There were 100's of these bees each constructing individual burrows, like this one. At the top, you can see the bee pushing the sand out with her head, and in the bottom photo, following that sand out.

Each tunnel is 4-6 inches long and as you can imagine, it is quite a task for a little insect about 1/2 inch in size, to do this heavy construction! You can see the sand mounded around the entrance hole. These bees are called Cellophane Bees or Plasterer Bees because they secrete a cellophane-like substance with which they line their nests before laying eggs. This waterproof lining protects against dampness and, so I have heard, against fungal attack.