A Geological History of the Adirondacks

Contrary to popular belief, the Adirondacks are not geologically related to the Appalachians. In fact, they are the only mountains in the eastern U.S. that aren't geologically Appalachian. They actually belong to a much older formation known to some as the Canadian Shield, as the Laurentian Shield to others, and as the Precambrian Shield to the rest. This is a huge formation, underlying about half of Canada (it was noted that Canadians more often refer to it as the Canadian Shield, while Americans prefer Precambrian Shield). The formation extends down through the Thousand Islands region of
The Trap Dike on Mt. Colden
from Avalanche Pass.
the St. Lawrence River Valley and into the Adirondacks. Adirondack rocks are most closely related to rocks found in what is known as the Grenville Province, an ancient formation north of Lake Ontario and east of Lake Huron. As a whole, the rocks making up the Adirondack region are amongst the oldest on the planet, around one billion years old. For the Adirondack bedrock to consist of the minerals that it does, it must have formed underneath 15 miles of overlying rock. Sediment building up on the bottom of an ancient sea, covering the present-day eastern seaboard, was forced lower and lower by more and more sediment, until it finally metamorphosed under the intense heat and pressure into the rocks we see today, a billion years later. After repeated uplifts spanning hundreds of millions of years, including one that is continuing today at a rate of 2 to 3 millimeters per year, and constant erosion of the sedimentary layer above, the ancient bedrock finally reached the surface. The rock that makes up most of the High Peaks region is Anorthosite. This rock, more often found well underground than at the Earth's surface, apparently is very common on the moon. The photo to the right shows the Trap Dike on Mt. Colden. This giant cleft, which runs nearly to the summit of the mountain (and, incidentally, is a possible climbing route from Avalanche Lake), was formed by the erosion of the softer gabbro that had intruded into the anorthosite bedrock. It is conceivable that the dike will only get deeper, judging by the large volume of water that, although hard to see here, was flowing through it at the time this photo was taken. The rest of the Adirondacks are made up of various different rocks, mostly some form of gneisse.

Another common misconception is that the Adirondacks are just old, worn-down peaks like the Appalachian chain. Yes, it is true that at one point Upstate New York was at an elevation rivaling, if not exceeding, that of the Himalayas today. However, most of the major features of the landscape that you see today are a result of the last Ice Age, 10,000 years or so ago, and the most recent uplift of the region, which probably began about 5 to 10 million years ago. The huge boulders often seen lying around the Adirondacks, sometimes in places you wouldn't expect to see them (such as the summit of Mt. Marcy or in an otherwise rock-free meadow), are by-products of the glaciers. Known as erratics, they are picked up by the moving ice sheet and dropped when the glacier receded. Since the mile-high summit of Marcy was itself covered by ice a mile thick, it was no more immune to receiving an erratic than a valley 4,000 feet lower. [As a side note, before there was widespread acceptance of glaciers and the Ice Ages, a popular explanation for these erratics was that a vast ocean covered these lands, and the boulders just floated to where they are now. Personally, I am more inclined to believe in two-mile thick sheets of ice than I am in a 15-ton boulder floating to the 5,344-foot summit of Mt. Marcy.] Erosion continues, exposing more and more bedrock. Major new slides form on the flanks of the taller peaks rather frequently, sweeping away large patches of soil and vegetation. Another major geological/geographical feature caused by the uplift of the region is the many fault lines throughout the Adirondacks. A simple glance at a topographical map will show that these faults, for the most part, all lie in a northeast-southwest orientation. Long Lake and Indian Lake, about 15 miles apart as the crow flies, run nearly parallel to each other. Nearly all the passes and valleys in the High Peaks run in the same general direction: Indian Pass, Avalanche Pass, Lake Arnold Pass, the Johns Brook Valley, the Ausable Valley, Hunters Pass. These valleys, lakes, and passes exist where faults weakened the bedrock, allowing it to be more easily eroded by glaciers.

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