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Between the Colorado and Nix, Lampasas County, where the deep-cut river approaches close to the western escarpment line, the areal outcrop of the beds is a narrow belt seldom exceeding a mile in width. The width gradually increases from Lampasas County northward as the river and escarpment diverge.

In approaching the gap in the Callahan Divide near the north line of Brown County the width and thickness of the Basement sands greatly increase and the surmounting escarpment is reduced to a narrow remnant of the Glen Rose limestone. The area widens down the Leon Valley, and then narrows again where the rocks disappear with the dip.

From Lipan, in the northwest corner of Hood County, the outcrop of the Basement sands narrows as it turns down the Brazos Valley and back again through Parker County, on the west side of the Weatherford quadrangle. From Comanche County north to Red River the average width of the belt of Basement sands is from 3 to 10 miles; they attain their greatest width in Wise and Montague counties, where they spread over considerable areas. They seldom exceed 10 miles in width, being widest in that portion which extends irregularly northward from Parker County to the mountains of Indian Territory. The total areal extent of the Basement sands along the western border of the main Cretaceous and north of the Colorado River in Texas can not be less than 1,200 square miles by the lowest estimate. In Indian Territory these sands vary from 1 to 15 miles in width, being 14 miles wide between Antlers and a point just north of Goodland.

Owing to the unconsolidated, pulverulent nature of these sands they are more rapidly eroded than the overlying limestone of the adjacent scarps or the underlying floor of older rock. Their outcrops therefore present no sharp or conspicuous features of relief, except when they form the base of a bluff supported by a protecting cap rock. There are three topographic phases of the Basement sands-vertical bluffs; wide, level, timber-covered flats overlooking the Paleozoic regions to the west; and wide valleys incised into the Glen Rose formation and bordered by sloping, terraced escarpments. The first is characteristic of both the Basement and the Paluxy sands, and depends upon the presence of an overlying cap rock of a more indurated character, while the second and third types depend upon the character of the foundation formation, which upholds the sands long after the cap rock has been removed.

The escarpment faces of sand are seen in steep westward-facing declivities wherever the upper limit of the sands are exposed south of Trinity River along the main western border. Most of the flats occur along the western border north of the Colorado and in the incised valleys.