Geology of the Lake George Region and the Adirondacks

The Lake George region and the Adirondacks did not form overnight. In fact, it took billions of years to become what it is today. The following is a brief outline of the complex and interesting history of the Lake George/Adirondack Region. Many links are provided throughout for those that are looking for a more in depth explanation or for those with a greater curiosity.

For help with Geologic Terms: Link to Iowa State University’s Illustrated Glossary of Geologic Terms http://www.geology.iastate.edu/new%5F100/gloss.html

The events we are talking about have taken place over a long period of time. To gain a better understanding of when certain events occurred, reference is made to a geologic time scale. The time scale enables the classification of events during Eons, Eras, Periods and Epochs. The different categories usually separate major events such as the Cretaceous-Tertiary boundary that occurred 65 million years ago. This boundary represents the mass extinction of the dinosaurs and ammonoids on Earth.

The Earth itself is approximately 4.6 billion years old and has experienced many changes throughout time. In our region, there is evidence of historic geological events dating as far back as 1.3 billion years before present (ByrBP). A shallow sea covered the area that we now call the Adirondacks during the middle Proterozoic approximately 1.3 ByrBP. The sea deposited sediments, precipitates and fossil remains to form sedimentary layers.

Between 1.3 and 1.1 ByrBP the proto-North American continental plate collided with another plate in a continent-to-continent collision. The collision started a mountain building process called an orogeny. This was called the Grenville Orogeny and was part of the Grenville Province. The Grenville Orogeny or Ancestral Adirondacks is similar to the present-day Himalayan Mountains, which are still forming as a result of the Indian continental plate and the Asian continental plate colliding. During the orogeny, the crust became very thick rising and sinking as it was compressed. The rocks were pushed down to depths of 30km and the pressure and intense heat from the Earth’s mantle metamorphosed the sedimentary rock. As the sedimentary rock was changing its composition, the layers were being folded up and over each other like pushing together two ends of many blankets that have been layered on top of each other. Faults were created in other areas of rock that did not fold.

At about the same time as the Grenville Orogeny was taking place, magma rose through the rock layers forming dikes. These intrusions as well as the folding and faulting that occurred can be seen today in some exposed outcroppings or road cuts.

The next 400 Myr following the end of the Grenville Orogeny (1.0 ByrBP) consisted of 25km of rock being eroded from the mountains.

That brings us to the late Cambrian period. From 550 MyrBP through almost the entire Ordovician Period (450 MyrBP) the eastern edge of proto-North America was covered by the Iapetus Ocean. The Iapetus Ocean again deposited sediments, fossils, and precipitates. These sediments covered almost the entire area of New York.

The mid-Ordovician throughout most of the Tertiary was a period of low tectonic activity for our region. New England and southern New York experienced orogenies, but the Adirondacks were largely unaffected by these mountain building episodes and continued to erode becoming nearly flat. In the late Tertiary or Miocene Epoch, the Adirondacks were uplifted. The cause of this uplift is speculated to be the result of a hotspot. The hotspot heated material under the crust. Once heated, the rock began to expand and rise pushing up to form a dome like structure. The dome has since eroded exposing older metamorphosed rock. This process is shown in figure 4.23 from Geology of New York A Simplified Account, Educational Leaflet No. 28. The uplift is believed to be continuing today. Please see the following link for more info on the rising Adirondacks.

The Lake George basin was influenced and formed by faults. There is a fault running along the eastern shoreline of the lake. The bed of the lake slipped down on the west side of the fault and on the east side of the fault the mountains remained at the same elevation or rose. On the western shoreline, the same has occurred with another fault forming the western shoreline. Actually there are three more faults, two on either side of the Tongue Mountain Range and a third that runs from Northwest Bay south along the shoreline in the Village of Lake George. The feature of a sunken basin surrounded by mountains on either side is known as graben (basin) and horsts (mountains). This feature is similar to the Basin and Range Province in the southwest U.S. (Utah and Nevada), but on a much smaller scale. At this time, the lake was not a lake. It was two rivers! One river flowed south out of Northwest Bay into the present day Hudson River and the second flowed north from the area of the Narrows into present day Lake Champlain.

