By Rich Heathcote, Geologist
Sand, in the sense of particles rather than masses, is present all through Whiterock. Sand comprises much of the Cretaceous Age Dakota Formation bedrock that forms the off-white and colorful cliffs along the rivers and streams. Sand also occurs as a dispersed component, together with pebbles and boulders, in a matrix of clay and silt that forms the pre-Illinoian and Wisconsinan Age tills deposited during the Pleistocene Epoch. Sand is also a component of Wisconsinan loess, which blankets the pre-Illinoian till, and sand is concentrated into massive deposits along the uplands of the Wisconsinan Bemis Moraine complex. Sand is also present in dispersed and concentrated accumulations in the flood plain, terrace, and channel alluvium of the Holocene Epoch found along the Middle Raccoon River and tributary streams. Given all the sand present in WRC, only a minuscule portion of the sand grains actually formed here.
Sand grains in the North American midcontinent, where WRC is situated, formed originally by disintegration and decomposition of much older rocks exposed far to the north and northeast. Those sand grains were carried into the WRC area by ancient rivers during Cretaceous time, and by glaciers during Pleistocene time. Here they have rested in storage until released by erosion.
Some of the grains have undoubtably passed through many cycles of transport, accumulation, and storage in sandstone masses. Consider the “life” scenario of a sand grain originally born by decomposition of a Precambrian Age granite in northern Wisconsin. The grain might have been first incorporated in a lower Paleozoic Era sandstone such as the Jordan or St. Peter Formations in what is now southwestern Wisconsin or southern Minnesota. Hundreds of millions of years later, when those regions were uplifted and eroding, the same grain might have been released from the sandstone during weathering, and entrained in a sediment stream that entered the Cretaceous Age river system where it was transported to what is now Guthrie County, Iowa. The grain then came to rest and went again into storage in the Dakota Formation sandstone. You can imagine a similar scenario for ancient particles broken by glaciers from the Precambrian or younger rocks, and transported into the region.
Today, under the grand principle of uniformitarianism, we can observe at WRC the re-release of ancient sand grains from very old and not so old storage, and their transport within modern fluvial environments, to come to rest again who knows where?
The photo above shows a deposit of sand in Long Creek Hollow that was released from the Dakota Formation during a gully-washer this past summer. The sand has buried some of the flood plain flora and likely some fauna as well. The deposit might stay at this location and be buried by subsequent deposits, or it might wash away in the next Long Creek flood. The photo below show a modern sand and gravel bar across the river from the Whiterock cliff near River House. The sand bar likely contains a mixture of sand grains that weathered out of storage in Dakota Formation, Pleistocene tills, and Holocene alluvium.