The Catskills, located in southeastern New York, are known for their scenery, deep river valleys, and steep peaks. Although often called the Catskill Mountains, their geological identity is frequently debated. Geographically, they are a subrange of the larger Appalachian Mountains, but their internal structure differs from their outward appearance. To classify the Catskills accurately, it is necessary to examine the definitions of major landforms and the region’s unique formation history.
Defining Plateaus and True Mountains
Geologists distinguish landforms based on their internal structure and the forces that created them. True mountains, or orogenic belts, result from intense tectonic activity, specifically the collision of Earth’s crustal plates. These mountain-building events, known as orogenies, cause massive structural deformation within the rock layers. Mountains like the Alps or the Rockies are characterized by highly folded, fractured, and faulted strata.
Plateaus are extensive, high-elevation landforms with a relatively flat top surface. They are distinguished from mountains by the lack of significant internal deformation. A plateau may be uplifted uniformly by tectonic forces, but its rock layers remain mostly level, preserving their original horizontal arrangement.
A dissected plateau arises when a high plateau is subjected to prolonged, severe erosion. Watercourses carve deep ravines and valleys into the uplifted, flat-lying rock layers, creating steep-sided ridges and peaks. Although these remnants appear mountainous, the underlying rock strata have not been compressed or folded by plate collision. The key difference remains the horizontal structure of the bedrock, despite the sharp surface relief.
The Geological Origin of the Catskills
The Catskills originated during the Devonian Period, 395 to 325 million years ago, when the region was a vast inland sea. To the east, the rising Acadian Mountains eroded rapidly. Rivers carried enormous amounts of sediment—sand, mud, and gravel—westward from these highlands into the shallow sea.
This accumulation formed the expansive geological feature known as the Catskill Delta. Over millions of years, this process created a sedimentary wedge up to 10,000 feet thick. This wedge consisted primarily of alternating layers of sandstone, shale, and conglomerate, which solidified into the bedrock of the Catskills today.
Later, the Alleghenian Orogeny caused a broad, regional uplift of the entire area. Unlike the intense folding further east, the Catskill region was lifted relatively uniformly, like a single block. This uplift raised the thick, horizontal layers of sedimentary rock to a high elevation without deforming their internal structure.
Once uplifted, the block was exposed to relentless erosion. Water runoff and rivers began to cut downward, creating the deep, V-shaped valleys and narrow passes that characterize the region. This process thoroughly “dissected” the high plateau surface, carving out the isolated peaks and ridges seen today.
The landscape was further sculpted by massive continental glaciers during the Pleistocene Ice Ages. These glaciers widened and deepened the existing stream valleys. This left behind the U-shaped troughs and rounded mountain tops common across the region, contributing to the dramatic relief.
Classification Based on Structure and Appearance
Geologically, the Catskills are classified as a mature dissected plateau, forming the northeastern end of the Appalachian Plateau system. The definitive structural evidence is the nearly horizontal layering of the bedrock throughout the region. Unlike true mountains, where rock strata are folded or thrust, the Catskills’ sedimentary layers remain flat.
This lack of structural deformation separates them from mountain ranges formed by orogeny. The peaks are not the result of the crust being crumpled, but rather the remnants of a high-elevation tableland severely eroded away. Connecting the highest points of the peaks approximates the original, flat surface of the plateau before erosion began.
The peaks are often broad and rounded, reflecting erosional processes, contrasting with the sharper ridges typical of true fold mountains. The common term “Catskill Mountains” is justified by their impressive height, steep slopes, and rugged terrain. However, the geological reality is that these striking landforms are a textbook example of a plateau carved into mountain-like relief.