Western San Gabriel Mountains, CA

Stop #6: Vasquez Rocks Park

The rocks in Vasquez Park are mainly composed of sandstone and conglomerate that was deposited 20 to
22 million years ago in an extensional basin. The sediment was eroded from nearby mountains. The
formations of these rocks coincide with the tectonic changes that took place in this region around that
time. Some blocks were broken apart, crumpled etc. some conduits for magma also opened up and basalt
lava flows are underlying and intercalated with the sediments at Vasquez. The coarse clastic sand and
gravel deposits at the base of the sequence suggest rapid uplift early on that eased-off later (marked by
siltstones and shales). You will still find faults and examples of graded bedding here. The sequence has
been tilted to varying degrees in the area and at some distance from the park itself we will come across nearly vertical strata