It came from the trees

This time of year, it's dangerous to sit outside under the white oak tree. I've been bonked on the head more than once by acorns, and believe me, they rocket out of the tree at a great rate and really hurt! So I expect to see acorns all over the terrace, but what are these little things? They are tiny nubs that could be bits of acorns, but there are too many of them to attribute to squirrels gnawing at them. Any ideas?