I would say that both are naturally formed. A volcanic void (air pocket) that formed in the rock before it cooled?
Neather rock seems to have the "determined" abrasion that would indicate that it was man made.
The second rock is the "first rock" after it has spent the last million years in a creek.
Either vessel "could" have been used as a container for Paint or pitch, or even as an oil lamp; none of which would leave any trace after a few thousand years in in the soil.
That's my best guess.