64 th highway geology symposium, north conway, …64 th highway geology symposium, north conway, new...

64th Highway Geology Symposium, North Conway, New Hampshire, September 2013

Page 30 of 87

Roadside Geology of the 2013 HGS Field Trip

By Wally Bothner and Frederick Chormann The following text and maps are provided as an accompaniment to the detailed narratives for each of the stops along the field trip route, presented sequentially following this section. The map on page 31 provides an overview of the bedrock geology traversed along our 120-mile loop through the White Mountains, clockwise from North Conway and back. The explanation for all the corresponding map units appears on page 32. Each of the subsequent 6 maps highlights the bedrock geology and other points of interest along a segment of the route centered on one of the field trip stops. Page 31: Overview map showing filed trip travel route, field trip stops and drive-by sites. Page 32: Explanation of bedrock unit symbols shown on field trip map.


Page 31 of 87


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Page 33 of 87

Page 34: Hotel to Drive-By Site A (North Conway to Soil Nail Walls Kancamagus

Highway) – Travel south from conference center to Conway on NH Route 16. Beneath us is typically pink coarse-grained Jurassic Conway biotite two-feldspar granite. It also underlies Cranmore Mountain to the east and reappears at Redstone to the south. Between these two mountains are Peaked and Rattlesnake Mountains, underlain by coarse-grained, greenish Jurassic Osceola (Jo1h and Jh) amphibole or pyroxene-bearing one-feldspar granite. To the west is Moat Mountain, the type locality of the Jurassic Moat Volcanics (Jmv). They consist of rhylolite flows, ash deposits, and breccias, some of intermediate composition. As elsewhere in the state, the Moat Volcanics are in contact with ring dikes supporting a cauldron subsidence as the principal means of preservation; see also Bartlett Mountain to the north.


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Page 35 of 87

Page 36: Drive-By Site A to Stop 1 (Kancamagus Highway Soil Nail Walls to Pemigewasset

Scenic Overlook) - We will see some of the Albany Porphyritic Quartz Syenite (J4hx) as we drive past Lower Falls of the Swift River heading west on the Kancamagus highway. We will also drive past Champney Falls trail leading to the top of Mt. Chocorua (Jo1h), and Sabbaday Falls trail (a must if you have a hour to spare!) that exposes typical Conway granite and a Jurassic (or younger?) diabase dike elegantly exploited by the stream. For the enthusiast this trail connects with another to the top of Mt. Tripyramid, underlain by one of the youngest central complexes with several ring dikes of syenite and an unusual and relatively rare gabbroic body, all of Cretaceous age. At the Pemigewasset Scenic Overlook, Stop 1, note the beautiful coarse grained, well-jointed Conway biotite two-feldspar granite in nearby road cuts and Mt. Osceola, the type locality for that one-feldspar granite (of the same age) is visible to the southwest. Here we are near the center of the White Mountains composite batholith. You should be able to see several landslide scars on the north flank of Mt. Osceola. Continue west and southwest crossing more Conway granite, then an outer belt (ring?) of Mt. Osceola Granite to about Lincoln where shortly w leave the White Mountains batholith.


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Page 38: Stop 1 to Drive-By Site B to Stop 2 (Pemigewasset Scenic Overlook to Loon

Mountain Bridge to I-93 Barron Mountain) – As we approach the center of Lincoln, we enter the “sea” of Silurian metasedimentary rocks of the Central Maine terrane. The rocks exposed along I-93 while heading south to Stop 2 are steeply dipping, high-grade schists of the Rangeley Formation. They are assigned to two members: Srl is the lower member and is typically a gray, thinly laminated pelitic rock with rare calc-silicate and coticules; and Sru, the upper (younger) member, is a rusty weathering metapelite and metasandstone with common calc-silicate pods and coticule. At the Barron Mountain rock cut, most of the rocks are coarse gray schists of Srl. In addition to the critical geologic engineering done to stabilize the cut, what mineral assemblage is obvious? A hand lens might be helpful. Note as we return north that the rocks just north of the North Woodstock are quite different from those seen so far… look for very coarse grained, dark gray porphyritic rocks of the Kinsman Granodiorite (Dk2x). The phenocrysts of alkali feldspar can reach nearly 10 cm and their cleavage often reflects sunlight easily as you drive-by… a ‘sparkling’ experience. These rocks of the Cardigan and Lincoln Mountain Plutons are also exposed through Kinsman Notch to the west and in an intrusive breccia at The Basin (see map on page 40 for location) should your travels allow such stops another time.


