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These large blocks of tightly cemented, Quaternary-age conglomerate are tumbling down undercut stream banks along Gypsum Wash along Northshore Road (near milepost 2). The "fanglomerate" (conglomerate formed from sediments deposited on alluvial fans) consists of gravel-, sand-, and silt-sized particles cemented together by calcite (calcium carbonate) and clays deposited by shallow, ephemeral groundwater from episodic storms in an arid climate. Dense, compact, desert soil that is rich in calcium carbonate is called calcrete (or caliche). Calcrete is common on older alluvial fan surfaces. Calcrete is also fairly resistant to erosion and commonly forms caprock or ledges along stream canyons cut into old alluvial fan deposits. |
