There were 100's of these bees each constructing individual burrows, like this one. 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.

I also saw several of the Tricrania blister beetles, pretty little red beetles which lay their eggs in the nest holes of these bees. To see the beetle: http://www.pbase.com/laroseforest/image/134170058