VARIABLE DEVELOPMENT OF WEATHERING PROFILES IN A ST. PETER SANDSTONE ROAD CUT NEAR RIVER FALLS, WISCONSIN

A road cut located on Wisconsin State Hwy 35, two miles north of River Falls (NW 1/4, Section 23, T. 28 N, R. 19 W), exhibits an unusual weathering profile. The road cut primarily exposes the St. Peter Sandstone formation, with some Quaternary deposits unconformably overlying the sandstone in the center and on the southeast end of the road cut. The overlying Quaternary deposits are a mixture of glacial till and loess, with a layer of carbonate blocks from the Glenwood formation. This unconsolidated sediment layer is staining the St. Peter Sandstone below, and creating a calcareous weathering rind on the sandstone. In the northwest part of this road cut where the overlying Quaternary deposits are absent, a distinct weathering profile has developed. The difference in weathering profiles along the road cut was quantified by surveying, during the fall of 2007, using a total station. This weathering rind is causing the St. Peter to weather more slowly in the section below the unconformity and Quaternary deposits. The weathering profile is less steep and features a distinctive undercutting layer that is 3-4 feet in height and has recessed about 2-3 feet. The weathering rind appears to form from calcium carbonate contained in the silt and fine sand washed down from above. Adding a calcium cement to a poorly cemented sandstone, causes it to become more resistant than the original formation.