© Lloyd, Charles Chandler

 

Hudson Canyon
http://upload.wikimedia.org/wikipedia/commons/7/7d/Hudson_Canyon.jpg

 

Monterey Submarine Canyon
http://throughthesandglass.typepad.com/.a/6a01053614d678970c0115701320da970c-pi

 

Canyons Formed Above Sealevel

 

These and many other canyons on continental shelves must have formed above the water level, because similar erosion does not occur under water. This means sea level must have been very low, or the continental shelves were initially higher, and then sunk down after the canyons formed.

 

Soft Continental Shelves

 

These canyons may have eroded through relatively soft sediment, as was apparently the case with the Grand Canyon.

 

Timing?

 

The canyons had to have formed soon after the Supercontinent Breakup, just as with the Grand Canyon.

 

Sea Level Change

 

The sea level probably did change, but did sea level generally rise very much, or did the continental shelves subside very much? If the shelves did not subside much, then there must have been less ocean water in the past. Cardona says most of the ocean waters were tied up in glaciation in temperate zones, although he says some water also came from the breakup of the polar column over the north pole.