The depressions do appear to be pecked out, so man made. I have seen similar stones in the creeks here where (I believe) they served as the anvil portion of a bipolar system for splitting up quartz and quartzite gravels and small cobbles in the creek bed. Were these found in a similar context? The "nutting stones" I have seen that seem more likely to be good for cracking nuts tend to be larger with deeper depressions and found in more upland sites where the people would have camps or villages. (My own experience using stones like this for both bipolar breakdown of quartz cobbles and breaking up hickory nuts seems to confirm this.)