Geology
The history of the Catskill Mountains is a geologic story come full circle, from erosion, deposition and uplift back to erosion. The Catskill Mountains are more of a dissected plateau than a series of mountain ranges. The sediments that make up the rocks in the Catskills were deposited when the ancient Acadian Mountains in the east were rising and subsequently eroding. The sediments traveled westward and formed a great delta into the sea that was in the area at that time.
The escarpment of the Catskill Mountains is near the former (landward) edge of this delta, as the sediments deposited in the northeastern areas along the escarpment were deposited above sea level by moving rivers and the Acadian Mountains were located roughly where the Taconics are located today (though significantly larger). The further west you travel, the finer the sediment that was deposited and thus the rocks change from gravel conglomerates to sandstones and shales. Even further west, these fresh water deposits intermingle with shallow marine sandstones and shales until the end in deeper water limestones.
The uplift and erosion of the Acadian Mountains was occurring during the Devonian and early Mississippian period (395 to 325 million years ago). Over that time, thousands of feet of these sediments built up, slowly moving the Devonian seashore further and further west. A meteor impact occurred in the shallow sea approximately 375 mya creating a 10 km (6 mi) diameter crater. This crater eventually filled with sediments and became Panther Mountain through the process of uplift and erosion.
By the middle of the Mississippian period, the uplift stopped and the Acadian Mountains had been eroded so much that sediments no longer flowed across the Catskill Delta.
Over time the sediments were buried by more sediments from other areas until the original Devonian and Mississippian sediments were deeply buried and slowly became solid rock. Then the entire area experienced uplift, which caused the sedimentary rocks to begin to erode. Today, those upper sedimentary rocks have been completely removed, allowing the Devonian and Mississippian rocks to be exposed. Today’s Catskills are a result of the continued erosion of these rocks, both by streams and in the recent past by glaciers.
Some traces of the most recent sedimentary layers remain for the discerning eye to discover, however. Even along the glacially-scoured eastern escarpment and in the upper Hudson Valley just below it—not to mention the glacial till-dumps and occasional terminal moraines of the southern-facing mountain slopes and valleys of the eastern and central Catskills—fragments of quartzite ranging from bright white, banded orange and tan, to deep red and dark gray are found. Many if not most of these are no more than 6" thick, have two flat sides and are without inclusions of other native rock, e.g., gray or blue sandstone ("bluestone"), most likely indicating the presence of a shallow, wave-beaten sandy delta or beach area at the base of the Acadian ranges in the delta's final stages of sedimentation. That sand layer, mostly free of silt (hence less opaque than older layers formed with higher concentrations of silt and mud under deeper water at more remote reaches of the delta) formed one or more upper layers of the delta. With compression and time, thin layers of sandstone formed of which only the here-mentioned fragments of sandstone remain now though in comparative abundance, if one measures their frequency against those of glacial erratics of similar size and shape which are typically metamorphic in origin (e.g., feldspars, granites, basalts), which most likely originated in the geologically complex region of the Adirondacks to the north. Such sandstones and erratics are frequently found collocated in cairns and other anomalous rock arrangements of the Eastern Catskills.
In successive Ice Ages, both valley and continental glaciers have widened the valleys and the notches of the Catskills and rounded the mountains. Grooves and scratches in exposed bedrock provides evidence of the great sheets of ice that once traversed through the region. Even today the erosion of the mountains continue, with the region’s rivers and streams deepening and widening the mountains’ valleys and cloves.
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