THE GROVER GRAVEL AND THE PRE-ILLINOIAN TILL BOUNDARY IN EASTERN MISSOURI

The Grover Gravel (one of the “Lafayette-type gravels”) is preserved atop upland remnants throughout St. Louis County, Missouri. The gravel rests upon Pennsylvanian strata and is buried by Illinoian till along the Mississippi channel. Thus, the age of the gravel is only poorly constrained; yet, it and its presumed correlatives are used extensively in geomorphic reconstructions. Most clasts are chert (carbonates are absent), but large (60 cm) rounded boulders of purple quartzite are present, along with granite, ironstone, jasper and Lake Superior Agate. These erratics clearly require extensive glacial transport, yet the Grover is present well beyond the pre-Illinoian till boundary, as currently mapped.

Most aspects of the Grover Gravel in its type area in Western St. Louis County are inconsistent with simple fluvial deposition. Thick massive beds lack clast imbrication, cross bedding, and channel-margin boundaries. The beds also grade vertically and laterally between clast and matrix support, and the matrix may exceed 10% clay. At the type section the Grover also defines ice-wedge casts within the substrate materials, meaning that deposition began while permafrost was present.

We interpret the Grover Gravel to be very old and weathered till/outwash mixed with residuum, which was redeposited as a series of alluvial/debris fans within a periglacial environment during a subsequent less extensive glaciation. Most importantly, the concentration of weathered siliceous materials means that the Grover Gravel is a good candidate for cosmogenic-isotope burial dating. This determination would finally provide a consise age of the deposit, along with subsequent rates of erosion and landscape development.