Western San Gabriel Mountains, CA

Stop #6:  Sedimentary rocks of the Vasquez Formation

The Vasquez Formation consists of sedimentary rock that is mostly sandstone and conglomerate (Dibblee, 1996b). Agua Dulce Canyon also contains outcrops of the Tvca member and Tvcs members of the formation. Most of the rocks in the canyon are of the Tvca member, a light grey conglomerate/fanglomerate//breccia with abundant anorthosite clasts. However, we will stop at a the Tvcs member, a pink to light grey conglomerate comprised of granitic clasts. The sediments that formed both conglomerates were deposited from a river around 24 million years ago (Frizzell & Weigland, 1993).

Although 24 million years might seem old by the standards of human longevity, 24 million years is a short time geologically. These are relatively young rocks.


The clasts (rounded rock fragments) within the Tvcs conglomerate can be used to decipher geologic history. The conglomerate is comprised of poorly sorted, subangular to subrounded granite clasts, derived from the late Mesozoic (65.5-145.5 million years ago) granite and quartz monzonite plutonic rocks to the west. Within the Tvca, the conglomerate are clasts of the anorthosite (1.2 billion years ago) exposed to the south and viewed at the last stop.

We can use the princpal of inclusions to help us interpret what happened 24 million years ago. First, it lets us know that the granite, quartz monzonite, and anorthosite rocks had been exposed at the surface since at least 24 million years ago. Those rocks had to have been at the surface for them to be eroded and eventually deposited to form these members of the Vasquez formation.

This also tells us something else: The San Gabriel Mountains must have already been significantly uplifted by at least 24 million years ago in order for the eroded intrusive igneous rocks to have traveled downstream to be deposited in either river beds or alluvial fans.


Both interpretations of the clasts comprising the conglomerates of the Vasquez Formation are an example of Uniformitarianism - the present is the key to the past. Where you are standing to look at these rocks is in an active stream channel. One of the functions of rivers is to bring excess water out of the mountains. In doing so, the water erodes, or breaks up, the rock it is traveling through and carries it downstream as well. The river therefore transports the broken rock, aka sediments, to new locations, where they are deposited. Sometime sediments stop in a location and stay to eventually become a part of the rock record, while others just pause there for a while before moving on.

Look down at your feet. See all those rocks and sand? They came from up in the San Gabriels. How do we know? Some of the clasts are from the Lowe Granodiorite, a diorite gneiss, and the granodiorite gneiss from Mount Gleason (Dibblee, 1996b), bodies of intrusive and metamorphic rocks found upstream. How did they get there? Erosion and transportation of those broken rocks via Agua Dulce Creek.


(T=Tertiary, v=Vasquez, cs=conglomerate & sandstone)

(T=Tertiary, v=Vasquez, ca=conglomerate with anorothosite clasts)