Page:Report on the geology of the four counties, Union, Snyder, Mifflin and Juniata (IA reportongeologyo00dinv).pdf/65

Rh crest of this axis that the lower Silurian limestone has been brought to daylight in Juniata county, the same limestone which makes the floor of the Kishacoquillas valley, and in but one other place, in the Spiegelmeyer valley in the northeast corner of the county, on the summit of the East Shade mountain, has the next higher formation—the slate No. III—being exposed. This brings about a significant point of difference geologically between Juniata and Mifflin counties, and the effect is largely due to the structure.

The Black Log or West Shade mountain axis crosses the river a mile above Mifflintown, still carrying the Ore sandstone on either side with dips of 15° and 30°, and as it gradually subsides on the west side of the river its crest becomes dimpled with a small synclinal roll making four outcrops of the Ore sandstone and Sand vein as far east as the Fayette township line.

Beyond this point it can only be identified by the indented outcrops of Lower Helderberg limestone and Oriskany sandstone in Flintstone ridge, where east of the Presbyterian church it shows north dips of 15° and south dips of 40°—50°. Its presence was not noted on Cocalamus creek at all.

13. The Mifflintown synclinal, which succeeds on the south, is the main axis of the northern part of the Juniata valley. It crosses the run a short distance below the county seat with converging dips of only 10° and 15°, and for 8 miles west to Allenville its course is marked by a trough of upper Salina marls and perhaps the lowest members of the Lewistown limestone, which have added so largely to the fertility of the valley.

Further west it dies away against Shade mountain. It is not continued directly eastward from Mifflintown, but is offset a little to the south en échelon with the line first described, and then passes on through Flintstone ridge gradually deepening until it receives the Marcellus slate No. VIII about 4½ miles from the river.

This trough line crosses Delaware run just north of East Salem with dips of 30°—35° in Marcellus slate, and Cocalamus creek about midway between the cemetery and the