The Ramona quadrangle is a central portion of the Peninsular Range of southern and Lower California. The geology of only a small part of this range is well known. It appears to be a large composite batholith with scattered roof pendants (chiefly metasedimentary rocks) and minor patches of Tertiary and Recent sediments along the margins. Tertiary volcanics are present in limited amount, but none crop out in the region here considered.
The oldest rocks of the area are Triassic(?) metamorphics, chiefly quartzite, quartz-mica schist, quartz-sillimanite schist, and amphibolite.
Associated with the schist is the Stonewall quartz diorite, which is older than all other intrusives of the region, but younger than the metamorphics. It intrudes the schist in a complex manner, the contacts between the two being indefinite and characterized by a wide zone of injection gneiss. Although much is a normal igneous rock, there are many places where the rocks show the effects of dynamic metamorphism.
Into the complex of schist and Stonewall quartz diorite was intruded a series of rocks ranging from gabbro to granodiorite, but with a predominance of tonalites. The order of intrusion is from mafic to felsic. There appears to be no systematic areal distribution except in a small central area where concentric arrangement suggests ring-dike structure. The batholith is of middle Cretaceous(?) age.
With the exception of the alluvium, the only postbatholithic formation in the area is the Eocene Poway conglomerate, small patches of which cap hills in the southern part of the quadrangle.