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Petrocalcic Horizon

posted Mar 11, 2016, 9:54 AM by Lynn Moody
If you're interested in earth science, geology, or soil science, this may interest you - if not, well...

Underlying the ridgetops on our ranch and in the surrounding area, is a soil horizon that must be very ancient - called a petrocalcic horizon, also known as caliche.  Actually there are several soils somewhat like this, forming a sequence on successively lower ridgetops that I think are remnants of Pliocene/Pleistocene alluvial fans.  The sediments of these alluvial fans make up the Paso Robles Formation, a geologic formation of mostly uncemented sediments containing lots of rock fragments of the much older Franciscan and Monterey Formations.  I guess old hills/mountains consisted of the Franciscan and Monterey rocks, and as they eroded, pieces of those were deposited as alluvial fans.  Then soil formation commenced, and lots of pedogenic (soil formation-related) calcite cemented the gravel pieces together into a hard soil horizon (layer).  Then a younger and topographically lower set of alluvial fans were deposited and stabilized, and similar soil formed again in this set of fans, and so on.  I think there are three petrocalcic soil horizons, each a little younger than the next.  They are associated, I believe, with three successively younger, and lower in elevation, sets of alluvial fans.   At any rate, you can see, on the highest ridges, outcroppings of the highest and presumably oldest petrocalcic.  Photo below, you can just make it out across the canyon, above and to the right of the brush..
The ridges are rapidly eroding (rapidly in geologic time) and below is a close up of a chunk of the petrocalcic, with rock fragments it has cemented together.  There are lot of lichens growing on this stuff here and there.
We find pieces of the petrocalcic horizons all over the floor of the canyons that dissect the ridges.