Ten Million Year Old Drainages Seen At The Santa Rosa Plateau: Overview

Perhaps the two most interesting features of the Santa Rosa Plateau are the Santa Rosa Basalt, along with a plant species that only lives on the basalt, the Santa Rosa Basalt Brodiaea, Brodiaea santarosae. Both of these are present only at the Santa Rosa Plateau, and have many interesting stories to tell. This page tells one of them, the story of ancient drainages preserved by the basalt.

The basalt erupted from a spreading center that flooded the entire area with lava roughly ten million years ago. Southern California had a quite flat landscape at the time, probably not much different from that of eastern Kansas today, with a maximum relief of only something like 200 feet at the location of the Santa Rosa Plateau. See Santa Rosa Plateau Geology Overview for more information.

The basalt froze that landscape in time, so that when we look at the basalt today, we can see exactly what the landscape looked like ten million years ago.

The most obvious feature of the basalt is noticed by everyone, the flatness of the top of the basalt layer on the mesas. When you walk along the Vernal Pool Trail on the top of the Mesa de Colorado, you are looking at a landscape that hasn't changed much in the ten million years after the last of the basalt finished flowing across the landscape.

The following picture is a 180° panorama taken from just south of the Vernal Pool Trailhead on the Mesa de Colorado (created using Autostitch). One scale is given by the cars parked at the Trailhead. The same north-south road appears on both the left and right edges of the photo. Note the two vernal pools filled with water at the right.

With just a little bit of imagination, you can easily reconstruct far-reaching views of what the landscape looked like ten million years ago. At the Vernal Pool Trailhead, you can look to Elsinore Peak, seven miles to the north-northwest, and to Oak Mountain at the northern base of the Agua Tibia / Palomar Mountain, 19 miles to the east, which are also covered with the same basalt. You can then imagine that whole extent covered with flat lava, with only shallow drainages interrupting the landscape. If you also mentally erase the prominent mountain ranges seen in the distance (San Gorgonio, Mt. San Jacinto, and Palomar Mountain), you will be seeing the landscape of ten million years ago.

A less obvious feature of the basalt is recorded by the bottom of the basalt layer. The bottom of the basalt layer recorded the landscape as it was before the lava covered the area, some 8-10 million years ago.

The ancient landscape can be seen directly by eye by observing the profile of the bottom of the basalt where it is exposed on the side of the mesas. It can also be reconstructed by analyzing maps of the current extent of the basalt below the mesas. See:

Ancient Drainages Seen In Profile By Eye

Ancient drainage reconstructed by analysis

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Copyright © 2007 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 24 November 2007.