On The Rocks page, I talked about the large granitic boulders that are prominently visible around Daley Ranch. I also described how these boulders originally formed below the surface and weren’t initially visible, because there was an overlying layer of older, basaltic material. So, what happened to those older rocks? And what might the fate of our big, granite boulders be?

The older, basaltic material has mostly weathered and eroded away. You can still see a few chunks of these older rocks up along the northeast arc of Boulder Loop, before you get to the intersection with the Cougar Ridge trail. These are the small, darker and fine-grained rocks that you’ll see on the trail (often surrounded by reddish/orangish, iron-bearing soil). Look closely at those rocks -- use a magnifying glass if you have one -- and you’ll notice that it’s very difficult to see any crystals. That’s because these basaltic rocks cooled at the surface quickly, and there just wasn’t the time or the right conditions for crystals to form. These meta-volcanic rocks formed about 150 million years ago -- and completely “reset the stage”, covering everything at the surface with molten material. Primary succession would follow, with the eventual development of new and continually-evolving ecosystems, leading to what we see today at Daley Ranch.

Primary succession includes the crumbling of that old basaltic material, and the eventual formation of soil. As the material continued to degrade and the surrounding topography of what would become Southern California continued to shift, uplift and fold, the basaltic rock eventually was washed away downstream. As the overlying material was removed, the underlying granitic boulders were revealed!

However, the granite boulders we see today won’t last forever, either. In fact, they’re slowly changing, right before our eyes (okay, you have to be really patient to see it happening!). You’ve probably heard of “decomposed granite”, right? DG is a common and popular construction and landscaping material, with a consistency almost like beach sand, just a little bit more coarse. The DG we buy today, though, probably wasn’t produced through a natural process, but rather was mechanically crushed and ground up. This “accelerated” decomposition is done at quarries all over Southern California -- but it mimics what’s happening naturally, at the pace of geologic time.

When you wander the trails of Daley Ranch, have a look down at the loose, sandy material on the trail. This sand wasn’t imported from the beaches of Encinitas! It’s made up mostly of the minerals quartz and feldspar; these are two types of silicates, and silicates are the essential materials of granite. The granite boulders you see around Daley Ranch are slowly decomposing, due to the factors and forces of weathering, and are turning into smaller rocks, and then finally to sand.

How does the weathering occur? These days here in Southern California we don’t usually experience the seasonal (or even daily) freeze/thaw cycles that are observed farther north, or back in the midwestern U.S. In those freeze/thaw cycles, liquid water trickles into fine seams in the boulders when it’s warm, then freezes solid when the temperature drops below freezing. Solid water (ice) is less-dense -- it occupies more volume -- than liquid water (that’s why ice floats in your glass). When the water freezes, it exerts a force in all directions, leading to more fracturing in the rock. We also don’t get a lot of rain, which might lead to mechanical erosion of the boulders, either by washing other, smaller stones against them, or by undercutting larger boulders and causing them to tumble downhill.
We do get bright sunshine, however. Combine the effects of heating just one side of a boulder with the fact that heat diffuses slowly from one side of the boulder to another, and you wind up with a rock that’s unevenly heated: hot on one side, cooler on the other.
This variance in thermal properties within the rock also lead to fracturing (though not as fast as the freeze/thaw effect). Thermal variation during the initial period of cooling in the fresh, molten material also leads to fractures. When the fracture occurs below the surface, water can carry dissolved minerals into the seam and fill it back in!
Eventually, the big boulders turn into smaller rocks, cobbles and sand. What does that look like? Big boulders slowly become smaller rocks...
This process of mechanical, physical and thermal weathering leads to the formation of grus ("groos"), the fine grains of silicates that you see on the trails of Daley Ranch.
You can do a little "accelerated" decomposing of granite on your own! During the next rainy season, when you're out walking the trails around Southern California (just NOT at Daley Ranch, for this bit), look closely at the rocks lying along the trails. Now and then, you'll notice one that just looks a little "soft", like it's crumbling or actually dissolving. If you step on it, it might even give way and crush under your boot, or even in your bare hand. That, folks, is grus, the fate of our big granite boulders.