Upper slope landforms and age of bedrock exposures in the St. Francois Mountains, Missouri: A comparison to relict periglacial features in the Appalachian Plateau of West Virginia

Abstract

We observed upper slope landforms in the St. Francois Mountains of Missouri and compared them to relict periglacial landforms in the Appalachian Plateau of West Virginia in order to determine if periglacial conditions existed in southeastern Missouri. The St. Francois Mountains are the exposed igneous core of the Ozark Plateaus and are about 50 km from the Illinoian glacial boundary in southern Illinois and 75 km from the pre-Illinoian glacial boundary in east central Missouri. Summits of rhyolitic knobs have the appearance of cryoplanation summits but are structurally controlled, stratified pyroclastics with no evidence of periglacial features. Most rhyolite summits have a discontinuous mantle of 30–70 cm of loess. No patterned ground was observed in the St. Francois Mountains, but was observed at a summit in West Virginia. Block fields and block streams in Missouri are forested matrix-supported diamictons of large blocks partially buried in loess fines. Appalachian block fields and block streams largely consist of contiguous blocks with little or no fines and no vegetation. We measured length, width, aspect, slope, and size of the largest blocks along the axis of the Missouri block streams and block fields and on two typical West Virginia block streams (one 36Cl dated 143 ka, late Illinoian). The Missouri block accumulations show no size distribution patterns, are generally steeper and closer to the block source outcrop than the West Virginia block streams, and do not have the morphology of Appalachian relict periglacial landforms. Bedrock exposures in the St. Francois Mountains are old enough to have been exposed to any periglacial environments during the Pleistocene. One rhyolite summit has a 36Cl age of 536 ka (pre-Illinoian) at zero erosion. A granite summit has a 36Cl age of 333 ka (pre-Illinoian) at zero erosion.