Back in January of 2022, we were invited for a winter hike within the Adirondack Mountain Reserve, and while we had a fantastic time, there is something more important underfoot that needs to be discussed. And that is the geology!
The Adirondack Mountain Reserve (AMR) is a small park located within the larger Adirondack Park near the small town of Keene, NY:
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| Location map for Adirondack Mountain Reserve (from the NYS DEC) |
The mission of the Adirondack Mountain Reserve is to preserve and protect the forests, lakes, mountain streams, and wildlife on our wilderness tract for the enjoyment of our members and, under the terms of a Conservation Easement in perpetuity to the State of New York, members of the public. We will preserve the forever wild character of this forest land and our property at St. Huberts in a manner appropriate to allow the enjoyment of their wilderness beauty. The mission also includes the operation of a club, with associated facilities, for the pleasure and respectful recreation of its shareholders.
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| Adirondack Mountain Reserve map. A digital version can be found the New York Almanac |
The park essentially starts in the northeast with the East Branch Ausable River valley. The entrance gate is located along this northeastern border of the Reserve. The park follows the river valley to the southwest through the Lower and Upper Ausable Lakes with another unnamed lake (or at least a lake I can't figure out the name of) situated at the southwestern point of the reserve boundaries.
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| Map of the park with some notable points highlighted. |
There are several spots within the park where the geology of the region can easily be seen. However, since we were there in winter, even those locations had less than ideal exposures. These spots that we stopped at are highlighted on the map above.
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| Beaver Meadow Falls |
Hiking southwest from the entrance gate one of the first locations that you can see the geology is at Beaver Meadow Falls. Waterfalls are always a great place to check out the geology.
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| Rainbow Falls |
Continuing towards the southwest along the trail, you then come across Rainbow Falls.
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| Indian Head Vista |
And eventually you can make your way up to the Indian Head Vista, with an absolutely gorgeous view of the Lower Ausable Lake.
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| View of the rocks on the Indian Head Vista. |
Pictured above are the rocks at the vista itself. However, if we look at the geological map below, it can be noticed that almost all of the rocks within the Adirondack Mountain Reserve are actually the same rock. They are nearly all the orange color, which is denoted as "a" on the
Geological Map of New York.
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| Geological Map of of the Adirondack Mountain Reserve. The black box indicates the extents of the map pictured above. |
Reading the Legend from the New York State Geological Map itself, I warn you, there is some heavily geological terminology in the paragraph ahead. Per the Legend, the rocks within the park, denoted as "a", are:
Black to light gray or greenish-gray metanorthosite and anorthositic gneiss (Marcy facies); overprint signifies mafic mineral content in excess of 10 percent (mainly gabbroic or noritic metanorthosite and subordinate anorthositic metagabbro or anorthositic metanorite-Whiteface facies). Subordinate members of this series grade through metagabbro or metanorite, to ilmenite-magnetite-rich metagabbro or metanorite, to pyroxenite rich in ilmenite-magnetite, to ilmenite-magnetite ore. Primary subophitic texture is variably destroyed by granulation and recrystallization resulting in a crystalloclastic fabric. Metamorphic minerals include garnet, clinopyroxene, quartz, hornblende, and a plagioclase less calcic than the original. Mafic mineral percentage is contoured in northwestern portion (St. Regis Quadrangle) of the Marcy Massif, contour value being shown on high side of contour line.
While that is a lot of technical jargon, I will try and break it down. The rock unit that the AMR sits on it known as the Marcy Anorthosite Massif, named after Mount Marcy, the highest point in NYS, which is located just to the west of the AMR. Anorthosite is an igneous rock made up almost entirely of the mineral plagioclase feldspar. And not only does this rock body cover the park, it is massive in size.
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| Large view of the Adirondack region of New York, showing the full extent of the Marcy Massif. |
Typically when we have mountain, we expect the geology to be jumbled up with many types of rocks overlaying each other. And that is actually what we see for the most part, outside the orange colored Marcy Anorthosite Massif. The Marcy Massif though sits at the center of the mountains, and it is in essence, the core of a dome structure that makes up most of the Adirondack Mountains.
The core of the Adirondack Mountains, the Marcy Massif, is ~1.1 billion years old (Ga), and covers an area of 3000 square kilometers (~1150 square miles). The rock was initially formed as an igneous intrusive magma body that cooled slowly over time, forming the plagioclase rich anorthosite. After formation this rock underwent periods of metamorphism where parts of the rock were altered, mostly along the outer edges. That is a lot of what is in the description of the rock unit above. These metamorphosed regions have various amounts of minerals that are specific to metamorphic rocks.
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Cross section of the Marcy Massif across the Adirondacks. Image courtesy of Regan et al., 2019. |
The formation of the Adirondack Mountains is where the unique status of the Adirondack Mountains comes into play. While many people may think that the Adirondack Mountains are just a subsection of the longer Appalachian Mountains, they are, in fact, their own distinct set of mountains. The Appalachian Mountains formed when North America slammed into Africa, forming the supercontinent Pangea, and lifting those mountains upwards as a result of the collision around 300 million years ago. Since the breakup of Pangea, these mountains have generally remained inactive and have been slowly eroding away over time. While the core of the Adirondacks formed during the Grenville Orogeny, over 1 billion years ago, the mountains themselves started to be uplifted ~20 million years ago. The reason for this uplift is currently unknown but the result was a dome structure with the Marcy Massif in the center. The way that domes form, it causes the younger rocks to erode away from the central area of the dome due to fracturing experienced during the uplift. This fracturing and erosion is the reason we have some of the oldest rocks in the Adirondacks exposed in such a huge degree.
The result is that the AMR sits near the center of a massive dome structure, comprised almost entirely of some of the oldest rocks found within the Adirondack Mountains and there are plenty of places within the park to check them out. (Note: there are some slightly older rocks found elsewhere in the Adirondacks, but not nearly as notable as the Marcy Massif.)
