The "mud-line," the depth of substantially increased silt and clay content commonly occurs near, but is only rarely coincident with, the shelf-to-slope transition. An evaluation of the mud-line off the United States East Coast and northern Gulf of Mexico highlights discrepancies between depth and position of this horizon and those of the shelf break. (1) Off Cape Hatteras and parts of the West Florida Shelf, off-shelf spillover of sand-size material results in a mud-line position well below the shelf break. Spillover at Cape Hatteras is a response to the powerful northeast flow of the Gulf Stream that tangentially crosses the shallow, narrow shelf. (2) The shallower depth of the mud-line off the Mid-Atlantic states between Norfolk and Wilmington canyons, and off the Panam City, Florida, margin identifies the long-term signature of energy concentrated on the sea floor; erosion results from the interplay of several mechanisms, including fronts, storm waves, tides, and breaking internal waves. The mud-line at these localities thus defines the position where, over time, shear-induced resuspension has largely exceeded the threshold of sediment transport. (3) The near-coincidence of the mud-line with the shelf break at the head of Hudson Canyon is a response to physical oceanographic parameters and to offshelf spillover; involved are the intersection of density fronts separating shelf and slope waters, and the channelizing effect of the canyon head cut deeply into the outermost shelf. (4) Considerable shallowing of the mud-line and a marked departure between t is level and the shelf break occur on margins where large amounts of sediment are supplied. Large asymmetric shoreward swings of the mud-line on the Gulf of Mexico

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margin west of De Soto Canyon record Mississippi and other river input and its extensive lateral dispersal by regionally important mass flow.

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