Editing Geology of the Appalachians (section)

=== Proterozoic era ===
==== Grenville orogeny ====
[[File:Grenville extent.gif|thumb|Land added to Laurentia during the Grenville orogeny]]
The first mountain-building tectonic plate collision that initiated the construction of what are today the Appalachian Mountains occurred during the [[Mesoproterozoic|Mesoproterozoic era]] at least one billion years ago when the pre-North-American craton called [[Laurentia]] collided with other continental segments, notably [[Amazonian craton|Amazonia]]. All the other cratons of the Earth also collided at about this time to form the [[supercontinent]] Rodinia, which was surrounded by [[Mirovia|one single ocean]]. Mountain-building referred to as the [[Grenville orogeny]] occurred along the boundaries of the cratons. The present Appalachian Mountains have at least two areas which are made from rock formations that were formed during this orogeny: the [[Blue Ridge Mountains]] and the [[Adirondack Mountains|Adirondacks]].<ref name="Thomas 2021">{{Cite journal |last1=Thomas |first1=William A. |last2=Hatcher, Jr. |first2=Robert D. |date=2021 |title=Southern-Central Appalachians-Ouachitas Orogen |journal=Encyclopedia of Geology |volume=4 |quote=The foundations of the Appalachian-Ouachita orogen were laid when the assembly of supercontinent Rodinia was completed. The collisional events were accompanied by high-grade metamorphism and magmatism during the Grenville orogeny in the time span of 1300–950 Ma. |via=Elsevier Science Direct}}</ref>

==== Breakup of Rodinia ==== 
After the Grenville orogeny, the direction of the [[continental drift]] reversed, and Rodinia began to break up. The mountains formed during the Grenvillian era underwent [[erosion]] from [[weathering]], [[Glacial period|glaciation]], and other natural processes, resulting in the leveling of the landscape. The eroded sediments from these mountains contributed to the formation of sedimentary basins and valleys. 

For example, in what is now the southern United States, the [[Toccoa/Ocoee River|Ocoee basin]] was formed. Seawater filled the basin. Rivers from the surrounding countryside carried clay, silt, sand, and gravel to the basin, much as rivers today carry sediment from the midcontinent region to the [[Gulf of Mexico]]. The sediment spread out in layers on the basin floor. The basin continued to subside, and over a long period of time, probably millions of years, a great thickness of sediment accumulated.<ref name=Clark>{{Cite book |last=Clark |first=Sandra H. B. |url=https://pubs.usgs.gov/gip/birth/birth.pdf |title=Birth of the Mountains: The Geologic Story of the Southern Appalachian Mountains |publisher=United States Geologic Survey |year=1996 |pages=4 }}</ref> 

Eventually, the tectonic forces pulling the two continents apart became so strong that the [[Iapetus Ocean]] formed off the eastern coast of the Laurentian margin. The rocks of the [[Ridge-and-Valley Appalachians|Valley and Ridge province]] formed over millions of years, in the Iapetus. Shells and other hard parts of ancient marine plants and animals accumulated to form limey deposits that later became [[limestone]]. This is the same process by which limestone forms in modern oceans. The weathering of limestone exposed at the land surface produces the lime-rich soils that are so prevalent in the fertile farmland of the Valley and Ridge province.<ref name=Clark/>

During this continental break-up, around 600 million to 560 million years ago, volcanic activity was present along the tectonic margins. There is evidence of this activity in today's Blue Ridge Mountains. [[Mount Rogers]], [[Whitetop Mountain]], and [[Pine Mountain (Appalachian Mountains)|Pine Mountain]] are all the result of volcanic activity that occurred around this time. <ref>{{Cite book |last=Rankin |first=James W. |title=The volcanogenic Mount Rogers Formation and the overlying glaciogenic Konnarock Formation: Two late Proterozoic units in southwestern Virginia |publisher=USGS |year=1993 |doi=10.3133/b2029}}</ref> Evidence of subsurface activity ([[Dike (geology)|dikes]] and [[Sill (geology)|sills]] [[Igneous intrusion|intruding]] into the overlying rock) is present in the Blue Ridge as well. For instance, [[mafic]] rocks have been found along the Fries Fault in the central Blue Ridge area of [[Montgomery County, Virginia]]. <ref>{{Cite book |last=Kaygi |first=Patti Boyd |title=The Fries Fault near Riner, Virginia: an example of a polydeformed, ductile deformation zone |publisher=VT Works |year=1994}}</ref>