Chapter 7 addresses Colorado’s fractured crystalline-rock, volcanic, valley-fill, and intermontane park aquifers. These aquifers occupy the central one-third or mountainous region of the state. The intermontane basins of central Colorado contain a network of hydraulically interconnected aquifers within basin-fill deposits. These unconsolidated to poorly-consolidated aquifers consist of sediments that were deposited by wind, water, and gravity from erosion of the surrounding mountain ranges. Colorado’s crystalline rocks are exposed at the surface within the Southern Rocky Mountians physiographic province. The crystalline rocks throughout this province are Precambrian-aged igneous and metamorphic rocks; largely granites, gneisses, and schists; and geologically recent (Tertiary age) volcanic and igneous intrusive rocks. Ground water in crystalline-rock aquifers is generally unconfined, and occurs where joints, fractures, and faults have crosscut the rock. The location and extent of the crystalline, volcanic, valley-fill, and intermontane park aquifers discussed in Chapter 7 are presented in Figure 4.3.