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Page 39 of 87

Page 40: Stop 2 to Stop 3 (I-93 Barron Mountain Rock Cut to Old Man Historic Site) – Both Kinsman and Conway intrusives are exposed along I-93 as we head north on our way to LUNCH. We pass through the central portions of two barely connected stocks of Conway granite – the Pemigewasset pluton and the Cannon Mountain pluton, the latter separated from the White Mountains composite batholith by a “screen” of Kinsman Granodiorite. Peaks to the west are underlain by Dk2x while those to the east are composed of “Mt. Lafayette” granite porphyry (J1hx) and porphyritic amphibole-bearing quartz syenite (J4hx), then a true screen of Dk2x and Sr, before entering the main batholith to the east. A path to the Old Man Plaza has numerous blocks of the now familiar Conway Granite along the way. Note also the landslide scars and landslide deposits at the south end of Profile Lake. The fieldtrip will break for lunch after Stop 3. Lunch will be held at the Peabody Slope

Base Lodge at Cannon Mountain.


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Page 41 of 87

Page 42: Stop 3 to Stop 4 (Old Man Historic Site to Carroll Visitors Center) - Our travels to Stop 4 take us over poorly exposed rocks of the Ordovician Ammonoosuc Volcanics (Oalx, bimodal volanics) and of Ordovician Oliverian Plutonic Suite (Oo1bx). The latter are variably foliated biotite granite gneisses.


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Page 43 of 87

Page 44: Stop 4 to Stop 5 (Carroll Visitors Center to Mt. Washington Scenic Overlook) - From the Twin Mountain stop, we travel east across more Conway Granite before reaching the Mt. Washington Hotel Scenic Overlook, Stop 5 (underlain by two-mica granite of the Bretton Woods Pluton). Refer to Figure 3 on page 24 for an idealized cross-section of the area during Early Devonian time and visualize the overlying Littleton Formation eroded away to expose the pluton.at the present level of the land surface. Traveling southeast on US Route 302 we pass Saco Lake at the headwaters of the Saco River and through “the gate of the notch” into Crawford Notch. Along the way we cross a bit of Silurian Rangeley Formation and Conway Granite before reentering the main composite batholith as we approach Stop 6.


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Page 45 of 87

Page 46: Stop 5 to Stop 6 (Mt. Washington Scenic Overlook to Willey House) – South of Saco Lake, rock of the Littleton Formation occurs on the flanks of Mt. Willey as a screen between the outer ring dike and the main batholith. At the Willey House we have just crossed the contact between the Conway and Osceola granites. By Notchland we cross the largest roof pendant (~8 x 3 km northeast trending block) within the batholith that contains the stratigraphic section Rangeley (Sr), Smalls Falls (Ssf) and Madrid (Sm) Formations in addition to a mass of Dk2x. If you’re still following along, find where US Route 302 turns east south of Notchland. Here we cross one of more interesting of the smaller ring complexes at Hart Ledge. It consists of partial ring dikes and stock of syenite, quartz syenite, sodium and iron-rich amphibole syenites and granites of Jurassic age (but younger than the Osceola Granite that it cuts). From here it’s a straight shot back to the conference center.


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Page 47 of 87

See previous map on Page 46: Stop 6 to Drive-By Site C to Hotel (Willey House to US Route

302 Sawyer River Bridge to North Conway) - The fieldtrip concludes after Drive-By Site C.

64 th highway geology symposium, north conway, …64 th highway geology symposium, north conway, new...

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