Diamond drill cores from an area 5 miles southwest of Frederick, Maryland, reveal significant stratigraphic and structural information.
The Antietam and Tomstown (Lower Cambrian) and Grove formation (Ordovician) are involved. The Antietam has some quartzite layers but is predominantly quartzose phyllite. The lower 40 feet of Tomstown is dolomite with sericite partings. The remaining 140 feet is impure, thin-bedded dolomitic limestone. The Grove is a light-gray slightly dolomitic limestone with red staining in the matrix of breccias and along fractures. The Antietam and Tomstown dip easterly 25°–30°. Overlying the Tomstown, above a fault which also dips 25°–30° easterly, is Grove limestone which is overturned, dipping 75°–85° easterly.
Along the fault are up to 5 feet of black clay gouge, 30 feet of breccia, and slickensides. The absence of the Frederick limestone and the angular relationship of the Grove bedding to the fault surface provide additional evidence of faulting. The fault, having younger rocks resting on older, could be normal, but the low angle plus overturning of the Grove limestone suggest a subsequent shear thrust.
The authors postulate that this fault or a series of similar faults may explain anomalous stratigraphic relations in the Lower Paleozoic rocks along the eastern edge of Catoctin Mountain. Locally, where the Antietam and/or Tomstown formations are missing, faulting could be the cause instead of facies changes or one or more unconformities.