- GeoRef, Copyright 2006, American Geological Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
Microscopic study of oriented thin sections of Wisconsin till from northeastern Ohio and northwestern Pennsylvania reveals three principal types of microfabrics. These fabrics are: (1) microfoliation, (2) coarse fragment orientation, and (3) veining. Microfoliation appears due to roughly parallel arrangement of silt flakes and elongate silt grains. Perfection of orientation generally decreases as the number of sand size particles increases. Microfoliation is roughly horizontal in till outcrop. Coarse fragment orientation is well developed in the more sandy till, and is particularly striking where small fragments of coal are involved. Studies indicate that coarse fragment orientation is parallel to microfoliation. Veining is a structure in which small thin veins or bands rich in silt flakes traverse the rock. Silt flakes roughly parallel vein walls, and veins may occur parallel to microfoliation or may cut it at a large angle. Branching and braided vein patterns are common. It is clear that veins postdate microfoliation. Veins are believed due to shearing of the till. The origin of the microfabrics is considered in detail, and problems worthy of further consideration are suggested.