Puente Chips

Well, I’ve been spending some time getting my garden soil ready for planting soon.  Every garden has its fair share of rocks and roots and sand and clay, but my garden soil has some fairly interesting and significant rocks that forgot about until I was turning the soil over last week.  You see by day I am a geologist so these kind of things fascinate me!  I figured I’d take a moment to explain…since it is garden-related.

These are the rocks I’m talking about.  They are 1”-2” long pieces of white siltstone.  These rocks originally were part of a large formation of siltstone rock found in the Puente Hills to the north of me.  The formation is called (duh!) the Puente Formation and is about 5-12 million years old.  These fragments of white siltstone first show up in a younger deposit called the La Habra Formation (which is about 1 million to 400,000 years old.  The reason these siltstone chips, called “Puente Chips” because they come from the Puente Formation, are important is because they are the signal of when the Puente Hills started getting pushed up.  As the Puente sediments were pushed up above ground, the white siltstone beds were exposed to weathering and the rock fragments washed down the river that was depositing the other sediments of the La Habra Formation during the Pleistocene.  So by dating the layers where these “Puente Chips” first show up, we can better pinpoint when the Puente Hills started to uplift (which is currently estimated at 1 million to 700,000 years ago).  This is important for understanding the history and activity of many earthquake faults in Southern California and other things only geologists really care about. ;-)

The “Puente Chips” in my garden are actually chips that weathered out of the La Habra Formation (making it the second time these fragments have made a journey from their original home, which is why they are a bit more rounded on the corners and fall apart a lot easier.
So that is the story of the 5 million year old rocks in my garden.