Spent some time in Joshua Tree last week. It has been a long time since I have played in so much granite. In fact it was way back in grad school and we had a professor who always brought the freshman students up to his idea of a "boulder factory" We would spend a little time discussing chemical weathering and then let the students climb around a little bit to experience the sharp quartz minerals left behind.
This was the same thing...only I was the one climbing around. You can clearly see how the once monolithic rock has jointed to make very regular blocks with the resulting acidic water rounding the blocks into spheres. In some cases, you would swear these were very large river rocks eroded down by the movement of water.