A Rolling Mud Ball Gathers Stones

Sphere and Flood in the Desert

            The region of southern California extending north from the Gulf of California is one of the most geologically active parts of North America, sporting detectable earthquakes (by seismograph) on average every 48 minutes, or 30 per day.  Most of the geological features of the Anza-Borrego Desert State Park owe their origins to the stretching of the continental crust by the same sea floor spreading that is currently separating the Baja California peninsula from the mainland.  Millions of years ago, this region had a gentle topography and was the delta of the ancient Colorado River, accumulating mud, sand, and other sediments thousands of feet thick.  Crustal stretching then (and still ongoing) cracked, rifted and rearranged this region through multiple fault lines, raising mountains and dropping rift valleys.  A large section of the Colorado’s deltaic deposits was elevated to about 1200 feet and tilted gently toward the north.  Over millennia, water erosion carved the soft deposits of this fault block into the Borrego Badlands, a wonderland of rapidly eroding, almost unvegetated hills and washes in a wide range of muted, earthen colors. Wandering these gently sloped washes is an unending pleasure that often reveals surprises.

The bed of this tributary dry wash draining the Borrego Badlands was littered with odd round “gravel balls.”

            One of these surprises occurred when I wandered eastward up a smaller tributary of the southward flowing Coachwhip Wash whose origin was in the Santa Rosa Mountains.  The tributary stream bed snaked among the barren mud walls making easy walking in its sand and dried mud bed.  The headwaters of this little drainage must have been within the Borrego Badlands, for the stream bed carried very few larger stones and only modest deposits of gravel.  It did carry a lot of strange round balls coated in gravel, like a chocolate truffle rolled in sprinkles.  Unlike the stones in the stream bed, most of them were almost completely spherical.  As there was no conglomerate upstream from which such objects could have been formed, their origin presented a puzzle.

Many of the gravel balls were nearly perfect spheres.

            The obvious first step in solving this mystery was to see what was inside the balls.  I quickly set aside the possibility that it was chocolate, and broke open one of the balls.  The inside, although chocolate-colored, was in fact a very fine-grained clay, and a clue to how these balls were formed.

Breaking a gravel ball revealed that the gravel was a coating on a ball of fine clay.

            Somewhere upstream in this little dry wash there must be a deposit of very fine, pure and (when wet) sticky clay.  As these deposits were eroded and undercut by the occasional flash floods, the fragments were pummeled and rolled, knocking or eroding off the points and projections and making the fragment ever rounder.  In addition, when the clay was wet, it was plastic and easily deformed, so that rolling it in a flood made it ever rounder, much like rolling a wad of clay between the hands to create a sphere (or a chocolate truffle).  The stickiness of these balls then resulted in a coating of coarse gravel that protected the ball from further rapid erosion.  So that is my explanation of these balls, and I am sticking to it (wink).

            I began wondering why such gravel-studded clay balls were absent from the Coachwhip Wash into which this wash opened.  Because Coachwhip Wash originates in the rocky Santa Rosa Mountains a few miles north, it is full of rocks of all sizes, from gravel to large boulders.  Any of my mud balls that got carried into Coachwhip Wash would be crushed, squashed, and pulverized in the next large flood.  If they had any sense, and apparently, they do, my mud balls would resist getting carried downstream by floods.