Pot-shaped depressions filled with sandy clayey silt are found in "Upland" gravels (previously termed Brandywine) of probable Miocene age, in northeastern Maryland and in Virginia near Washington, D.C. The pots are about 7 ft (2m) deep and commonly are about as wide. In plan, many are strongly elliptical. Sides are steep or even bulbous, and the filling in some pots shows faint stratification paralleling the sides. Strata in the enclosing gravel commonly bend downward and are thinner beside and below the pots. The gravel deposits are remnants of alluvial deposits of the ancestral Susquehanna and Potomac Rivers. All the pots are at the present gravel surface. We suggest that the pots originated when seasonal frost in an overlying layer of sandy clayey silt provided a confining upper layer. The freezing plane in the silt moved downward, reaching the gravel at some points before others. Water in the gravel moved toward the freezing plane by capillarity or by cryostatic head, forming ice lenses in the silt. Each winter, ice growth forced unfrozen clayey silt a short distance downward and outward into the gravel, increasing the irregularity of the silt-gravel contact and promoting more rapid movement. The pots probably reflect some centuries of growth, probably during the Illinoian Glaciation.
