Whether it’s your first or hundredth time visiting Estes Park, your eyes can’t help but turn toward the dramatic ridge of gray rocks that tower up to 800 feet above the north side of town. To the Arapaho tribe, which inhabited the area for centuries, this distinctive fin was known as Thath-aa-ai-atah, or “little lumps.” Euro-Americans adopted a similar name, Lumpy Ridge, to describe the series of more than two dozen prominent domes that create its uneven profile.

Lumpy’s bumps are all composed of granite, which crystallized from molten magma 1,400 million years ago. But this magma didn’t erupt from a volcano (like Hawaii’s Kilauea has recently been doing). Instead, magma rising through the Earth’s crust stalled several miles below the surface, where the surrounding rock insulated it, allowing it to slowly cool over thousands of years. The resulting network of large, interlocking crystals created a strong rock with a rough surface that’s ideal for rock climbing.

Even though the granite is strong, it’s riddled with planes of weakness, which geologists call joints. Because they affect the rock’s resistance to erosion, the joints are ultimately responsible for creating the ridge’s famous bulges.

The lumpy granite behind the town of Estes Park first cooled from molten magma 1,400 million years ago. Photography by Lon Abbott and Terri Cook

Lumpy’s joints formed in three different ways. The first developed during the rock’s birth; as the hot magma solidified, it became brittle enough to crack as it contracted. More joints appeared during the vast sweeps of time that have passed since the granite’s formation as Earth’s tectonic plates repeatedly jostled with one another, sometimes compressing and other times tugging the granite apart.

The third way joints formed was by decompression; as erosion progressively stripped off the miles of rocks that once covered the granite, the removal of this weight caused the rock to expand. Because this didn’t happen equally in every direction, the granite developed sets of arching cracks that resemble the layers of an onion. Lumpy Ridge hosts lots of these joints, including the cracks that form the folded “wings” of the Twin Owls, the ridge’s most famous landmark.

Once erosion finally exposed Lumpy’s granite at the Earth’s surface, the same processes immediately began to quarry it. Where the joints were present, water seeped in. When this water froze on cold nights, it expanded and slowly pried the rock apart. Because the sections without joints were better able to resist Mother Nature’s onslaught, this tougher granite gradually formed the prominent domes between the areas of more subdued topography, such as the forested hillsides between domes, where the joints were present.

Other joints that haven’t broken down as much form the finger-, hand-, and body-sized cracks that climbers use for upward progress on many of the ridge’s hundreds of climbing routes. These cracks, which are the result of the repeated freezing and thawing of rock that first formed from magma 1.4 billion years ago, are part of what makes climbing on Lumpy Ridge so spectacular.

Terri Cook
Terri Cook
Terri Cook is an award-winning freelance writer based in Boulder. More of her work can be found at down2earthscience.com.