The geomorphology, characteristics, and origin of the freshwater marl sediments in the Great Limestone Valley, Maryland, USA

Certain calcareous soils occupying alluvial landscape positions in the Hagerstown (Great) limestone valley of western Maryland have developed from highly calcareous (60–100 g 100 g−1) marl sediments of Holocene age. The limestone valleys of this region are located in the Ridge and Valley Province, which transcends the western portions of Maryland and Virginia. The sediments range from 0.5 m to over 8 m in depth. The soils developed from the marl have been mapped in the Great Valley in units named for the Warners series (fine-silty, carbonatic, mesic Fluvaquentic Haplaquolls) and the Massenetta series (fine-loamy, carbonatic, mesic Fluvaquentic Hapludolls) and typically have a high pH (7.5–8.5), low bulk density (1.00 g cm−3), and high porosity (0.5 to 0.6). The carbonate in the marl was developed through inorganic and biogenic processes. The marl was formed in ponds, as evidenced by the presence of certain gastropods, bivalves, algae, and the extremely high CaCO3 content. The ponds are now extinct, but occurred in alluvial landscape positions during parts of the Holocene. Grain analyses and micromorphological evidence indicate that certain algae (Chara sp.) capable of accumulating carbonate internally and externally, were largely responsible for forming the marl. Periods of intermittent marl development are evidenced by the buried surface horizons within the marl sediments. The mechanisms responsible for damming and ponding of these alluvial landscapes were apparently destroyed, and then reformed prior to subsequent marl development.