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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
The Influence of Local Bedrock: An Important Consideration in the Interpretation of Textural and Mineralogic Analyses of Till
John P. Szabo, Michael P. Angle
ABSTRACT
Bedrock produces variations in texture and composition of tills not only on a regional scale but also on a local one. Bedrock influence is evident in Wisconsinan tills in an area as small as a township in the Allegheny Plateau of northeastern Ohio. Generally till sheets of the Plateau fine upward with decreasing age. Within the Mogadore and Northampton tills, samples from borings terminating in sandstone bedrock on the upland contain significantly more sand and have higher quartz-feldspar ratios than those from the same tills overlying lacustrine silts and clays in the valley. On the upland samples from the Northampton till, overlying sandstone contains slightly more sand than that from the same till overlying shale and siltstone. However, quartz-feldspar ratios increase down ice from sandstone bedrock highs. A few kilometers of transport may have been required for ice to crush plucked sandstone fragments which added more quartz to the fine sand fraction of the Northampton till. Geologists mapping tills using soil associations should be aware of textural and lithological variations produced by local bedrock.
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