Data from a contour map of the submarine topography surrounding the Near Islands, from the companion map of the Rat Islands by Gibson and Nichols (1953), and from the geology of the western Aleutian Islands suggest that the submarine topography reflects the structure of the western part of the Aleutian Ridge.
Four principal topographic provinces are recognized: (1) The Crest of the Aleutian Ridge contains the Aleutian Islands, the Insular Shelf at depth ranging from present shore lines to 70 fathoms, and the Ridge Shelf at a depth of 100 to 500 fathoms, all apparently the result of subaerial and marine erosion since the middle Tertiary and of glaciation in the late Pleistocene. (2) The Insular Slopes form the sides of the Aleutian Ridge. The North Insular Slope is a long, steep, linear scarp that probably marks a major fracture in the earth's crust. The South Insular Slope appears to be a broad, faulted and warped arch containing numerous steep-sided linear sea valleys and canyons. Many of these traverse the south slope at an angle to the maximum regional gradient, and several line up with observed faults on the islands. These linear topographic features probably mark fault zones. (3) The Aleutian Bench is a prominent step in the general slope from the islands to the Aleutian Trench, and its inside edge may be the trace of a thrust fault. (4) The arcuate Aleutian Trench has a steep north side, a flat floor at a depth of about 4000 fathoms, and a south side containing an en echelon topographic pattern. The Trench perhaps marks a major thrust zone dipping north beneath the Aleutian Ridge.
A structural interpretation of the submarine topography suggests that the western part of the Aleutian Ridge is an arched and faulted asymmetrical wedge bounded by a northward-dipping normal fault on the north and by a northward-dipping zone of reverse faults on the south. Formation of this wedge probably began with major uplift and faulting of the western Aleutian area during the middle Tertiary, and the many earthquakes and active volcanoes in the Aleutian arc today indicate that deformation is still continuing.