Adirondack Geology: A close look at the High Peak Region

• The oldest rocks that can be found in the Adirondacks are from the Grenville Series.

• They are 1500 to 1300 million years old.

• This rock is anorthosite, which is metamorphosed strata, formed by pressure and heat.

• It is the predominant strata found in the Adirondacks (Jaffe, 3).

Hannah and Aaron explore exposed anorthosite on Baker Mountain in Saranac Lake.

• Anorthosite metamorphosed with other strata to form other minerals such as wollastonite and monticellite marble.

• Syenite and hybid gneisses formed from partially melted Grenville strata (Jaffe, 3).

Landscapes change over time. The Adirondack Mountains formed from a combination of uplift (caused by plates

converging) and erosion (Jaffe, 5).

This picture shows some of the High Peaks, it was taken from the top of Baker Mountain in Saranac Lake. Photo by Jaime Armstrong.

Landscapes change over time.

Water is a major force in changing the landscape over time. Hurricane Irene caused the landscape to change dramatically in the Adirondacks. The picture below shows the washed-out road in Keene. The Ausable River rerouted itself, causing havoc, as a result of the hurricane. Photo by Phil Brown, used with permission.

Landscapes change over time.

Duck Hole was a favorite spot for many Adirondack campers. Flood waters from Hurricane Irene caused a breach in the Duck Hole Dam. What was once “one of the most scenic water bodies in the High Peaks Wilderness”, according to the Adirondack Mountain Club, is now a mudflat. Hundreds of years from now, geologists will be able to tell that this area was once covered in water due to the fossils left behind. Photos used with permission, by Phil Brown. (DEC won’t rebuild Duckhole Dam)

Angular Unconformity.• These rocks, toward the

top of Owl’s Head Mountain, are at an angle due to the uplift. An angular unconformity would be present if sediments formed a layer over this rock. In that case, it would be clear that the layer below was deposited first, uplifted, then deposited over.

Intrusion

• This rock appears to have an intrusion.

• The grey rock that “intruded” came after the pinkish rock was already there.

• Therefore, the pinkish rock is older than the grey rock.

Pitchoff Mountain Trail

The view from “Balance Rock” on Pitchoff Mountain. Photo by Jaime Armstrong.

Pitchoff Trail: Van Hoevenberg Gneiss

• According to Jaffe, “Geology of the High Peaks Region”, the first lookout to the Cascade lakes, on the Pitchoff Mountain trail contains Van Hoevenberg gneiss (a.k.a. gabbroic anorthosite augen gneiss) (Jaffe, 54).

Photo taken on the trail to balance rocks.

Pitchoff Trail: Van Hoevenberg Gneiss

• Van Hoevenberg gneiss contains blue crystals, which are andesine-labradorite plagioclase feldspar; “white groundmass”, which is plagioclase with augite, garnet and hornblende (Jaffe, 54).

• Lichen, which was covering much of the rock, made the detail difficult to see.

This picture was taken where the guidebook described Van Hoevenberg gneiss would be. However, from the description I think that the photo on the previous slide (found further up the trail) is correct.

Pitchoff Mountain

• The trail opens up to the exposed rock. Here boulders are covered in lichen. This makes them difficult to identify.

• According to the location description in the guidebook, this is a hornblende-magnetite quartz syenite (Jaffe, 54).

• Please continue on to Part 2 of 3.