Pollen-covered chaps

A few days ago I encountered these wonderful bees on Bodega Head.  At first I was simply impressed with the extremely dense, long brown hairs on the legs — they somehow reminded me of chaps.

 

And then one flew in covered with bright yellow pollen.  

 

Just after it landed, I was trying to photograph this striking individual when it started to dig into the ground.  What happened next was a little surprising.  The bee disappeared below the surface within about 30 seconds!  Here's a sequence:

 

I looked around for another bee to see if the same thing would happen.  Sure enough, here's another sequence of a different individual (next image).  Once the bee was below the surface, you'd never know it was there.  The only telltale mark was the loose soil (similar to an anthill) in the general area.

 

I'm afraid I don't know what type of bee this is yet.  Nor do I know exactly what's going on here.  I'm wondering if this could be a species of Andrena?  Perhaps the bees with pollen are females digging to access burrows where they will deposit and provision eggs?  How long will they stay underground?  What type of burrow or tunnel system is down there?

I'll have to do more research and report back later.  In the meantime, enjoy the mystery of the burrowing bees with the pollen-covered chaps!