Hanging Rocks - Geology

Geology

Hanging Rocks is arranged in the form of three anticlinal arches, of which the most eastern spans 250, the second 550, and the third 220 yards in width. Hanging Rocks consists of anticlinal stratified sandstone and limestone layers. The upper stratum of rocks is Monterey and Oriskany sandstone. Immediately below the Monterey and Oriskany (Ridgeley) sandstone lies a layer of cherty limestone known as Lewiston chert-lentil which consists of a conglomeration of brachiopods. Atop Hanging Rocks is a level bench of land devoid of stone and containing fine rich soil.

At the western end of the Hanging Rocks formation lies an exposure of fine black to drab shales also containing small concretions and some fossils. Contained in one of the shale layers are numerous specimens of Phacops cristata Hall. A volume of The Journal of Geology published by the University of Chicago in 1915 noted the following additional species collected in the layers of Hanging Rocks shale:

• Stropheodonta sp.
• Chonetes cf. lepidus Hall
• Dalmanella lenticularis (Vanuxem)
• Cyrtina hamiltonensis (?) Hall
• Ambocoelia umbonata (Conrad)
• Styliolina fissurella (Hall)

Of the aforementioned fauna, Dalmanella lenticularis (Vanuxem) is confined to the Onondaga formation; Cyrtina hamiltonensis Hall occurs in the Onondaga, Hamilton, and Portage formations; Ambocoelia umbonata (Conrad) from the Onondaga to the Chemung inclusive; Styliolina fissurella (Hall) in the southern Onondaga shale, Marcellus, Genesee, and Portage black shales; and Phacops cristata Hall elsewhere in the Onondaga. The Journal of Geology concluded that the Romney shales present at both Hanging Rocks and Mechanicsburg Gap represent the southwestern continuation of the Onondaga limestone, Marcellus shale, and Hamilton formation of New York.

The Hanging Rocks formation lies within a deep and narrow gap in Mill Creek Mountain formed by the South Branch Potomac River. The distance through the gap at Hanging Rocks is five-eighths of a mile. The South Branch flowed in its present course as Mill Creek Mountain formed and slowly cut away at the mountain to expose Hanging Rocks.

The gap at Hanging Rocks is one of four gaps in Mill Creek Mountain, the others being Mechanicsburg Gap, the Lower Hanging Rocks gap at Blue Beach, and the gap at the North Branch Potomac River to the west of its confluence with the South Branch to form the Potomac River.