I wonder if anyone can help me with this rock formation on the Back Shore. What is the black in the granite. It looks like it was poured in.
The rock formation on the back shore looks like an intrusion of a molten rock that moved through cracks or zones of weakness in the Cape Ann Granite and cooled more rapidly to give a finer crystalline texture and it it called a dike – probably a basaltic dike and possibly nearly as old but definitely younger than the older granite which has been dated about 430 to 450 million years ago –
Henry N McCarl, retired geologist
Good Morning Gloucester 