The Rio Grande has cut its way several hundred feet through these deposits, and its broad valley is covered with river alluvium. Practically all of the surface material is Recent, being now in the process of formation. Pleistocene fossils have been found in the upper part of the bolson deposits, but the age of the lower part of the unconsolidated material, reported by deep well records, may be Tertiary. The bolson deposits consist chiefly of gravel, sand, and clay, derived from the disintegration of the rocks of the highlands. The material extends up the mountain sides several hundred feet above the general level of the lowlands, toward which the debris inclines in graded slopes.
Coarse material abounds near the mountains, and finertextured deposits compose the.
Surface of the lowlands. Erosion by gullies adjacent to the mountains has exposed numerous sections and a number of deep wells have been sunk into the mesa, so that the general composition of the Quaternary deposits is well known. The deposits are very diverse both in composition and distribution. Near the mountains there are accumulations of bowlders, gravel, and sand, in many places heterogeneously mixed, though locally they are cross-bedded and rudely stratified. Next to the southeast end of the Franklin Range an erosion outlier of the bolson plain is covered with a veneer of coarse detritus which overlies 25 feet or more of flat lying stratified clay.