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Analysis of surface features in south Florida indicates a clear pattern of structural control (faulting, fracturing, or bending). This control is visible in lake outlines, stream channel directions, stream patterns, stream occupation of a non-stream trough, drainage pattern discontinuities, ground surface offsets, coastal offsets, confluence geometries, and ground slopes. The two dominant linear orientations are N50°E and N65°W; these form acute angles, facing east and west, of about 65°, and are taken to be the shears in a first-order strain ellipse. The intersection of two of these trends, immediately west of Lake Okeechobee, provides five or more meters of relief, shapes the western shores of that lake, distorts drainage patterns to the east, north, and est, and produced the basin which the lake occupies.
The long trough, through which the Caloosahatchee River flows, is parallel with one of these first-order alignments, and is marked by a down-to-the-southeast asymmetry. An extension of the northern boundary of the trough is thought to account for the coastal offset in the vicinity of Sanibel Island.
The fault-and-fracture pattern deduced here indicates north-south tension, in accord with the known northward migration of the continental block. Subsurface data indicate that the tectonic pattern is an old one, but the surface features which can be seen today date from Pliocene-Pleistocene time. The maximum rate of deformation in the last few million years has been calculated to be about one millimeter per millenium, and the actual rate has probably been somewhat less.
Second- and third-order orientations, predicted by use of the Moody-Hill hierarchy, have been observed in several stream patterns.
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