Gravel—uncemented rock or mineral fragments that have diameters greater than 2 millimeters (about ⁸/₁₀₀ of an inch).
Gulf Coastal Plain—an area that extends, in Texas, from the Gulf of Mexico to the [Balcones fault zone] and in which [Quaternary], [Tertiary], and Upper [Cretaceous] strata crop out at the surface (see [p. 42]).
High Plains—an area in northwest Texas extending from the Pecos River valley north to the Oklahoma-Texas boundary (see [p. 42]).
Igneous rocks—rocks formed by the cooling and hardening of hot, molten rock material.
Intrusive rocks—[igneous rocks] that have formed below the surface of the earth.
Lava—molten rock material that has poured out onto the earth’s surface from volcanoes; also the rock that is formed after the molten material has cooled and hardened.
Llano uplift—an area in central Texas where [Precambrian] and early [Paleozoic] rocks occur at the earth’s surface (see [p. 42]).
Magma—hot, molten rock material from which [igneous rocks] form.
Massive—in a mass, without a regular or complete form.
Mesozoic—an [era], one of the great divisions of geologic time (see [p. 3]). This era began about 230 million years ago and lasted until about 63 million years ago.