These mountains are characterized by long, even ridges, with long, continuous valleys in between. From a great enough altitude, they almost look like corduroy, except that the widths of the valleys are somewhat variable and ridges sometimes meet in a vee.
The ridge and valley system presents an important obstacle to east-west land travel even with today's technology and was a nearly insurmountable barrier to railroads crossing the range and especially to underfunded migrants traveling west to settle the Ohio Country, Northwest Territory and Oregon Country, before the days of motorized transportation. In the era when animal power dominated transportation there was no safe way to cross east-west in the middle of the range, only nearer its extremes save for the presence of a few rough passages opened mid-range during the colonial era such as Braddock's Road and Forbes Road, later improved into America's first National Roads (respectively Cumberland Road, Lincoln Highway or designated U.S 40 and U.S. 30 in later years).
These curious formations are the remnants of an ancient fold-and-thrust belt, west of the mountain core that formed in the Alleghenian orogeny (Stanley, 421-2). Here, strata have been folded westward, and forced over massive thrust faults; there is little metamorphism, and no igneous intrusion.(Stanley, 421-2) The ridges represent the edges of the erosion-resistant strata, and the valleys portray the absence of the more erodible strata. Smaller streams have developed their valleys following the lines of the more easily eroded strata. But a few major rivers, such as the Delaware River, the Susquehanna River, and the Potomac River are evidently older than the present mountains, having cut water gaps that are perpendicular to hard strata ridges. The evidence point to a wearing down of the entire region (the original mountains) to a low level with little relief, so that major rivers were flowing in unconsolidated sediments that were unaffected by the underlying rock structure. Then the region was uplifted slowly enough that the rivers were able to maintain their course, cutting through the ridges as they developed.