CompuServe Messages

Don, >> On both sides we have these tidy folded mountains that are purported to have eroded only a little — only a few thousand feet of sediment missing — in 300,000,000 years. I don't have any problem with localized uplifts, but I have a lot of trouble with the selective erosion that seems to be necessary to make the model work. << Based on the above, I'm not sure you quite understand. My refs all describe the sequence of events in eastern NA as follows: 1. Taconic orogeny, mid-Ordovician, creates a mountain range something like the modern day Andes: lots of igneous rock, lots of volcanoes. 2. Taconic Mountains are worn down by erosion, which process is basically finished by the early Devonian. At that point, eastern NA is an old landscape, rolling hills, old meandering rivers, very little relief. 3. Acadian orogeny, late Devonian through early Pennsylvanian, raises a new range of fold-type mountains somewhat east of where the Taconics were. 4. Acadian Mts. worn down by erosion, but that process may not have been complete by the time the Old Red Sandstone continent hit Gondwanaland to form Pangaea. 5. Formation of Pangaea in Late Missippian/Early Permian causes third (Alleghenian) orogeny, a third range of mountains in eastern NA. Lots of folding, lots of thrust-faulting, especially in the south. In some places, coastal rock was thrust-faulted inland for dozens of miles. 6. During the Mesozoic, the relief in eastern NA was again worn down by erosion, to an old landscape with rolling hills, old rivers, and little relief. 7. During the Cenozoic, exactly when I'm not sure, epeirogenic activity (one text describes this as 'arching' due to upwelling from the mantle, possibly a very very early stage of pre-rifting activity) and isostatic lift (after the ice ages) gently lifted the entire eastern part of the US. This lifted the remnants of the Taconic, Acadian, and Alleghenian ranges by two or three thousand feet, enough for serious weathering to start up again. Streams formed and started downcutting, faster in soft rock than in hard, and so on. Because all that was left of those ancient ranges after so long was the folded, faulted roots, that's what was uplifted, and that's what is now eroding. So they haven't eroded "just a little in 300 million years" — they eroded a lot, in fact just about all the way, and what we see today is the product of maybe ten to thirty million years of erosion. Maybe less. Maybe a lot less. Jon W.