On the High Plains, the sediments transported by the Brazos River are secured from Tertiary outcrops but at the eastern edge both Triassic and Permian rocks supply materials. The deposits in the Brazos River consist of clay, sand, pebbles, cobbles, and small boulders. Most of the boulders are pieces of "cap rock" caliche. The cobbles consist chiefly of quartz and are well rounded. The abundant minerals in the sands are quartz, biotite, muscovite, magnetite, ilmenite, garnet, epidote, tourmaline, hematite, rutile, and zircon.

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