The Horseshoe Gulch area includes 16 square miles in the
Eastern Paleozoic Belt of the Klamath Mountains geologic province
north of Callahan, California. Fossiliferous Late Ordovician and
Silurian limestones and unfossiliferous greywackes, arkoses, shales,
mudstones, schists, and phyllites, and other terrigenous clastic
rocks are exposed in a broad fault zone in the area of study. It is
concluded that the limestones represent a clear, stable, and relatively
shallow water environment of deposition. These limestones
are found associated with a geosynclinal sequence of rock units.
Eleven mappable units grouped into upper and lower thrust
plates, are recognized.
The fault zone in the Horseshoe Gulch area, approximately one
mile wide, is characterized by a prominent folded thrust fault that is
part of the Mallethead Thrust. The Mallethead Thrust, whose age
is at least post-Late Silurian in the area, has placed upper plate
unfossiliferous, pre-Cretaceous age chlorite grade metamorphic
rocks over the Early Paleozoic unmetamorphosed limestones and
clastics of the lower plate. The trace of the Mallethead Thrust is
very sinuous and irregular.