I am fairly certain this bee belongs to genus Megachile. Possibly the same species as that I photographed earlier in the summer. This was one of the most interesting finds on any of my insect expeditions. The bee kept going into its burrow head first. It would emerge a few seconds later tail first with its mandibles full of sand grains. It would then fly some distance away, returning about thirty seconds to a minute later to repeat the cycle. After a few minutes it completely ignored me. I was able to get quite a few good shots before my camera died, apparently a victim of the extreme heat and rapid fire shooting. I found it interesting that the bee did not simply drop the sand right outside the burrow like an ant would. Carrying the sand off seems like a large waste of energy but I assume there is some benefit (better concealed hole?) to doing so.