Matthews and Johnson were the first geologists to determine that the slate was in the core of the fold, and after mappng its southwestern part in Harford County, interpreted the formation as a syncline (Maryland Geological Survey).  The Peach Bottom Slate deposit extends approximately appoximately 10 miles.  Known as the "Slate Ridge," the 150 feet high and high mile wide formation runs northeast to southwest in three parallel belts.

The belts are 75 to 120 feet thick, and cross both the Maryland/Pennsylvania border as well as the Susquehanna River.  They exyend from York County, Pennsylvania abour three miles into Cardiff, Maryland.  Although the deposit's total depth is approximately 1,000 feet, it seldon exceeded 200 feet in the quarry.  The width is about 1/5th to 1/2 mile. When the Conowingo Dam was constructed, about 11/2 miles of the slate was discovered underneath the river.

Underlying and outlining the Peach Bottom slate is the Cardiff Conglomerate, an unusual quartz-pebble unit named by Matthews for the village of Cardiff.  The conglomerate surrounds the slate except for a nine-mile stretch on the northwest side where the slate is in contact with the Upper Pelitic Schist of the Wissahickon formation.  The name "Peach Bottom Slate" was common by the first geological students of the area (Maryland Geological Survey).