Cape Cod Bay, lying on the Massachusetts coast partly enclosed by Cape Cod, is in a glaciated region of low relief. Coarse sediments generally occur in areas exposed to wave and current action as in shallow water near shore or on shoals, and in the deep channel north of the tip of Cape Cod, which is swept by tidal currents. Fine sediments are restricted to the deeper water in the central portion of the Bay, and to the small, well-protected embayments of the shores. Most of the sediments are well sorted as compared with other shallow-water marine sediments. Within the Bay, the coarser materials generally have the highest degree of sorting while the finer sediments invariably are more poorly sorted. Fine-grained materials contain a small amount of organic matter and generally are stratified in layers 1 to 2 cm. thick. Sphericity of the pebbles varies but little and in a random manner, but roundness decreases with increasing depth of water. A previously described hard bottom zone in Massachusetts Bay was traced into Cape Cod Bay. This hard bottom consists of a concentrate of pebbles produced by wave and current erosion of glacial drift on the bottom under present conditions. The sediments studied are similar to those in Buzzards Bay, Massachusetts Bay and San Francisco Bay, but they differ somewhat from those of the continental shelf, a more exposed environment, and from those of Barataria Bay, a more protected environment.