Western San Gabriel Mountains, CA

Stop #7: Vasquez Rocks Park

First:  Satelite image of area.  Second:  Overlay of geologic map

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.

Recall the Prinicples of Relative Dating. Sediments are typically laid down in horizontal layers (Principle of Original Horizontality). But, the beds in the Vasquez Rocks area are most definately not horizontal! Dips are as much as 55º SW in the northern part of Vaquez Rock Park and shallow to around 20º W in the southern portion.

If the sediments were originally horizontal, how, once they lithified into rocks, did they become so highly tilted? Earthquakes on the nearby Elkhorn Fault and Agua Dulce Fault caused the rocks to be tilted into the position they are in today.