 Footnotes to the Geologic History of Virginia and the Mid-Atlantic States
GRENVILLEFootnote A-1: An overturned anticline is one in which the rocks have been folded up into an arch and then shoved some more until the rocks on one side of the arch are overturned (i.e. upside down.) Footnote A-2: "Africa" is in quotes because it is not clear at this time just which continents collided with the east coast in the Proterozoic. Likely parts of South America were a major block that docked with eastern North America at this time. The supercontinent Pangaea, so familiar in the Mesozoic, did not exist at this time. EARLY DIVERGENT CONTINENTAL MARGIN Footnote D-1: Compass Directions: Since continents move around all the time, all compass directions are given by today's locations. In the early Cambrian eastern North America acutally lay south of the equator and "east" was in fact "south." DEVONIAN ACADIAN OROGENY Footnote J-1: Deltas As late as the 1960's the term "delta" had two meanings. The first was the "tectonic delta" and referred to any large accumulation of sediment at the base of a mountain. The Catskill clastic wedge fits this definition. The second is what we might call a "sedimentation delta", that is sediment accumulating at the end of a river where it enters a standing body of water. Clear understanding of the processes in these deltas did not appear until the 1960's. The two uses of the term "delta" are not compatible, and it has been clear since the 1960's that the Catskill "delta" has no "sedimentation deltas" in it. Today the concept of the "tectonic delta" is passe; we no longer use it in any geologic sense. Instead the more accurate "clastic wedge" has been substituted. Still, generations of geologists learned about the Catskill "delta" and a fondness for the name persisted until recently. Footnote J-2: Rates of Subsidence We do not want to leave the impression this was one swift movement. Geologic events of this size do not work that way. They may be geologically rapid, but they take place through thousands, 10's of thousands, of small, incremental events, sometime separated by years. Similarly, these events probably occur in pulses, a sudden surge of uplift/subsidence lasting a 100,000 years, followed by an interval of relative calm.) Footnote J-3: Prograde Prograde originally referred to a shoreline, as in "a prograding shoreline" which meant that the shoreline built out into the sea by the accumulation of sediment. Or, land was created where there was water by filling in the water with sediment. Prograde now refers to any environment that builds out toward the basin by the accumulation of sediment. Prograding is a type of regression. The sea is said to "regress." that is move away from, retreat from, the land and move toward the basin. So we can say a regressing shoreline, meaning the shoreline is moving toward the basin. But progradation and regression are not always the same. A progradation is a regression, but not all regressions are caused by sediment building out. A regression could also be caused by a drop in sea level (or a rise in the land); both have the same geographic effect, the shoreline moves toward the basin. |