I need some help. A couple of days ago while hooking-up the drip irrigation to my chinkapin oak, I discovered a series of what I can only call "reverse ant-hills". Instead of being mounds above ground, these are small depressions, about 1/2 inch deep & about an inch in diameter. What made call them reverse ant-hills, are the small grain-like debris, similar to what you see around the top of ant-hills. No actual insects seen or damage. Any ideas what is causing these? and what to do about them,if anything?

It might be ground wasps.

do a google search on antlion.

Sandh- Thanks for the tip. I got on the University of Nebraska at Lincoln website and found a photo what I was describing. According to them, antlions are totaly harmless. Thanks again for your help.