🚀 go-pugleaf

jlg@cochiti.lanl.gov (J. Giles) writes:

>And, in the other direction, the sea level along the eastern seaboard
>of the US was once much higher (or the continent was lower).  The existing
>coastal plain is former continental shelf.  The line of the former coast
>is marked by a so-called "fall-line" - a line where there's an abrupt
>rise in the land.  This is the line at which all the rivers on the
>east coast had rapids and falls so they were not navigable beyond

The so-called continental shelf is really just part of the continent which
happens to be underwater at the moment.  Sea level does not cause the fall
line either.  Think of it like this:  A continent is a slab of granitic
"scum" floating on the more dense mafic material of the mantle and sea
floors.  This slab has fairly abrupt edges.  Now, we add water to the
picture:  Depending on how high the water is, and how thick a given
continent is, the shoreline may be at the steep "edge" of the continent,
or may cover part of the continent to produce a continental shelf.