CompuServe Thread

<<I suppose that is what I saw northwest of the Grand Canyon from my airliner (a pair of ridges forming a long straight line on an otherwise flat and featureless section of the Colorado Plateau>> Arrrgh! Out of 457 geologic paradises to check out when I was in the SW two years ago, I had time for all too few, and this was one. Time to go again. I'll check the maps. <<If your (c) undercutting is a big factor, the tough top layer might fall into a hole in the center and it would look somewhat like a fault block situation on the surface (except I guess the top layer would typically fall as rubble instead of a block)>> I always wondered about rubble in the classic karst topography. Where does it go? Easier to understand around Central PA are the boulder fields, where on-edge sandstones and quartzites at the tops of the ridges are quarried out by ice and other erosion and weathering and form boulder slopes. These boulders litter the central ridge of my synclinal valley (the one I was chatting with Kamron about) and make it very difficult to determine strike and dip from a casual observation. Another one frequently spills blocks onto US Route 22 in the gorge the Juniata River has cut between here and Harrisburg. <<But as long as the undercutting is only a contributing factor, I figure these center-cut folds should be "easily" distinguished from fault block mountains because the top layer would be different between the outside flank of the ridges and the lower center area.>> True. Faulting is not common in the PA section of the Ridge & Valley province. Much moreso to the south. We have some thrust faulting here, but it is more often at depth than at the surface. The anticlinal valley surrounding Penn State University is complexly faulted at depth, but all you see at the surface is a broad valley of smoothly arched Cambrian and Ordovician limestones running along a SW to NE axis, bounded by more or less parallel ridges of resilient Silurian and Devonian sandstones. When the anticlines and synclines plunge one way or another along the strike, complex V-shaped features pop out. Central PA's Nittany Mountain rises abruptly out of the limestone like the bow of a ship, then stretches to the NE in a pair of gradually diverging sandtone ridges enclosing a synclinal valley, the syncline plunging in that direction. Another feature of that sort sticks its nose into the Susquehanna River just north of Harrisburg, with the preserved layers of tough quartzitic sandstone standing vertically in the river, concealing some very large and feisty smallmouth bass <grin>. ed–