TRANSPORT AND WEATHERING OF LARGE LIMESTONE BLOCKS ON HILLSLOPES IN HETEROLITHIC SEDIMENTARY LANDSCAPES

The conversion of bedrock into mobile regolith is a fundamental concept in landscape evolution. However, for landscapes formed in heterolithic rock that weathers into weak regolith and hard blocks we have a poor understanding of the transport and evolution of such large rock blocks on soil mantled hillslopes. The properties of such blocks may determine both the transport process and movement rate downslope.

We focus on hillslopes in the Flint Hills of Kansas in the United States, where we study the role of block shape and size in soil mantled hillslopes under a limestone cliff that breaks up equally into cubic and tile-shaped clasts. Based on previous studies on the transport of rock clasts we hypothesized that the shape of blocks should lead to differences in transport leading to different distributions of block size and orientation with increasing distance downslope from the cliff.

We find complex relationships between block size and distance downslope from cliff face, we propose that this is due to weathering via fragmentation. This process produces discrete smaller fragments from a larger parent block. We find that there are statistically significant differences in overall block size and surface weathering in relation to block shape for our landscape. We propose that fragmentation occurs differently in accordance with block shape. We also find, block shape has significant influence over how soil collects and is stored on the hillslope near blocks. Yet, few cubic blocks appear to be transported through tumbling in contrast with assumptions in recent modelling work.