The smaller rocks embedded in this boulder appear to have been rounded and smoothed by the erosive action from being in a flowing stream (river rock) for a long time. Then somehow those rocks got embedded in a layer of solid rock. I would later discover that there is an entire layer of this type of rock exposed in the canyon wall - photo #8. My knowledge of geology is very limited but I'm guessing it's a sedimentary rock where this aggregate has been compressed into a solid by geologic pressures.