Found these holes in a flowerbed along the back of the house today.

There are about 20 of these little 3/4cm holes, all grouped together on a mound that's about a foot & a half across. The pic is a bit blurry, but if you look closely you can see that the holes have little built-up rims around them.....some are very faint, but others (like the one at the business end of the pen in the photo) are built up nearly half an inch tall.

Can any of you experienced gardeners help identify them? Initial research shows that they might be from Cerceris wasps, but other sources say they might be from a type of bee or even (yikes!) termites.

thanks in advance!