The Mechum River Formation in Virginia is a metasedimentary unit consisting of alluvium, glacial till, and rounded boulder conglomerate, all of which are very difficult in a marine setting. The rock is neoproterozoic, ~700 Ma. Google for more details, but here's a quick site:
Page not found | William & Mary
Other info is here:
http://geology.geoscienceworld.org/...tent/abstract/26/7/623
http://jsedres.geoscienceworld.org/...tent/abstract/44/3/862I don't have access to this journal, but you may. Give it a try.