Red and drab-colored sediments of the Patapsco Formation were studied to determine if color changes, clay mineralogy, and iron content of the sediments are related to the present ground-water geochemistry (Back and Barnes, 1965) and the present ground-water flow system (Mack, 1962). The red color of the sediments becomes darker in the direction of the ground-water flow, particularly in local discharge areas. This color change is related to the amount and kind of iron oxides in the sediments. Hematite and goethite coexist in most of the red and mottled samples; hematite is more abundant than goethite in red sediments but is rarely found in drab sediments. Large amounts of amorphous or poorly ordered iron oxyhydroxides in these sediments indicate that much of the iron has been introduced by diagenetic processes. Lower iron oxide values prevail toward the center of the outcrop belt, and higher values prevail in the recharge area. Detrital kaolinite and illite are the most abundant clay minerals. Vermiculite and mixed-layer illite-smectite are almost always found in red colored sediments and are probably products of post-depositional diagenesis.
The mineralogic and chemical variations correlate regionally with the observed ground-water flow pattern and with observed changes in Eh and dissolved iron content of the ground water. These results suggest that fluctuations in the ground-water flow system in conjunction with Eh and pH conditions caused precipitation of iron hydroxides that, on aging, have crystallized as goethite and (or) hematite.