William H. Langer 1974 <p>This map shows the distribution and thickness of material overlying 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 material overlying the principal clay unit commonly consists of sand.&nbsp; In places this sand is overlain by gravel or sand and gravel, and may also be overlain by fine materials not related to the principal clay unit.</p><p>Locally the actual thickness of material overlying the principal clay unit may differ from the mapper value because the clay grades upward into the overlying material.&nbsp; The contact in these areas is poorly defined and is an estimate.</p> application/pdf 10.3133/mf487C en U.S. Geological Survey Map showing thickness of material overlying principal clay unit, Hartford South quadrangle, Connecticut reports