08/14/2026
Then we went nearby Colorado Springs to this roadcut. The lower part is salmon-pink colored granite (Precambrian; ~1.1 billion years old) overlain by tan and brown sandstone (Cambrian; ~500 million years old). The contact, visible at the obvious overhang of rock layers at the change in color and texture, is what we call “the Great Unconformity.” More than a billion years ago, Colorado had been crumpled into part of a great mountain belt, composed of rocks like the granite at the bottom of this roadcut. Then, for more than half a billion years, weathering and erosion wore down the mountains into a relatively flat plain near sea-level. And then, during the Cambrian, sea-level rose and flooded much of North America (including Colorado) under a shallow sea with sandy beaches and sea floors. Those are now preserved as the Cambrian sandstones resting on top of the granite. So, unconformities are surfaces like this in the rock record where an enormous amount of time is missing. This one is so widespread across large parts of North America, and it typically represents enormous lengths of time, that it has well earned its name of “the Great Unconformity.”
