Page:Report on the geology of the four counties, Union, Snyder, Mifflin and Juniata (IA reportongeologyo00dinv).pdf/39

Rh Tuscarora mountain forms the south wall of this valley from the Juniata east of Thompsontown to the Huntingdon county line. Composed of the same No. IV rocks as the various other mountain ranges of the district, it produces similar topographical effects. From the river west to the Liberty Valley gap south of Honey Grove it forms a comparatively straight wall without a single water gap for about 22 miles. It is of anticlinal structure, one-half of the mountain being in Juniata and the other half in Perry.

South of Honey Grove it is offset southwards, enclosing the Liberty valley of Clinton rocks between its two divisions, and extends for 12 miles more in the district to Huntingdon county.

Turkey ridge forms the north wall of the Tuscarora valley and the Juniata-Perry county line east of the Pennsylvania railroad all the way to the Susquehanna river; but it is composed of Hamilton sandstone No. VIII, a much higher geological formation than the Medina No. IV and here forming a ridge almost as high as the mountain.

The Oriskany sandstone, colored yellow on the map, makes a series of zigzag ridges through the eastern portion of Juniata county enclosing the valley of the Cocolamus and Mahantango creeks, composed of the Marcellus, Hamilton, Portage and Chemung rocks of No. VIII, a valley in many respects like the Tuscarora further west. Like it, its width is only a mile between Port Royal and New Mexico on the Juniata; along the Mahantango, south of Richfield, 7 or 8 miles wide; and on the Susquehanna 10 to 12 miles wide, where it deepens to receive a small boat-shaped area of the Catskill rocks north of Port Trevorton.

For reasons already fully explained in treating of the Tuscarora valley, the choice farming lands of eastern Juniata and southern Snyder all lie behind the Oriskany sandstone ridges, and owing to the repeated folds in these rocks and the elevation of the main basin going towards the Juniata, large areas are occupied there by the Lower Helderberg limestone and Salina marls and shales, so that magnificent farms occupy most of the territory between the Tuscarora and East Shade mountains.