Recently acquired sidescan sonar images reveal, for the first time, the landscape pattern and the detailed geomorphology of the North Atlantic shelf-slope break off the United States and other areas extending from the edge of the continental shelf to the upper slope. Long-range (GLORIA II) images that have swath widths of as much as 40 km show small-scale regional features; midrange images that have swath widths of as much as 5 km show features as small as 3 m across.

The midrange images show that the outer edge of the shelf, deeply embayed by submarine canyons, outlines a nearly flat surface marked by complex ripple patterns. In places, the slopes converging toward adjacent canyons are also complexly rippled; this feature suggests that the shelf surface and the upper canyon slopes are mutually stable under common

End_Page 966------------------------------

y10hydrodynamic conditions. In general, canyons in the upper slope have relatively broad flat thalwegs distinguished by relatively bright surfaces; tonal gradations indicate that the thalweg surfaces lap smoothly onto sharply etched border terrane. Terrane bordering the canyons typically has a simple gully and ridge trellis pattern; one or both sides of this bordering terrane may be spectacularly etched and extended. Individual gullies typically extend up to faceted back slopes. The extent and pattern of etched terrane, as shown in the midrange images, varies considerably from canyon to canyon suggesting the influence of multiple erosional events involving different mechanisms, and a substratum of varied erosional resistance.

End_of_Article - Last_Page 967------------