Thursday, March 7, 2013

Anza-Borrego geology

Anza-Borrego Desert Narrows Earth Trail in “Powder Dump Wash”. 

In the 1930’s road crews called this wash “Powder Dump Wash” because this was where they kept explosives. Below is a photo of part of a concrete and boulder wall from the building where they were stored. Noticed the fault line next to it.

Now this wash has become the Narrows Earth Trail with good geological examples of how this region was formed.  I thought Anza-Borrego was all about flowers so I was surprised to find a geological trail.  Here is what I found and learned. Over 100 million years ago hot liquid rock, called magma thrust into the earth’s crust forming igneous rock.  As it cooled, it crystallized into minerals and became granite.  Later erosion exposed the granite and broke it down even further into sand.

This region had experienced and continues to have earthquake faults and because of that there are sections of older sedimentary rock next to the granitic rock.  The sedimentary rock had pockets of softer material here and there and over time, rain and wind removed the softer rock, leaving these holes.

Anza-Borrego has thousands of small faults like the one below. The rounded edges of the contact zone caused by constant movement and the different kinds of rock material are distinct features of a fault. The fault lines in this valley create cracks in the earth that allows water to seep up and provided the needed water for the many palm oases.

The rock below is metamorphic, which means it has been changed by pressure, heat or chemical action.  When this rock was at the bottom of the sea, about 450 million years ago, this cliff was located south of what we now call Guaymas, Mexico. You can see how the layers have been pushed, squeezed and turned on their sides.

As I left I turned around for a parting shot of the alluvial fan sloping down the mountain caused by heavy rain as it carried the sand and gravel down the mountain.
This short walk was a reminder of how we are always surrounded by active processes of geology. I could have found many of these same features in my home state of West Virginia that I have described here.  I am always amazed at how our earth has exploded, bent, oozed, and moved all around. Once I began to notice geological features, it is hard to go anywhere without noticing and it makes my journey all the more interesting.



1 comment:

DeniseinVA said...

This is a fascinating post, and the photos were all enjoyable. I have had vacations in that part of the country and have always been fascinated by the desert. Hope to get out there again one day.