Colorado by Nature: North and South Table Mountains

Golden’s laid-back and free-spirited ambiance is due in part to its marvelous foothills location and the separation from Denver granted by North and South Table Mountains. Because the two mesas rise high above the city, it would be natural to assume that, like the nearby Rockies, some great geologic upheaval thrust them above the adjacent plains. In this case, however, no such turmoil happened; instead, the Table Mountains are wonderful examples of a geologic process called topographic inversion—a process by which low spots in an ancient landscape become today’s high points.

Both North and South Table Mountains have steep, rubbly slopes consisting of soft sedimentary rocks capped by a sheer cliff of basalt, a dark volcanic rock. The sediments are layers of debris eroded from the Rocky Mountains as tectonic processes began to elevate them 66 million years ago. They are part of a thick pile of sediment that once blanketed the entire Golden area above the height of today’s Table Mountains.

About 66 million years ago, the lowest spot around Golden was the bed of a stream that flowed southeast across the area where today’s Table Mountains now stand. When a volcano erupted at the site of Ralston Reservoir about 64 million years ago, it sent a lava flow like those that pour from Hawaii’s Kilauea volcano down this ancient river valley. As the lava cooled, the 10-mile-long flow solidified into the hard rock that caps North and South Table Mountains today.

Ongoing erosion of the Rockies soon buried this flow in more sediment and continued to raise the height of the Great Plains until about 5 million years ago, when Clear Creek and other rivers began to slice into this landscape. Although the rivers had no trouble removing the soft sedimentary rocks, their work became much more difficult when they encountered the old lava flow. The smaller streams were no match for the tough basalt; they could only carve through the soft rocks east and west of the ancient, basalt-filled valley, gradually lowering the elevations of Golden and Arvada to today’s heights.

Thanks to the tough basalt, the area covered by the old lava flow eroded much more slowly, gradually converting what had once been the lowest point in the landscape into the highest. Only Clear Creek was powerful enough to slice through it, successfully cleaving the old lava flow into today’s twin Table Mountains. These city icons are astonishing reminders that the forces of nature can, given sufficient time, literally turn a landscape upside down.