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It has been said in the previous pages that the rock sheets composing the two Cretaceous series were comparatively uniform in their relative inclination towards the sea, and that successively lower and lower (older) strata succeed each other to the westward. In places the uniformity and continuity of structure is somewhat disturbed by jointing, faulting, and denudation of the strata. In the vicinity of Austin these joints and faults are especially conspicuous, no less than five of them being readily discerned between the eastern limit of the city and the top of Mount Bonnel. Their general direction is north, 20 degrees east, but occasionally there are complemental directions to these. The downthrow of these faults is usually to the east, but in two exceptional cases it is reversed. The amount of this downthrow in most of these faults is less than 100 feet, but that of the main one, which runs from Mr. Huck's mansion parallel with the river, is over 500 feet, probably 750, so that the rocks underlying the region east of the Bonnell ridge have fallen down from their former higher position. Accompanying the downthrow of this fault are a large number of abnormal folds, usually dipping in every direction, forming numerous low anticlines and synclines when exposed. The rocks in the vicinity of these folds are usually metamorphosed into marble, and their exact origin and relation to the proximate faults and volcanic phenomena are seen seven miles southeast of the city, where the uniformity of structure is again broken by an extrusion of basaltic rock at Pilot Knob.

The main, or Mount Bonnell, fault is one of the most conspicuous features of the region, belonging as it does to a line of fracture extending from the San Gabriel to the Rio Grande, and which in the southwest bears the appropriate name of the Balcones. This fracture in the earth's crust is marked by an escarpment of from one hundred to two hundred and fifty feet in height, the eastward face of which is usually composed of the basal, or alternating, beds, while the summit, which is the plateau of the main Grand Prairie, is composed of the hard Caprina limestone. The downthrow, or region to the east, as seen in the country between Austin and Oatmanville, or between Austin and Mount Bonnell, is more or less crumpled and broken by minor accompanying fractures, accompanied by much metamorphism.