[Rockhounds] Northeast US Rift Basin... was New Madrid fault / Rivers

[Mr EMan]

Mark Sigouin <marksigouin at verizon.net> wrote:
The Hudson River also occupies a failed rift.

I thought that technically the Lower Hudson , down through at least eastern New Jersey, overlies a "successful" rift but, you are right. The Palisades; a Basalt escarpment on the west rim was part of that rift--the other half is in Spain, Portugal and/or Morocco, I believe.

As a result of rift formation we get graben faults, half grabens etc.  Any resulting escarpment pretty much insures a river channel will align parallel to the fault trace. Such is the escarpment along the western bank of the Delaware River between the NY State line and East Stroudsberg, Pa.  Mineralogically unremarkable save for occasional pyrite cubes but oh so full of trilobites, water falls, and dead falls. Each spring at the bottom of the escarpment at the end of fresh skid marks I find scores of carcasses including deer, coyote, raccoon, snake, and box turtle.

Paleo drainage basins are an interesting sector of geology more so than of general rockhounding interest but they did cut natural gaps which, until human road building enterprise, were the main windows into bedrock. Thus providing access to minerals for early prospectors, miners, and paleo-rockhounds.

Back to the Hudson Valley which was reworked and rerouted by glaciers in more recent times, research shows that the paleo-Hudson actually drained through the present day Delaware River channel and cut the massive Delaware Water Gap. The Paleo-Delaware was responsible for the abandoned channel that today is known as Wind Gap, Pennsylvania where for a time it cut through Sliurian rocks of the Blue Ridge.  For anyone familiar with those gaps and the surrounding lowland, it is awe-inspiring to think that the wind gap was the bottom of the river channel and all the adjacent missing cubic miles of material represents how much material was actually removed from the region by erosion for the 300 my after uplift cut off the paleo-Delaware shifting it westward into the Lehigh River before emptying into the present-day Delaware--By some estimates: 4000-8000 feet of sedimentary layers.  Sometime more recently the Delaware jumped back into the present day channel.

It is also awesome to think that the very river that cuts through a mountain range had to be running in that spot before the mountain/ridge building began. As in ancient times it is true today, rivers do not run up hill to cut gaps in mountains.  It had to be there sawing away the bedrock faster than the mountain was being built. If the uplifting ever got ahead of the river, the channel was abandoned and a wind gap is all we have to know that "once a river ran through it"...

Elton

--- On Sat, 3/28/09, Mark Sigouin <marksigouin at verizon.net> wrote:
> The Hudson River also occupies a failed rift. From my
> geology classes years and years ago, when a rift forms
> causing two plates to move from each other, at points along
> the successful rift there occur third unsuccessful rifts
> where the continental crust doesn't pull apart.  But the
> rift presents a weak point for erosion to work on creating a
> river valley that often ignores other hydro-geologic flow
> structure controls.  So you get rivers that cut through
> mountain ridges rather than going around them.