The red rocks of this coulee are marine concretions. We have been informed that they are the largest in the world.
Concretions come in many shapes, sizes and vary in mineral content.
In most cases they take a long time to form, and it has to be admitted that science has not been able to come up with an explanation of how or why. Some organic process was at work here, but no one should split them to find a fossil core or anything of that sort. Many have broken in recent time, and no apparent different material is to be found as a core, although a few do include some fossil fragments here and there in their outer part. Most are slightly flattened spheroids. The average time of emergence from the soft muddy shale bedrock, plus the action of frost and water, causes erosion making the appearance of a pot with a lid, though not every concretion will show this effect.