The Murrieta Hogbacks are underlain by
Cretaceous granitic rocks of the
Peninsular Ranges Batholith. On the north half of the hogbacks, by
foliated biotite-
hornblende tonalite and on the southern half by hornblende
gabbro. The basalt is
potassium-argon dated to 10.4 to 10.8 million years and is a remnant of a channel-filling basalt flow, overlying a thin deposit of unconsolidated gray stream gravel, indicating that the basalt had filled in a former water course. ==References==