Lake George Geology, History and Formation
A unique lake due to its formation and location!
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 the present (ByrBP). A shallow sea covered the area that we now call the Adirondacks during the middle Proterozoic era, 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. 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. Faults were created in other areas of rock that did not fold.
Image from New York State Museum
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.
During the next 400 Myr, following the end of the Grenville Orogeny (1.0 ByrBP), 25km of rock 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.
Grabens, such as the valleys of Lake George, were created by stretching of the crust
Image from Mike Story
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. On the east side of the fault, the mountains remained at the same elevation or rose. On the western shoreline, the same process occurred. 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.
Lake George as we know it today was formed as the glaciers receded about 10 to 12,000 yrBP. 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.