One of the most striking features of the terrain throughout Southern California is the geology. Plants change throughout the seasons, and animals come and go moment by moment. The rocks, however, are as dependable as, well, rocks...
Most of the big boulders you'll see exposed around Daley Ranch -- and the rest of Southern California, for that matter -- are just big, ol' hunks of granite (or, more accurately, granodiorite). These boulders were created about 80 million years ago during the Cretaceous Period, when blobs of magma were coming up from deep below the surface. That new, igneous material didn't quite make it to the surface (at the time, the land was covered by older basaltic material). These boulders formed below the surface, and are known as intrusive formations. Now and then, you'll notice small pockets in the sides of boulders where the magma intruded into a subterranean pocket, but didn't quite displace all of the older material. The magma cooled slowly underground, and under pressure from the weight of the overlying material, allowing for the formation of larger crystals.

Those earlier magma flows were extrusive formations; as the magma/lava made it to the surface, the molten material was able to cool more quickly, and prevented the formation of large crystals. This earlier material weathered more easily (and earlier than the underlying granite) and subsequently eroded away, flowing downhill with the rainwater.

Thus, the big granite boulders we see today have been slowly revealed as the overlying material erodes away.
Click the image below for a tour of the rock formations around Daley Ranch!
If you're interested in seeing the detail of the geology of Daley Ranch and the surrounding areas, pull down a copy of the USGS Geological Map by clicking the button below.

USGS Geological Map

Valley Center
What's your favorite reference for Rocks and Minerals? Audubon? Peterson's? Any good iPod/iPhone/iPad or Android apps you want to share? Do you use a pocket guide? Please let me know what you use -- I'll add it to the list!