There are Pleistocene-Recent deposits around the south side of Baltimore. Also, shelly silt is widely used as gravel for roads, etc. in the coastal plain. If you can identify the species, that would help figure out the age of the deposit. The Maryland Geological Survey has some helpful references; most are out of print but might be in a local library.
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
[log in to unmask], 301 862-0372 Fax: 301 862-0996
"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!