By: magic-gigapans
Image: Robin Rohrback, Mid-Atlantic Geo-Image Collection
License: Creative Commons Non Commercial ⧉

Uploaded: 13 May 2020
Last Updated: 15 May 2020

3.87 gigapixels
154,464 x 25,036 pixels
514.9 in X 83.5 in at 300dpi

This image shows three generations of Potomac riverbed, called straths. How does a river deepen its channel? One way is through nickpoint propagation. A nickpoint develops when the river's base level drops. (Base level dropping could be caused by sea level dropping, or the land uplifting, or other reasons.) On the river, you'd recognize a nickpoint as a waterfall or a series of rapids. Great Falls is a nickpoint along the Potomac, for instance, and so is Little Falls. As water falls over the nickpoint, it excavates the rock which underlies the waterfall. As rock is removed, the nickpoint retreats in an upstream direction. If base level were to drop again, perhaps due to sea level drop like seen here, it would cause a new nickpoint to develop close to base level. As time goes by, that nickpoint will work its way upstream too. As each nickpoint propagates upstream, it carves a new channel, a new gorge. These gorges get longer because of nickpoint propagation, but they get wider due to mass wasting. The river chews its way downward, but that leaves large cliffs unsupported by the rock that used to hold them up. As a result, they tend to collapse into the river, which carries away the debris. So as time goes by, each nickpoint serves as the lead for a new valley carved in the middle of older, wider valleys. You could visualize these valleys as a nested series of "rowboat" shapes, each with a nickpoint at its bow. In the area of the Billy Goat Trail, Mather Gorge is the innermost and narrowest of these levels. Why is Mather Gorge so straight? One possibility is that a major crack in the Earth's crust, a fault, runs along the bottom of Mather Gorge. This fault would have crumbled up the rocks along its surface, making for an easy spot where the river could erode away rock.