captured on a sensi-tive negative film (KODAK GOLD 400 ASA).

5. Results

5.1 Simple microscopic observations of weathering rinds

Most of the exposed surfaces studied here do not have well-developed thick weath-ering rinds, probably because development of such features, given the hard and low porosity granite, would take thousands of years to form. However, there are clear differences in the presence and nature of weathering rinds on the samples studied as reviewed in table 3, and much evidence that lichen thalli are penetrating into the surface (a form of biological weathering). Such differences in weathering rinds can be used to produce a rough, preliminary, sequence of samples according to the de-gree of weathering. Sample K from the fresh rock scar at Putucusi Mt, and sample B from the SE face of Intiwatana hill both have the freshest appearance with no visible evidence of a weathering rind. Sample F from the tectonic zone at the N end of Main Plaza, sample G – a boulder from near the tectonic zone at the N end of Main Plaza, and sample D from the summit of Huayna Picchu, all constitute the most intensively weathered group. Between those extremes, there are two groups. Firstly, there is a slightly to intermediate weathered group of samples C – from the fault plane in the Cliff cave area near the top of Huayna Picchu, and H – a boulder at the N end of In-dustrial Sector), and secondly an intermediate weathered group of samples (A – an-other boulder at the N end of Industrial Sector, E – from the face of Rodadero Cliff near the temple of Condor, and J & I – boulders from the “Quarry”).


Tab

le 3

C

ompa

rativ

e ta

ble

of s

ampl

es a

naly

sed

in O

xfor

d (A

–K)

and

in P

ragu

e (M

P1–M

P8).

Site

Oxf

ord/

Pra

gue

sam

ple

num

ber

sFA

1G

BM

1EM

1FC

M1

Wea

ther

ing

rind

(m

m)

Wea

ther

ing

stat

us2

Not

es

Old

faul

t lin

e ne

ar

top

of H

uayn

a Pi

cchu

MP1

**

***

*Sa

mpl

es fr

om r

ock

outc

rop

pres

umed

to

have

bee

n ex

pos

ed b

y ol

d ro

ckfa

ll ev

ent,

S an

d SE

faci

ngC

No

*

Top

of H

uayn

a Pi

cchu

MP2

***

*Sa

mpl

es fr

om to

p of

pea

k, p

resu

med

ex

pos

ed b

y ol

d ro

ck fa

llsD

0.5–

1 m

m**

‘Qua

rry’

site

, b

ould

ers

MP3

***

****

Sam

ple

buri

ed u

nder

ca.

10

cm o

f soi

l

MP7

***

**Sa

mpl

es fr

om b

ould

er s

urfa

ces

not c

over

ed

by s

oil.

‘Qua

rry’

ass

umed

to h

ave

bee

n cr

eate

d by

old

roc

kfal

l eve

nt. I

= N

faci

ng,

J = S

faci

ng

I0.

5–1

mm

**

JN

o*

Out

crop

s at

N e

nd

of P

laza

, nea

r R

oca

Sagr

ada

MP6

***

*B

urie

d un

der

1.3

m s

oil

G0.

5–1

mm

**O

utcr

ops

assu

med

to h

ave

bee

n ex

pos

ed a

t le

ast s

ince

beg

inni

ngs

of a

rcha

eolo

gica

l site

, G

= li

chen

cov

ered

, H =

spa

rse

liche

nsH

<< 0

.5 m

m*

Fres

h ro

ckfa

ll fr

om

near

by P

utuc

usi M

tM

P8**

**Fr

esh

surf

ace

rece

ntly

exp

osed

by

rock

fall

(with

in la

st 2

yea

rs).

