See Ten million year old drainages seen at the Santa Rosa Plateau: Overview for an introduction to this page.
From the trails of the Santa Rosa Plateau, if you look closely at two of the mesas, you can directly see ancient drainages, thanks to erosion which has cut a cross section through the basalt covering that ancient landscape.
Ancient Valleys in Mesa de la Punta
The most obvious ancient drainages are two small valleys seen in the Mesa de la Punta, as seen from the Punta Mesa Trail 0.29 miles from its junction with the Adobe Loop Trail. (See location and direction of photo on a map of the entire Santa Rosa Plateau Ecological Reserve.) These small drainages were the first part of the landscape to receive lava, and you can make out a number of the individual lava flows.
The following picture shows the two valleys as seen from the trail, with the view framed by the plants on the trail. The visible portion of the Mesa de la Punta is about 0.2 miles long. This is roughly the view you'd see with your eye. (The drainage on the left is better seen a short distance farther along the trail.) Can you see them? (Don't puzzle over this if you can't; they are labeled in a picture below.)
Maybe it will help to blow up the picture a bit, and outline the top of the landscape from ten million years ago:
As indicated in the above picture, there might be a third small valley in the middle of the above picture, but since there is no exposed lava, it is hard to be sure about that third valley.
The following picture is from a 3x telephoto view of the largest valley, labeled #1 in the picture above, which spans perhaps 0.1 mile at its top:
Now you can make out the individual lava flows! The next picture marks some of the more obvious flows, with black lines separating some of what appear to be different flows. Of course, since I haven't actually examined those layers up close, it is hard to be sure what is due to erosion alone and what comes from erosion acting on the different layers. But in many cases one can make out linear features in the picture above that are almost surely from different flows.
Individual lava flows, especially the top of each one, will weather differently since the lava is never exactly the same, and, in many cases, the top of each lava flow will suffer some erosion before the next lava flow was deposited, creating a weaker layer at the top.
Some sagging of the upper layers is apparent in the photograph, which may simply be due to the perspective from which this picture was taken. Due to erosion, the center of the ancient valley might be at a larger distance from the observer, which makes it appear to be at a lower angle.
Ancient Valley in Mesa de Burro
There is much-wider ancient valley recorded at the Mesa de Burro, but it is so wide that it is harder to see unless you are aware of it. (See location and direction of photo on a map of the entire Santa Rosa Plateau Ecological Reserve.) The visible portion of Mesa de Burro in the following picture is about 0.6 miles long; the valley is about 0.4 miles long at its top.
The bottom line in the picture above shows the profile of the ancient valley. The top line shows that the basalt on top of Mesa de Burro has been significantly eroded in the middle. It is not a coincidence that the modern-day erosion of the top of the basalt roughly coincides with the ancient valley!
The following picture shows a close-up of the central part of the ancient valley. The contrast has been enhanced in order to show the basalt better:
The ancient river valley shows a small amount of inverted topography, since the bottom of the ancient river valley is now a shallow ridge toward the direction of the photographer, similar in profile to the topmost black line. The topmost black line above shows the current top of one of the first lava flows to fill the valley.
Erosion has removed all subsequent lava flows for a short distance behind the outcrop of this basalt. The following picture, taken from the north side of the ancient valley, shows the flattish top of this oldest lava flow, covered with grass, as well as the stair-steps caused by erosion of successively-higher basalt flows:
Amazingly, the immediately-lower part of this ancient valley is recorded in lava still remaining today on its downstream slopes! See Ancient drainage reconstructed by analysis.
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 25 November 2007.