Ebony Spleenwort (Asplenium platyneuron) there are 4 ssp.

2006-9-4 Judge Morris Estate, New Castle County, DE
We were surprised to see ferns growing on this wall. We had always believed that ferns only grew in moist soil and liked humid conditions.
-This fern can grow with very little soil
-Often found in rock crevices
http://www.maine.gov/dacf/mnap/features/asppla.htm
http://www.marylandbiodiversity.com/viewSpecies.php?species=387