Corpus Christi is at the center of an area of 35 by 45 miles covered by detailed topographic maps with scale of 1 to 62,500 and 5-foot contours. This area is studied in detail and illustrations from a group of similar maps in Kenedy and Cameron counties, on the south, are chosen. The topographic features are described, with physiographic origins and history. Wells drilled on "unusual topography," or in "broken country," or on surface mounds, without regard to physiographic origins, will continue to produce a high percentage of failures here.

Only one topographic feature of local diastrophic origin is found--a fault scarp or fault-line scarp bounding present development and subsurface diastrophic structures (as now known) in two oil and gas fields. This lies east of the White Point producing area and divides the Saxet producing area into two parts. Some gas has been found across this supposed fault in each field, but with no records of steady production. Dry holes have been drilled on the scarp not far from producing wells in the Saxet field. Oil wells also occur on it. The subsurface relations of the fault have not yet (June 15, 1933) been studied, but cores showing a fault plane are reported (see description of Figure 15).

This fault offsets the upland (Beaumont) deltaic plain and two terraces of Nueces River by 15 feet, vertically, producing a disturbance of gradients in the valley for 7 miles upstream. A distributary of ancient Nueces River turned at right angles to its former course on meeting the White Point structural uplift and continued along the bearing of the supposed fault for 8 miles away from the present entrenched valley to Taft.

The positions of flank dips on the structural noses at the White Point and Saxet fields, as postulated from somewhat inconclusive subsurface data, are coincident with topographic slopes of diverse origins, revealing an alignment believed to represent partial diastrophic control of topography in the immediate areas of the fields but lack of control at short distances away from them. Thus, meander scars associated with flank dips are continued into areas where they can not be expected to coincide with buried structures.

The general topographic features of the area are described from the physiographic viewpoint. Some of these, of non-diastrophic origin, have been thought by some to indicate diastrophic structures and oil occurrence. These include round bays, distributary ridges, segments of a ridge of windblown sand (mature offshore bar) and intrameander mounds. The occurrence of oyster, clam, and snail shells of marine types on high bluffs on the ground surface and in soil is of general, not local, distribution along tidewater bluffs, both high and low, and is solely of Indian origin. Asphalt on gulf beaches either floats ashore from oil seepages in the gulf, from oil waste of ships, as asphalt eroded from South American asphalt lakes, or in barrels from lost cargo and is of general distribution alon the offshore-bar beach. Sharp contact of fresh and salty waters in shallow sands along the west side of the mature offshore bar for 100 miles along the coast is not due to faulting, but to accumulation of rain-water in the dune-sand ridges

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and its downward and outward penetration into underlying sediments to depths of 100 feet and the resulting contact with the normal saline waters of the area. Large playa basins in the South Texas sand plain south of the area, with associated mounds of clay-dune origin, are wind-scooped and wind-deposited features. The saline ground waters have accumulated by evaporation of in-flowing surface run-off from higher prairie areas and by salt blown inland with silt from coastal flats and with fogs.

Negative evidence from physiography is not considered adequate to condemn any locality in the area as an oil and gas prospect, because we do not yet know the diastrophic history of the area, including the dates of the last deformation at different points. The occurrence of concentric or extensive linear alignment of slopes on topographic features of diverse origins is considered to be strongly suggestive of diastrophic deformation in the subsurface, with reflected surface expression. An example of fresh fault scarps a short distance west of the area shows that other parts of South Texas may hold other such evident topographic effects of diastrophic disturbances.