Bubble rock?

[BearCreek]

Found this interesting little rock in our NW Georgia creek today. Any ideas on what caused the bubble look on one side and the "moon crater" look on the other side? I've never seen anything like it in our creek. Thanks for any ideas!

 

[DDancer]

Your specimen appears to be a sedimentary rock, something like sandstone. The bulges and pockets, your bubbles, are likely concretions *the bulge* and the dissolved centers of other smaller concretions *the pockets*. The hollows, look like thumb indention's, would be impressions of other concretions that have eroded away. In the first picture on the lower left corner you can see a concretion that has eroded in half to reveal the structure of the interior.