Curious small piles of dirt

The other day I noticed some curious small piles of dirt in Juli's garden among the Fragaria (Strawberry) plants.  In the picture below, little earthen piles are located midway between top and bottom at left and right sides of the photo.  I thought they were earthworm castings at first. 

 

It turns out that there was a small flying creature (yellow, perhaps a wasp) that inhabited them. Here it is, a bit out of focus due to my cell phone camera's autofocus "feature".

I don't think it was the type of wasp (yellowjacket) that we put up traps to mitigate: it was smaller than I would expect had it been one and appeared to be solitary rather than group nesting; perhaps a mining bee?