Science! Basalt tree mold

There I was, walking merrily along the Fish Lake Trail, when I looked over at the wall of basalt that lines the trail and saw this

'Hm,' thought I. 'That looks like a tree stump grown into the basalt.'

'Hm. That looks like a tree mold in the basalt.'

I looked at the shape of the basalt in the holes

And the shape of the hole

Tree mold