The Lower Ordovician series of the Great Valley area of western Maryland is made up of about 3600 feet of carbonate strata, divisible into three formations comprising the Beekmantown group. The Stonehenge limestone lies at the base of the group and is correlated with the Gasconade dolomite of the Ozark region and its equivalents. The Stonehenge is succeeded by a thick sequence of limestones and dolomites named Rockdale Run formation, which contains strata of Roubidoux through Smithville age. The uppermost formation is the Pinesburg Station dolomite of questionable Early Ordovician age. Deposition appears to have been essentially continuous from Late Cambrian into Middle Ordovician time. A brief study of the tectonic setting of these strata indicates that flowage in a has not appreciably changed pre-deformation stratigraphic thickness. Faulting, probably of the high-angle reverse type, and flexure folding have caused about 30 per cent crustal shortening, thus distorting spatial relationships of stratigraphic sections. Palinspastic reconstruction of reference points is attempted. Evidence that the Beekmantown sediments were deposited on a subsiding shelf in warm, shallow, marine waters is presented. The environment of deposition appears to have been an intertidal mud flat during most of Beekmantown time. Most of the sedimentary materials probably were derived directly from the sea by chemical precipitation and physico-chemical reactions that took place at or near the depositional interface. The action of sediment-binding algae appears to have been important in Beekmantown sedimentation. The Lower Ordovician faunas comprise gastropods, cephalopods, brachiopods, arthropods, echinoderms, sponges, and corals including at least 49 genera and 69 species, of which 30 genera and 43 species are described and illustrated in this paper. One new genus and 15 new species are formally presented.