William H. Langer 1974 <p>This map show the distribution and thickness of the principal clay unit.&nbsp; The clay unit consists of a thick, massive bed of relatively pure clay in some areas; in other areas it consists of discrete layers of clay alternating with layers of silt and very fine sand (varved clay).&nbsp; The upper layers of the clay unit commonly grade into a layer of silt and very fine sand.&nbsp; Small, isolated clay deposits not associated with the principal clay unit are not mapped.</p> application/pdf 10.3133/mf487B en U.S. Geological Survey Map showing thickness of principal clay unit, Hartford South Quadrangle, Connecticut reports