Oriskany Sandstone and Shale No. VII.
This double formation, colored yellow on the map, forms a most important geological and topographical break all through the district, occurring between comparatively softer measures, the Marcellus slate above and the Lewistown limestone strata below.
The Oriskany sandstone can be traced everywhere by the sharp low ridge it forms, no less than by the character of the soil made by the disintegration of its rocks. It is perhaps the most variable and uncertain formation of the Palaeozoic series of Pennsylvania; but it is well developed in this central district of the state. Sometimes the rough weathering sandstone has dwindled down so much in thickness that the underlying Lewistown limestone makes the crest of the ridge, with the Oriskany (Stormville) shales and a little broken down sandstone making the outer flank. Elsewhere, especially in Mifflin county, the sandstone swells out to 125′-150′ in thickness; creates high and rough ridges, with the limestone making a valley at the base.
The Oriskany (or Stormville) shales are rarely well exposed, being generally covered with débris from the overlying sandstone, and their thickness is equally as variable. Through the larger part of the district the Oriskany sandstone No. VII exists as a hard flint or chert rock, breaking up into sharp irregular pieces, which cover the crests and slopes of the ridges, and assist in locating this formation. When in such a condition this sandstone is useless and creates a hard dry soil, very light and but rarely farmed. But the typical sandstone is made up of grains of sharp quartz, loosely cemented together and quarried in many places for use as glass-sand.
In Union county the Oriskany sandstone has but a slight representation. It makes a thin band of loose sand through Gregg township in the north end of the county, where the low dip of the measures has spread its outcrop considerably; but its thickness is not great.
