FOLDS -- FOLDED SEDIMENTARY ROCKS

Both anticlinal ridges and valleys and synclinal ridges and valleys are less common that other types of valleys and ridges in terrains of folded rocks.  The diagram shows why.  Imagine a stack of folded sediments with one resistant bed in it.  Anticlinal or synclinal ridges and or valleys could only occur very near the levels of the green lines below -- anticlinal ridges/synclinal valleys at the upper green line, and reversed topography at the lower.  Erosion in any other place would either produce no valley and ridge topography (upper red line) or ridges formed only on the limbs of the folds (lower red line).  Because these ridges would have beds dipping only in one direction they are called homoclinal ridges and the valleys beside them are called homoclinal valleys.

 

The composite image below shows Germany valley, WV from the south.  The lower panorama shows the entire valley north of US33.  Though there are numerous hills and ridges in the photo, the two important ones are North Fork Mountain at the extreme right and the River Knobs with numerous of its gaps indicated by arrows.  the high ridges to the left of it are part (the "Front") of the Allegheny Plateau, and are not part of the fold.  The top right inset shows the crest of North form Mountain, where a resistant sandstone bed is apparent, with a gentle dip to the right (ESE).  Examination of this bed indicates that it is the Silurian Tuscarora Sandstone, and that it is stratigraphically upright.  The 3 insets to the left of it show the same sandstone bed as it appears in various gaps of the River Knobs.  In each place the bed is seen to be vertical, as is the case all along the River Knobs.  Its original top faces WNW toward the Allegheny Front in this ridge.  The obvious inference is that the two ridges are on the limbs of a breached asymmetric anticline, a hypothesis that is easily seen to be correct at the point indicated as the nose of the fold in the bottom right inset.  This is an area called the Smoke Hole (after a cave that occurs there and that 'exhales' steamy warm air in the winter) and the asymmetric fold can be mapped in its entirety there because it is not breached.  Thus, both the River Knobs and North Fork Mountain must be homoclinal ridges and Germany Valley must be a homoclinal valley.

The background image for these pages (reproduced below as a larger panoramic shot) also shows homoclinal ridges.  The crest of the ridge from which the picture was taken (Peters Mountain on the Virginia/West Virginia border) is Tuscarora Sandstone, dipping to the right (ESE), thus Peters Mountain is a homoclinal ridge.  (However, the mountain was thrust westward over the rocks in the valley by a fault at its base, so the slope in the photograph is also an obsequent fault-line scarp).  There are two obvious low ridges in the valley below.  The nearer is Little Mountain and the farther is Chimney Ridge.  They are homoclinal ridges of a breached anticline in Mississippian rocks.

The final photograph is a view northwestward from the Blue Ridge Front on Mt. Rogers, VA across the Valley and Ridge.  Straight and Grosses Mountains are homoclinal ridges of a breached anticline, Big Knob Ridge is an anticlinal ridge near the middle of the Shenandoah Valley, and Walker Mountain is a homoclinal ridge with a fault-line scarp on its west face (so there is no matched ridge on the other limb of a fold).  The southern equivalent of Peters Mountain (seen in the photograph above, and called East River Mountain this far south) is the third ridge beyond Walker Mountain.  Both Walker and East River Mountains have tunnels blasted through them for the passage of I-77 as it goes north from Wytheville, VA into West Virginia.  Historically the various ridges of this region, along with the edge of the Allegheny/Cumberland Plateau, have proven formidable obstacles to westward movement because the early settlers couldn't scrape up enough dynamite to build such tunnels.  Thus Dan'l Boone had to show them how to get through the Cumberland Gap.  Washington and Jefferson, among others, (pipe)dreamed of settling the west along the Potomoc, a process that was finally accomplished via western New York and Pennsylvania on the one hand and the Mississippi Valley on the other.

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