Moles, Voles, or ???

My yard - can you see the holes?

Help.  I have something ruining my lawn, just in one place!  My first thought was moles, so I did some research.  Seems moles and voles are very close, but with significant differences.  Moles are carnivorous, while voles are vegetarian (moles=meat, voles=veggies.)  They both leave well-defined runways just under the surface of the ground where they burrow.  But moles can also go deeper and when they do, they leave a mound of earth that they've excavated from the ground.

Therein lies the problem.  There are numerous little holes in my yard, but no mounds and no runways.  So what am I dealing with?

Here's a photo of an actual hole in the adjacent flower bed:

And here's the mulch that apparently floated out during last night's storm:

Any ideas about what my pest is and what I can do about it?

Be sure to visit the Master Gardener booth at the Germantown Festival this weekend!