The Catskills are technically a plateau, not a mountain range. Geologically, they are a dissected plateau: a flat, elevated landmass that has been carved by streams and glaciers into something that looks and feels like mountains. The peaks reach over 4,000 feet, the valleys are deep and dramatic, and hikers will tell you they sure climb like mountains. But the underlying rock structure tells a different story.
What “Dissected Plateau” Actually Means
A true mountain range forms when tectonic forces push rock upward, folding and faulting it into peaks. The Catskills didn’t form that way. Instead, they started as a massive, relatively flat deposit of sedimentary rock that was later lifted as part of the broader Appalachian region. Over millions of years, streams, rivers, and glaciers cut downward into that flat rock, carving valleys between what remained. The “mountains” you see are simply the parts of the original plateau that haven’t eroded away yet.
The key physical evidence is in the rock layers themselves. The bedrock of the Catskills consists of nearly horizontally layered sedimentary rock. In a true mountain range, you’d expect tilted, folded, or faulted layers pushed up by colliding plates. In the Catskills, the layers are still close to flat, just like they were originally deposited. That horizontal layering, now carved into ridges, valleys, and peaks, is the defining signature of a dissected plateau.
How the Catskills Were Built
The rock that makes up the Catskills dates to the Late Devonian period, roughly 385 to 360 million years ago. At that time, a mountain-building event called the Acadian orogeny was pushing up a range to the east. Erosion from those ancient mountains sent enormous volumes of sediment westward, forming what geologists call the Catskill Delta. Braided streams deposited coarse gravel and sand on alluvial fans closer to the mountains, while slower, winding rivers spread finer sand and mud across broad plains further out. Layer after layer of this sediment accumulated over millions of years, eventually hardening into the sandstone, siltstone, and shale that make up the Catskills today.
The original Acadian mountains are long gone, but the sediment they shed became the raw material for the Catskills. The irony is that the Catskills are, in a sense, the eroded leftovers of erosion from an even older mountain range.
Glaciers and Streams Shaped the Landscape
Two forces did most of the sculpting. First, streams carved downward into the plateau over millions of years, cutting deep valleys and leaving ridges standing between them. Then, during the Pleistocene ice ages, continental glaciers covered the entire Catskills region, grinding across the bedrock and widening valleys. Exposed bedrock in the uplands still shows the scour marks left by glacial ice.
When the glaciers retreated, meltwater carved even more aggressively. Kaaterskill Clove, one of the most dramatic gorges in the Catskills, was cut by stream erosion after the last continental glacier melted. The combination of slow stream erosion over geologic time and relatively recent glacial carving gave the Catskills their rugged, mountainous appearance.
How the Catskills Compare to the Adirondacks
The distinction becomes clearer when you compare the Catskills to a real mountain range nearby. The Adirondacks, about 100 miles to the north, are made of billion-year-old metamorphic and igneous rock pushed to the surface by tectonic forces during the Grenville Orogeny. That rock formed deep in the earth’s crust during a continental collision and was later uplifted. The Adirondacks are still rising today, making them a geologically active dome.
The Catskills, by contrast, are made of much younger sedimentary rock that was deposited in horizontal layers at the surface. No collision pushed the Catskill peaks upward individually. The whole region was elevated as part of the Appalachian Plateau, and then erosion did the rest. Same state, similar elevations in places, completely different geology.
Part of a Larger Plateau System
The Catskills sit at the northeastern edge of the Allegheny Plateau (also called the Appalachian Plateau), which stretches from New York southwest through Pennsylvania, West Virginia, and into Ohio and Kentucky. The entire plateau is made of Paleozoic sedimentary rock, and the Catskills represent the thick wedge of sediment shed from the Acadian orogeny. Because the Catskills are on the eastern, higher portion of this plateau and have been more deeply eroded by streams draining toward the Hudson River, they look more like mountains than their flatter cousins to the west.
Why Everyone Calls Them Mountains
Slide Mountain, the highest peak in the Catskills, tops out at 4,180 feet. There are 35 peaks over 3,500 feet. The valleys between them can drop well over a thousand feet. When you’re hiking up 1,780 feet of elevation gain to reach the summit of Slide Mountain, the geological classification feels academic. The name “Catskill Mountains” has been in use since at least the 1800s, and New York State manages much of the region as the Catskill Park. Even the USGS has historically referred to them as mountains in certain contexts.
The terminology isn’t wrong in everyday use. “Mountain” describes how the landscape looks and how people experience it. “Dissected plateau” describes how it formed. Both are accurate in their own context, but if you’re asking about the geology, the answer is plateau.