The Dismal Swamp, on the border between eastern Virginia and North Carolina is one of the few remaining large (approximately 210,000 acres) areas of wet wilderness in the eastern United States. There has been much speculation concerning the hydrologic conditions that led to the formation of the swamp.

Oaks and Coch (1973) recently completed a detailed investigation of the geology and morphology of the area. An analysis of their geology and the pollen work of Whitehead (1972) has lead the authors to the following hypothesis concerning the hydrologic conditions that led to the formation of the peat in the swamp.

A permeable sand facies of the Norfolk Formation underlies Dismal Swamp. This facies was originally completely covered by the Sand Bridge Formation, which is a confining layer, and underlain by the impermeable Yorktown Formation. Movement of water eastward within the Norfolk Formation from the outcrop area on the top of the Suffolk Scarp was further restricted by a less permeable facies of the Norfolk east of the swamp; thereby creating an artesian head within the permeable sand facies of the Norfolk Formation.

Erosion during the Pleistocene age breached the Sand Bridge confining layer and allowed upward seepage of water along the shallow stream valleys. This seepage, combined with the abundant rainfall and naturally sluggish surface drainage, may have been sufficient to trigger the formation of peat along stream valleys about 9,000 years ago. The peat further inhibited surface drainage, which in turn, accelerated the accumulation of peat until the interfluve areas were covered. The present role of the Norfolk Formation in the hydrology of the swamp is not clear, but it is considered to be one of the most important aspects of the hydrology to be studied in future investigations.

Surface inflow is from small streams draining from the west. The flow of these streams varies widely, being generally less in the summer than in winter. Outflow is primarily through the Feeder Ditch-Dismal Swamp Canal system, which discharges at South Mills and Deep Creek locks.

Rates and direction of surface flow within the swamp are partly controlled by gates on many of the ditches. Inadequately controlled ditches penetrating the Norfolk Formation plus withdrawal of water from wells along Suffolk Scarp have altered the flow of ground water under the swamp. These modifications and the loss of water through the Dismal Swamp Canal have probably resulted in a generally drier swamp as indicated by changes in the vegetation. It is feasible to preserve Dismal Swamp, but more detailed studies of the hydrology are needed to aid in future management.