Potholes in the Susquehanna River 

Curved rocks at Three Mile Island
A series of linked potholes in large boulders of Triassic diabase in the Susquehanna River near Three Mile Island, Pennsylvania.


Potholes at TMI
Intersecting potholes at Falmouth, off the southern end of Three Mile Island, in the Susquehanna River, Pennsylvania. They were exposed during the summer of 1999 when the water level was low.


Potholes in cliff side
Another view of the Holtwood potholes
A cluster of potholes at the Holtwood Gorge along the Susquehanna River, Pennsylvania, formed in a steep cliff  of Wissahickon Schist with quartz boudins.  Potholes in the vicinity range in size up to 9 metres deep and 4-6 metres in diameter. References to previous studies can be found here.


Water marks on rocks show the normal water level
Potholes at Falmouth, off the southern end of Three Mile Island, Pennsylvania. Holes occur in the rocks where adjacent potholes intercept. The potholes typically widen with depth. Water marks show the level of the Susquehanna River was lower than normal. The form of the partial potholes is clearly unrelated to the water levels, suggesting currents were not the cause of the potholes.

Small linked holes in a boulder
Small linked potholes form grooves in this boulder.

Potholes were not formed by currents
Careful observation shows that current action has not carved these potholes; eddies in streams are not anchored to their positions, but move along with the current and disappear, while new eddies are continually formed. The eddies or vortices do not drill down into the rocks of a stream bed. It is suggested the potholes were formed due to the effects of the release of vertical stress during the removal of overburden by flood waters in catastrophic conditions. The river merely exposed them by eroding away the pothole contents. 

Credits

Photos by Mark Brinkman. Used by permission.

Links

Fred & Barney
Mystery of Pothole Origins
On the Interpretation of Potholes

Peculiar Potholes at Lion's Head

Copyright © 2003 by Douglas Cox
tcc@sentex.net