SE

faci

ngK

No

0

Indu

stri

al s

ecto

rM

P4**

***

**Fr

om la

rge

bloc

ks a

t E o

f site

– S

ampl

e 4

from

sur

face

, sam

ple

5–10

cm

dee

p in

soi

lM

P5**

****

**

Key

:1 *

= p

rese

nt, *

* =

com

mon

, ***

= a

bund

ant,

FA =

feld

spar

alte

ratio

n, G

BM

= g

rain

-bou

ndar

y m

icro

crac

king

, EM

= e

xfol

iatio

n m

icro

crac

king

2 0 =

fres

h, n

o ev

iden

ce o

f wea

ther

ing

on th

e su

rfac

e; *

= s

ome

evid

ence

of w

eath

erin

g on

sur

face

, **

= lic

hen

wea

ther

ing

evid

ent o

n su

rfac

e


5.2 Microfractographic and petrographical analyses

!e microscopic examination of thin sections provides a more precise approach to the understanding of rock weathering process. !e decoration of pore space (both equigranular voids and flat microcracks) allows one to distinguish different types of microcracks (e.g. grain boundary, cleavage, exfoliation etc.) but also signs of chemi-cal weathering that is manifested by e.g. formation of micropores in feldspars due to its transformation to clay minerals.

Grain boundary microcracks are present in all studied samples (Tab. 3, Fig. 3). !ey are a typical weathering feature of granitic rocks occurring probably due to thermal stress (changes in temperature of the exposed surface and different thermal expansion of present rock-forming minerals). Grain boundary microcracking was common on all samples, even those from very freshly created rock surfaces. Because grain boundary cracking was common on all samples, even those from very freshly created rock surfaces, we suggest that they are not a useful diagnostic feature of the degree of weathering at this site.

Exfoliation micro-cracks are found only in two samples – MP1 from the fault plane in the Cliff cave area near the top of Huayna Picchu, and MP5 – a boulder in the N part of Industrial Sector, but sample MP1 shows a particularly intensive distribution of these microcracks (Fig. 2). !e exfoliation microcracks are typical

Fig. 2 Dominant exfoliation microcracks (bright, more open lines horizontally cross-cutting the image) in sample MP 1. This rock shows moderate grain-boundary cracking and alteration of feldspars. The width of the image is approximately 2 mm. (Ultraviolet reflected light, photo by R. Přikryl.)


transgranular microcracks oriented parallel to the exposed surface. !ey are o6en ascribed to one of three possible causes (Kranz 1983), i.e. to thermal stress (changes in temperature of the exposed surface and different thermal expansion of present rock-forming minerals), to stress release through unloading of the overlying layers, or to formation of salt or ice crystals. !ese exfoliation cracks may, thus, be diagnos-tic of a considerable period of subaerial weathering.

Cleavage microcracks are common on all samples, even those from very freshly created rock surfaces. !ose microcracks are found on feldspars and can be consid-ered as precursors of later alteration. Chemical alteration features are found only in feldspars (Fig. 4). Total alteration of feldspars occurs in sample MP2. Intensive altera-tion of feldspars is found in MP3, MP6 and MP7. Medium alteration of feldspars can be found in samples MP1, MP4 and MP5. Sample MP8 shows almost no alteration of present feldspars. !e alteration represents evidence of hydrolysis (argillitization). !e presence of feldspar alteration indicates intensive chemical weathering in humid and warm conditions (subtropical and tropical), removal of K+ and gradual forma-tion of clay minerals, namely of kaolinite. !us, feldspar alteration is also suggested to be diagnostic of a considerable period of subaerial exposure at the site.

6. Overall assessment of weathering intensity and its implications

As can be seen from table 4 the two analytical techniques carried out on the two sets of samples have produced similar results. !erefore, a joint ranking of the degree of weathering can be produced.

!e most weathered rocks are represented by both samples from the top of Huayna Picchu (samples MP2 and D), by all samples from the N end of the Main Plaza – from outcrops near to a tectonic zone (samples MP6 and G), as well as di-rectly from that zone (sample F). Two out of the four “Quarry” site samples (MP7, I) also belong to the most weathered group. All listed localities are assumed to repre-sent rock surfaces or rock mass parts exposed to weathering for the longest time.

Table 4 Ranking of weathering intensity based on (a) simple microscopic observations of we-athering rind and weathering status and (b) petrographic analysis of degree of feldspar altera-tion.

