Page:Report on the geology of the four counties, Union, Snyder, Mifflin and Juniata (IA reportongeologyo00dinv).pdf/61

Rh Jack’s mountain in this county, always forming the southern boundary wall of the Kishacoquillas valley, shows a straight monoclinal ridge, whose summits rise to the elevation of 2000′ above tide. This remarkable mountain creates a complete topographical and geological barrier between the Kishacoquillas valley on the north and the Lewistown valley on the south, two totally different valleys in their topography, geology and structure.

Topographically both have high bounding walls north and south. The Kishacoquillas valley however, is largely an open plain from 2 to 4 miles wide, broken at its eastern end by mountain spurs, but open on the west. The Lewistown valley, stretching similarly from one end of the county to the other, is broken by many slender ridges and narrow subordinate valleys, presenting a fairly smooth and homogeneous appearance from the summit of Jack’s or Shade mountain, but in reality very deeply cut up longitudinally. The effect produced in both valleys is directly due to their geology and structure.

The Kishacoquillas valley is composed of the Lower Silurian limestone and slate below the mountain sand rocks No. IV; the Lewistown valley contains only rocks above the Oneida and Medina sandstone, the upper Silurian and Devonian Nos. V, VI, VII and VIII, and both groups of rocks have a more or less characteristic topography.

Then again the structure of the northern valley is normally a great anticlinal: the southern valley is synclinal: and while both are modified by subordinate axes, the Lewistown valley is crumpled into a dozen rock waves, duplicating and repeating the same rocks several times, and consequently accounting for the diversified topography due to the successive elevation and depression of such rocks as tend to create the ridges and valleys.

The detailed structure of the Lewistown valley has been so thoroughly delineated in Report F, page 47 et seq., that it would seem useless to repeat the description here. The map coloring and the dip arrows will sufficiently explain the structure, even without the aid of the several carefully drawn cross-sections of Report F along the Kishacoquillas