This area was formerly used to quarry diabase rocks aka trap rock, used as railroad beds and highway bases.
Diabase tends to be found in long vertical sheets called dikes that cut through other, more common continental rocks. Diabase dikes are found all along the East Coast of North America. They are considered evidence of the rifting that broke Pangaea apart about 275 million years ago.
Unlike bassalt rock it never reached the surface. The rocks here were overlain with shale which has eroded to expose the rocks.