The Pierre Shale Formation
Pierre shale - Shale layers of a changing seabed.
The Pierre Shale
By Jack Barkstrom
In this photo, alternating layers of tan and grey reveal something of the existing conditions of the ocean over periods of time. The layers now are nearly vertical, a result of the uplift of the Rocky Mountains. This area is within 500 feet or so of the beginning of the Fox Hills Formation (not included in this picture but to the left of the area shown). This represents the last two million years of the Cretaceous Sea or, more accurately, the last 1,875,000 years of the Sea's existence. (The calculation is the ratio of what 500 feet is to the total 8,000 feet of the Pierre Shale, compared to the 30-million year existence of the Cretaceous Sea.)
While the layers run parallel, they are not of uniform thickness. In places the tan layer is thicker; in other spots the grey or brown is wider. One explanation might be the existence of a giant floodplain, onto which a river emptied silt and sand. Yet it seems unlikely that the mouth of any river would extend as far as what would have been the middle of the Cretaceous Sea. A more likely possibility is that stable climactic conditions favored the growth of certain kinds of plant or animal life which trapped sediment for long periods of time. Another possibility is that sporadic volcanic activity to the west and northwest carried debris over wide areas of ocean at particular times. When the volcanoes became dormant, the volcanic ash was covered by a layer of clay and silt, eventually turning to shale.