The Rio Grande depression, a lowland through which the Rio Grande flows, varies in width from 10 to 50 miles or more and extends southward about 450 miles from the head of the San Luis Valley in southern Colorado across central New Mexico to near El Paso, Texas. This report concerns only that part of the depression extending from the vicinity of Taos, New Mexico, on the north to a short distance north of Las Cruces, New Mexico, on the south. Both on the east and on the west the depression is bounded by series of rugged mountain ranges.

The Rio Grande area is covered by as much as 21,500 feet of rocks, late Cambrian to Recent in age, which rest on a pre-Cambrian basement. Gently tilted or folded Cenozoic formations occupy the surface throughout the depression. Older rocks are exposed in the bordering narrow uplifts and in many places are involved in complex folds and overthrusts.

The pre-Cambrian generally consists of granitic or metamorphic rocks.

Formations in the Paleozoic part of the section are primarily marine carbonates and clastics; non-marine clastics and evaporites are found principally in the Permian. Cambrian through Mississippian beds wedge out northward in the southern part of the area; isolated Devonian and Mississippian remnants occur in the northern and central parts, and Pennsylvanian and Permian formations are present in most of the area.

The Mesozoic is largely marine and non-marine clastics with a minor amount of evaporites in the Jurassic and thin marine carbonates in the Jurassic and Upper Cretaceous. Triassic and Jurassic strata are confined almost entirely to the northern and central parts of the area and thin to absence southward. Upper Cretaceous beds occur in most of the area.

Consolidated and partly consolidated non-marine clastics, evaporites, lavas, associated pyroclastics and intrusive rocks, constitute most of the rock types of the Cenozoic.

Epeirogenic and possibly local orogenic disturbances in the Rio Grande area during Paleozoic and most of Mesozoic time, culminated in the late Cretaceous and early Tertiary Laramide overturning and thrusting. Subsequent events probably included high-angle faulting and mild folding in Miocene(?) time, late Pliocene(?) faulting, tilting and uplift, and Quaternary faulting. The main

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outlines of the depression may have been formed in Miocene(?) time, but its present configuration is chiefly the result of late Pliocene(?) deformation. Structurally, the depression is a series of north-trending grabens arranged en echelon north-northeasterly.

Slides showing possible thickness and distribution of the Cambrian-Ordovician-Silurian, Devonian-Mississippian, Pennsylvanian, Permian, and Triassic-Jurassic-Upper Cretaceous in the area are presented; also a schematic stratigraphic section north-south across the area and a map showing Laramide and younger tectonic features.

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