Precambrian rocks exposed in the core of the northwestern Uncompahgre Plateau, Utah and Colorado, may be subdivided into four main units, each of which has distinctive lithologic or geophysical properties: (1) an oldest sequence of complexly folded biotite and feldspathic gneiss, amphibole gneiss, and amphibolite, exposed in Westwater Canyon of the Colorado River and tributaries; (2) gneissic granodiorite, exposed along Coates Creek and the upper Little Dolores River; (3) a batholith of coarse porphyritic biotite quartz monzonite, which extends from Ryan Park, Colorado, northwest to Spring Canyon, Utah; and (4) a pluton of metagabbro or metadiorite, 2 miles in diameter, exposed in the lower canyon of the Little Dolores River and in Marble Canyon. The quartz monzonite and etagabbro intrude the older gneissic sequence.

The quartz monzonite and metagabbro are relatively magnetic and cause positive aeromagnetic anomalies; the metagabbro is relatively dense and causes a local gravity high. The gneiss and amphibolite are relatively non-magnetic, and their densities exhibit wide variations.

Regional gravity values increase by 50 mgals. from Sagers Wash syncline, in the Paradox basin, to the crest of the Uncompahgre Plateau. This large anomaly is attributed to three combined factors: (1) structural relief of about 16,000-19,000 feet from the Precambrian surface in the Paradox basin to the crest of the plateau; (2) the northeastward wedge-out of about 12,000 feet of Paleozoic sedimentary rocks, including several thousand feet of Pennsylvanian evaporites, against the paleotectonic highland that was ancestral to the present Uncompahgre Plateau; and (3) density variation of Precambrian rocks, which are relatively more dense near the crest of the plateau than they are on the southwestern flank. Juxtaposition of the relatively magnetic quartz monzonite and the non-magnetic sedi entary rocks along the front of the uplift causes an aeromagnetic high of about 500 gammas. The metagabbro pluton gives rise to a residual gravity high of 6-8 mgals. and an aeromagnetic high of 300 gammas. The sequence of gneiss and amphibolite is characterized by gentle magnetic gradients and low-amplitude anomalies.

On the basis of the geophysical anomalies, quartz monzonite is inferred to extend beneath the cover of Mesozoic rocks from Spring Canyon northwest to Cisco, Utah; a concealed mafic pluton, about 2 miles in diameter, is inferred in T. 20 and 21 S., R. 24 E., northeast of Cisco; and a belt of quartz monzonite is inferred to extend northwest from T. 11 S., R. 104 W., Colorado, to T. 19 S., R. 25 E., near Harley Dome, Utah. The remaining areas of the northwestern part of the plateau are inferred to be underlain by relatively non-magnetic gneiss and amphibolite.