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Geologic Division ---- Coastal and Marine Geology ---- Woods Hole Field Center

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Continued Differential Ground Subsidence

Suffolk Scarp We see evidence of continued differential subsidence of the land surface over the crater in the geology and topography of the modern land surfaces around the bay. First look at the geology. Notice that the boundary between the older rocks (orange) and the younger rocks (yellow) coincides with the position and orientation of the crater rim on the three peninsulas that cross the rim. The cross section at the bottom of the illustration shows how the older beds have sagged over the more rapidly subsiding breccia, and the younger rocks have been deposited in the resulting topographic depression. The topography also reflects the differential subsidence. The Suffolk Scarp and the Ames Ridge are elevated landforms (10-15 m high) located at, and oriented parallel to, the crater rim.

River bed channels Even the courses of the modern rivers in the lower bay region point to the continued influence of differential subsidence over the crater. Most of the rivers, like the Rappahannock, flow southeastward to the Atlantic. But in contrast, the York and James Rivers make sharp turns to the northeast near the outer rim of the crater. We infer that the topographic depression over the crater has been maintained recently enough to have been a significant determinant of the modern courses of these rivers. This continued subsidence also may play a role in the high rate of relative sea-level rise that is well-documented for the Chesapeake Bay region. One of the locations of highest relative sea-level rise is at Hampton Roads (the lower part of the James River), located over the crater rim.

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