One of the things I love to do on this blog is share photographs of geology I've taken while doing field work, attending field trips/conferences, or simply traveling. I started this series of sharing a photo on Fridays pretty soon after I started blogging back in 2006.
Today's photograph is from a trip my wife and I recently took up to Point Reyes National Seashore, which is about an hour or so north of San Francisco (see this post I wrote for QUEST last week for more information about the geology).
I chose this photo because it relates to what I wrote about the other day concerning the nature of time in the rock record. While it may not be apparent at first glance, if you take a closer look at the photograph you'll notice a difference in the character of strata towards the bottom of the cliff compared to strata above. Do you see it?
Right around the level of my wife's head is a surface separating Miocene (~15 million years old) rocks below from Pleistocene (< 2 million years old) rocks -- well, more like sediments -- above. Note how the layering in the Miocene strata is crumpled up and folded a bit (like a rug on a hardwood floor) in the bottom-center of the photo. The younger strata above is flat. Also note how the younger deposits are much coarser grained -- you can see the chunky texture even from this distance.
The surface separating these deposits, called an unconformity*, represents about 13 million years of time. The time value of that surface is an order of magnitude greater than the time represented in all of the overlying Pleistocene deposits.
Happy Friday!
Image: Beach cliff exposures, Pt. Reyes/Brian Romans on Flickr
* I suppose one could call this a disconformity, which is a type of unconformity, in this location. But if you walk a couple hundred meters in either direction the Miocene strata is folded/tilted and planed off as angular unconformity.