(a) (b)

Most weathered Outcrop at N end of Plaza – exposed surface (lichen-covered)Top of Huayna PicchuQuarry site – exposed boulder (N facing)

Outcrop at N end of Plaza – buried sampleTop of Huayna PicchuQuarry site – exposed boulder

Intermediate Quarry site – exposed boulder (S facing)Old fault line near top of Huayna PicchuOutcrop at N end of Plaza – exposed surface (sparse lichens)

Quarry site – boulder buried in soilOld fault line near top of Huayna PicchuIndustrial sector blocks

Least weathered Fresh rockfall, Putucusi Mt Fresh rockfall, Putucusi Mt


Fig. 4 Characteristic cleavage cracking of feldspars that facilitates later chemical weathering (hydrolysis) that results in the formation of numerous microvoids around newly formed clay mi-nerals sample MP 5. The width of the image is approximately 2 mm. (Ultraviolet reflected light, photo by R. Přikryl.)

Fig. 3 Intensive grain boundary microcracking of sample MP 4. Note wide opening of most of the boundaries. Micas (dark grain at the bottom) do not exhibit any cleavage cracking in this case. The width of the image is approximately 2 mm. (Ultraviolet reflected light, photo by R. Přikryl.)


!e group of samples exhibiting intermediate weathering shown in table 4 could be, rather speculatively, further subdivided. Samples C and MP1– from the fault plane flanking the Cliff cave area near the top of Huayna Picchu, and H – boulder at the N end of Industrial Sector, belongs to the less weathered subgroup, whilst the subgroup of well pronounced intermediate weathering encompasses samples A – another boulder at the N end of Industrial Sector, E – from the face of Rodadero Cliff near the temple of Condor, and J & MP3 – boulders from “Quarry”.

!e discrepancy between the degree of weathering found on samples from the top of Huayna Picchu Mt. (MP2 and D – most weathered) and the ones coming from the fault plane flanking the Cliff cave area near the top of Huayna Picchu (C and MP1 – intermediate, less weathered) can be used to suggest that the rockfall event which occurred along the Huayna Picchu fault line near the top of Huayna Picchu Mt. came a6er the process which exhumed the samples taken from the top of Huayna Picchu. !is suggestion is in full agreement with the placement of a large rock fall scar at the SE facing wall of Huayna Picchu in the geomorphological map (Vilímek et al. 2005).

It is interesting that the lichen-covered and soil-covered samples (G and MP6) from the outcrop at the N end of the Plaza are both classified as most weathered, whilst another sample (H) from the same site, which has only sparse lichen cover, falls into the not so well, intermediate weathered sub-category. !e sample with sparse lichen cover was taken from adjacent to a fracture in the rockface and may thus have suffered from spalling or other physical removal of the upper weathered rind. !e soil-covered sample MP6 was extracted from 1.3 m depth of soil and thus may reflect the impacts of long-term burial which, according to Birkeland (1999), leads to an increase in weathering rate to the same or more than that on subaerially exposed surfaces.

Comparison of exposed sample H from the south facing “Quarry” area boulder with the sample J which was facing north could indicate the influence of aspect. !e former belongs to the lower intensity subgroup of intermediate weathering, whilst the latter sample belongs to the well developed one. However, the S facing sample also had less lichen cover which may also be an influencing factor.

It is hard to place the “Quarry” site event in to a clear relative dating framework, because of the variability of weathering intensities recorded there. But there is cer-tainly some evidence that it is older than the rock fall event that affected the cliff at the fault line near the top of Huayna Picchu. Moreover, this evidence (the occurrence of most weathered rock surfaces in the form of samples MP7 and I in the ‘quarry’ site) does not support the interpretation that these block field landforms are a product of stone quarrying by the Incas, as is the official archaeological explanation (Fig. 5).

