Page:Report on the geology of the four counties, Union, Snyder, Mifflin and Juniata (IA reportongeologyo00dinv).pdf/382

354 F³. The principal structural features consist of the Blue ridge anticlinal on the north; the Black Log—Shade mountain—Lost creek ridge anticlinal in the center, and the Academia ridge anticlinal on the south. Between the two former is the Licking creek valley synclinal of lower Helderberg, Salina and Clinton rocks; and between the two latter anticlinals is the Mifflintown synclinal valley largely composed of lower Helderberg and upper Salina rocks, with the lower Salina and Clinton measures on the flanks of the bounding ridges. The Tuscarora synclinal axis lies largely south of the creek and Beale township, so that the rocks on the south side of the Academia ridge all dip southward into this basin and are largely Salina and lower Helderberg strata.

Both Blue ridge and Shade mountain show outcrops of the red and white Medina No. IV sandstone, but the township and county line along Blue ridge lies wholly upon the south (white Medina sandstone) ridge until along the Juniata river this rock gradually sinks over the dying anticlinal with dips of 60° N. W. and abontabout [sic] 50° S. E.

The Shade mountain axis on the south side of Licking creek valley keeps the red Medina (No. IVb) rocks on its crest for about 4 miles east of the Tuscarora township line, where the anticlinal sinks and permits the two white Medina crests to come together and form one anticlinal ridge about 3 miles in length, showing dips upon the road north of Johnstown of 25° S. E. and 40° N. W.

East of the Medina sandstone knob the ridge is triple-crested, outcrops of the Ore sandstone forming separate ridges on the north and south flanks of the anticlinal with the Iron sandstone and lower Clinton shales making the central backbone and anticlinal.

The north Ore sandstone ridge has received locally the name of the “Forge ridge;” while the southern outcrop making a similar ridge is known as the “South ridge.” Both, together with the central backbone, create the Lost Creek ridge anticlinal.

Where the Shade mountain Medina sandstone is elevated, the Ore sandstone ridges become merged with it as terraces