The Adirondack Park is the largest wilderness east of the Mississippi River. At more than 6-million acres, it’s the size of Yosemite, Yellowstone, Grand Canyon, Glacier, and Great Smoky Mountains National Parks combined.

Within the park’s boundary (commonly referred to as the blue line) are more than 3,000 lakes, 30,000 miles of rivers and streams, hundreds of mountain summits (two that exceed 5,000 feet in height: Mount Marcy and Algonquin Peak), and an exceptional variety of eastern hardwood and boreal forest habitats, including rare, old growth forests and freshwater wetlands (marshes, peatlands, bogs and fens, swamps and open river corridors).

Unlike any other U.S. national or state park, more than a hundred small towns and villages can be found inside the Adirondack Park. In fact, according to 2020 census data, more than 123,000 people live inside the blue line, year-round (5% fewer than in 2010). Still, population density within the park is the lowest found anywhere east of the Mississippi River.

Interestingly enough, more of the land within the blue line is privately owned than publicly owned (3.4-million acres and 2.6-million acres, respectively). The park also contains several farms and agricultural businesses, as well as a wide range of small businesses and an active timber harvesting industry.

The boundary of the Adirondack Park more or less matches that of the Adirondack Mountain range; encompassing roughly 5,000 square miles.

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A geographically unique mountain range

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Although it’s commonly assumed that the Adirondack Mountains are part of the Appalachian Mountain range, they’re not.

In fact, they’re the only mountains in the eastern United States that aren’t geologically Appalachian.

The Appalachian Mountains were created by colliding tectonic plates. And tectonic plate boundary collision processes are relatively well understood. But the Adirondacks were uplifted, forming a more or less circular dome about 160 miles in diameter and 1 mile high. Uplift mechanisms are much less clear. And the Adirondack doming process, or uplift, has long been a geological mystery.

Interestingly enough, it’s still happening. The Adirondacks are still growing, rising by around 2- to 3-millimeters every year, a rate which is faster than the rate of erosion.

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A hot spot

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It’s been widely accepted that, approximately 20 million years ago following a series of continental collisions and separations, an uplifting of, until recently, undetermined origin began.

Although the Adirondack Mountains lack areas of geothermal activity (e.g. hot springs, geysers), it has long been widely accepted that beneath the crust, a “hot spot” of some kind. Perhaps a bubble of magma was responsible for pushing up the old basement rock and overlying sedimentary rock (sandstone, limestone, shale) that formed the Adirondack dome. But tools for evaluating the formation mechanism, or rising, of the Adirondack Mountains simply didn’t exist.

Recently however, researchers, using an advanced seismic imaging method and the most up-to-date data available, discovered what they’ve described as a “pillow” of low-density, relatively light rock material which, possibly expanded by heat, formed the dome-shaped Adirondack Mountain range. Addressing the results of their research, UMass Amherst geoscientist and postdoctoral fellow Xiaotao Yang said: “We propose that geologic processes triggered the flow of a lower density, relatively light and buoyant, low-velocity material from a large reservoir into a fracture or a vacant space and it accumulated there in a column and was lifted up. It also may have been hot and thermally expanded.”

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New mountains, old rocks

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The rocks we see on the surface today predate the rising, or forming, of the Adirondacks. They are the ancient bedrock, risen to the surface.

Sometime around 1 billion years ago, under the intense heat, pressure and buckling and folding of colliding continental plates, the preexisting sedimentary rock began slowly transforming into a mixture of metamorphic and igneous rocks. For example, quartzite — a metamorphic type of rock that we often see lying on the ground — was originally pure quartz sandstone. The more common metamorphic rocks found in the Adirondacks include quartzite, gneiss, schist and marble.

Igneous rocks formed from hot, molten rock that crystallized before solidifying. Common igneous rocks include granite, obsidian and basalt.

Over the next several million years the uplifted sedimentary rock was slowly worn away to expose the billion-year-old metamorphic and igneous bedrock that rests upon the surface of the Adirondack Mountains today.

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Mountains of moon rocks

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It’s interesting to note that the rock which makes up most of the High Peaks region is Anorthosite, an igneous rock that is 90-100% feldspar and, apparently, very common on the moon.

It’s also worth mentioning that we can find sedimentary rock containing fossilized plants and animals that lived 400- to 500-million years ago along the perimeter of the Adirondack range.

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Erratics

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The huge boulders we often see along trails and in unexpected places, such as the summit of Mount Marcy or Mount Skylight, are known as (glacial) erratics. They were lifted by the movement of the advancing glacial ice that once covered the entire region and deposited where they now sit, when the glacier receded. Glacial erratics can be carried for hundreds of miles and often differ significantly from other rocks in the area in which they rest.