Beginning 1.6 Myr BP the climate began to cool and glaciers soon covered the northeast. The Pleistocene Epoch began with the introduction of the glaciers. Throughout the Pleistocene several ice sheets advanced over New York, each erasing evidence of the last. The last ice sheet was called the Wisconsinan ice sheet. As it crept over the surface, many surface features that we see today were formed. The glaciers bulldozed, scraped and crushed the surface soils and rocks, gouged out river valleys and carried materials forming a new landscape. As the glaciers retreated deposits were left behind which allowed the Lake George basin to form. Some of the glacial features that are visible in New York include: glacial striations, eskers, drumlins, moraines, kettle lakes, and a lot of glacial till. The National Snow and Ice Data Center (NSIDC) has a link for a glossary of terms on glaciers.

Lake George as we know it today (for the most part) was formed as the glaciers receded about 10 to 12,000 yrs BP. The ice sheet paused at the southern end of the lake just north of Glens Falls as it receded. This pause left behind a recessional moraine made up of boulders, stone, and sand forming a dam at the south end of the lake. A similar event occurred at the north end resulting in the basin that we currently see. The departing glacier left some ice behind to help fill the basin with water.

Presently, the main water source for Lake George is from precipitation throughout the year with some additional water entering the lake from springs. The basin is not a closed basin, the water from Lake George flows out the LaChute River into Lake Champlain, the Saint Lawrence Seaway, and eventually into the Atlantic Ocean.

Geologic maps

Surface geology and bedrock geology from NYS Museum created in MapInfo.

Click on the names below to view the coordinating maps:

[Adirondack Uplift] [Map of NYS]

Take a Geologic Tour!

The following is a map that Jason Brechko used during his 2000 LGA Lake Talk to indicate some interesting geologic sites that you can get to in the area. (The numbers on the map correspond to the following descriptions.)

1: Lake George Park: At the entrance to the bike path you find Ordovician limestone. This entrance and Long Island contain some of the few Ordovician deposits left in the lake . This is especially important because of the surrounding hills made of Precambrian gneiss.

2: Exit 21 Anticline: Layers of gneiss can be seen here that were originally horizontal and were bent when they were still several miles beneath the surface.

3: Second Lookout on Prospect Mountain Road: This lookout can be reached by hiking 0.5 miles up the Prospect Mountain Trail. The rock cuts here expose excellent PreCambrian metamorphic rock including metagabbro and granitic gneiss. Look for large hornblende and garnet crystals in the rock. Notice the horizontal joints or cracks in the rock that occurred from glacial rebound. Also soak in the view of the Lake George Graben with its surrounding horsts.

4: French Mountain: The sheer face of this mountain is an excellent example of the eroded East Lake George Fault.

5: Glen Lake: This lake is a classic example of a kettle lake formed by a block of ice that was surrounded by glacial deposits. When the ice melted the hole was filled with water. Notice the steep low ridges in this area. These are eskers, a type of glacial deposit formed by streams running within the glacier.

6: Round Pond: Another example of a kettle lake surrounded by eskers, kames, and other glacial deposits.

7: Ridge Road Landfill: While dropping off your trash, recyclables, and compost, notice the areas of unsorted glacial deposits and further back the areas of sorted sand from glacial outwash. The ridge that includes this site and the Glen Lake site dammed the south outlet to the Lake George Graben.

8: Lake Sunnyside: Another example of a kettle lake.

9: Buck Mountain Summit: Besides spectacular views of the Lake George Graben, central Adirondacks, and the high peaks, you can also find excellent garnet specimens in the granitic gneiss of this peak.

10: Assembly Point Anticline: Directly south of the Assembly Point Road you will find some interesting folded gneiss layers that were once horizontal.

11: ACC Limestone: the outcrops in front of ACC are the same rock as the Ordovician limestone in Lake George Village.

For more information about Geology Related Links, see our Related Links Page.

** Please note: A Geographer and not a-Geologist outlined this page. If you notice any inaccurate data, please contact the GIS Coordinator. Thank you!