CompuServe Messages

<< The picture I am forming in my mind is of a great range of fold mountains (as big as the Himalayas?) forming during the collisions as/before Pangaea formed, with correspondingly great roots below them…Then, sort of like I explained to Kamron, as the tops wore down, the roots floated up correspondingly>> I see what you're saying here, and I think you're basically right, but I don't see your "roots" going down very far. Remember, these are sediments which have overflowed the edge of the continental craton being pushed back by the plate collision like you'd push back the cuticle of your fingernail (ouch…!) So while the margin of the craton could be expected to subside a bit because of the additional mass of the folded, faulted and crumpled sediments on top of it, we're really talking about only a few thousand feet of overthrusted sediments across a relatively narrow band parallel to the continental margin, and that can't make TOO much difference along a continental margin that has been accumulating sediments since the end of the Precambrian. I rather think the bulk of the uplift in any plate colllision, or at least a good portion of it, is from the upturning of the continental crust as the colliding plate plows under it. The difference in the density of oceanic crust and continental crust with sediment on top of it CAN'T be so great that the oceanic crust subducts without plowing up the continental crust as it goes. I think you're right though that isostatic adjustment constantly renews the erosion process and exposes deeper and deeper folded and faulted layers to the elements, unless something happens that either causes the whole region to subside or causes sea-level to rise and thereby allow deposition on top of the eroded stubs. << the water gaps where rivers cut straight through these mountains, mentioned in other messages in this thread, are a bit annoying.>> Danged annoying <g>. I live at the head of one gap and downstream of another, cut by two separate streams. We tend to think that a stream which breeches an anticline consequently pre-dates that uplift. I try to think that it merely pre-dates the most recent EXPOSURE of that structure. But then we look at WV's ironically misnomered New River, which flows the wrong way directly across the Appalachian range. My thought is that it must have risen in a localized chunk of high country to the east which pre-dated the additional pushing and shoving which progressed to the west. FWIW, Western PA's Youghiogheny River also runs the wrong way, and cuts two spectacular gorges through Laurel Hill and Chestnut Ridge, two afterthought anticlines — still intact, not inverted — along the western flank of the Allegheny Front. These low ripples apparently occurred after a larger uplift to the east and the already-established river cut them as they formed.