!e ranking also confirms the obvious fact that samples from the fresh rockfall from Putucusi Mt. (K and MP8) are the least weathered of all those sampled. How-ever, another fresh looking sample (sample B) was taken from the Incaic terrace-free part of east face of Intiwatana hill. !is sample indicates that the bare face of the southern end of the east slope of Intiwatana hill could be the result of a relatively very young event. !is is again in good agreement with geomorphological and en-


gineering geological findings. According to them, this slope part belongs to an un-settled, linearly prolonged zone predisposed by tectonic shattering. !is zone had probably caused problems even to Inca builders by producing unequal settlement of the Main Temple grounds. According to archaeologists (F. Astete, personal com-munication) that temple was abandoned and its walls become cracked before it was finished. Nowadays, the disturbance of archaeological structures by open cracks and shattering of their building blocks are traceable along a line starting at the Intiwatana site, through the Temple of !ree Widows and to the building adjacent to the latter at SW, and finally heading towards the “Quarry” area. Both the rock outcrop making the southern end of Intiwatana hill just behind the Main Temple, and the bare face of the southern part of the main rock face of Intiwatana are places of active microde-formations (Vilímek et al. 2005). !e form of the bare rock face, the character of its contact with terraced slope part and the existence of an accumulation pile at the toe of the bare rock face lead us to the conclusion that the destruction of Incaic terraces at this part of Intiwatana was caused by a small size rockfall. Unfortunately, there is no confirmation for this very important weathering finding by another sample analysed by the microfratographic and petrographic method.

However, the Intiwatana rock fall conclusion is in a disagreement with the inter-pretation of archaeologists (F. Astete, personal communication). !ey deny the exist-ence of a rock fall accumulation pile there. Instead, they propose that the basement

Fig. 5 Photo of the so-called quarry. Inca people split some blocks, but the whole site is natural conditioned. (Photo V. Vilímek)


terrace had originally possessed an irregular form. Following that idea, they have rebuilt the pile of natural rock blocks mixed with manually shaped ones (remains of fallen terraces) into a new terrace wall protruding forwards into the Main Plaza.

!ere are several limitations to our investigation which represents a preliminary attempt to utilise the weathering record on natural rock outcrops to help constrain the timing of geodynamic events at Machu Picchu. Firstly, we were not able to sam-ple any of the stones carved by the Incas, thus it is difficult to constrain the dating of fresh and medium weathered samples in any more detail in relation to the Incaic period. Secondly, we have only carried out simple observations of weathering status from a small range of samples. Further analyses are proposed to investigate other parameters related to weathering intensity, e.g. rock hardness using the Schmidt- hammer (see Swantesson 1992) and surface roughness (Pope 2002) measurements. !ese further analyses should help to provide a more confident assessment of weath-ering intensity, and also will enable us to investigate in more detail the influence of lichens and aspect on weathering rates which might confound any simple attempt to date surface exposure using weathering intensities.

7. Conclusions

Different types and degrees of weathering of granites have been found by two dif-ferent methods on samples from the Machu Picchu archaeological site. Good agree-ment between the results of both these methods has enabled a preliminary attempt to utilise the weathering record on natural rock outcrops to help constrain the tim-ing of geodynamic events there. !e results of that attempt agree with geomorpho-logical and engineering geological interpretations of individual landforms and the hypothesis that there have been multi-generation and multi-scale slope movements in the area in question. Sometimes, as in the case of the S part of the eastern slope of Intiwatana hill, the weathering records support the geomorphological-engineering geological interpretations of environmental history here against the official archaeo-logical ones. Further systematic sampling of a wider range of weathered surfaces together with a study of other parameters related to weathering intensity (e.g. rock hardness, and surface roughness), are needed in order to confirm or refute the ex-planations and interpretations presented here.

Acknowledgements

�e authors would like to thank the Ministry of Education, Youth and Sports of the Czech Re-public (Projects MSM 00216 20831 and INGO, LA 157) for their financial support. Special thanks belong to INC Cusco and INRENA Cusco for scientific and personal support as well as to Dr. V. Kachlík (Charles University in Prague) for the petrological analysis and Pia Wind-land (Oxford University) for sample preparation. �e porosity study was possible through the A3046401 project (Grant Agency of the Academy of Sciences of the Czech Republic).


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intensity of weathering as a relative exposure age indicator of mass

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