PA Susquehanna Bedrock Terraces

 Autumn comes to the Susquehanna River. Water levels are low, this is part of the annual cycle of the Lower Susquehanna River Basin (LSRB). The last good high water flow was weeks ago and with it went the annual plants that have hardly a foothold. Seeds from upstream were flushed downstream to lodge into wrack or flood plain sediments and may germinate next spring. Now we see the this harsh bedrock environment for what it is, exposed to the sun, scoured by eons of floods, and soon, hopefully, ice floes will come in winter. The bedrock terrace is a tough environment and now it has shed the lush summer mantel while only the toughest plants survive. 

When we visited Great Falls on the Potomac River this summer and walked out over boulder fields, across wooded islands and ravines, we noted the park service interpretive signs that pointed out the ecological uniqueness of these types of bedrock habitats. We could as well erect similar signs here in the bedrock flats of our own river to alert people to its rare and challenging landscape.  Deformed schist and gneiss is the bedrock complex, a sharp, granular metamorphic rock that formed from ocean bottom sediments during the great squeezing together of tectonic plates.

A sandstone boulder erratic, not native to this bedrock type, but flushed downriver on floods. 

The origins of this bedrock terrace is the Iapetus Ocean  that existed before the Atlantic.  Kenz wonders if there might be fossils here but I assure her that any relics of ocean life have been squeezed and flattened, folded and made semi-molten as continents drifted towards each other. But she looks anyway. There is plenty of sandstone washed down from New York and occasionally we found shattered cobbles that contained impressions of mollusks and other shells.  So yes, there are fossils here and I stood corrected. 

Pickerel Frog

Pools of water trapped between sharp ridges of bedrock yield hundreds of tadpoles and crayfish that dart into bottom mud to escape detection. Occasionally we disturbed Pickerel Frogs that launched themselves five feet or more to escape our clumsy footfalls. The seasonal assemblages of plants included grasses in seed, autumn standards like Thoroughwort and Boneset, and the most delicate of fall flowers,  Slender False Foxglove that is only in bloom for a very short period of time. This is a brave and tough little plant that survives in the most inhospitable places and I admire it year after year for its tenacity.  

Lock 12 Susquehanna Tidewater Canal

We come across places where 19th century canal builders quarried rock and hauled it upslope to build Lock 12. Bore holes from hammer drills and blasted sheaves of stone can be found on the slightly higher terraces above the bedrock. What high river floods haven't swept away was embedded silts or piled up against the rise to the next terrace in waste piles of split and shaped material. 

Geological map courtesy of the Susquehanna River Basin Commission. 

The lifespan of an old river can be read in its terraces and the Susquehanna has been much studied for these broad shelves of rock that step-down to the level of the river today. They tell us that the river was able to scour its way down through the bedrock at a slightly faster pace than the land was rising around it as North Africa crunched into North America.  The Susquehanna does not meander like many western rivers or rivers of the coastal plain. Instead it juts this way and that, following along the folds and fault lines of metamorphic rock through the LSRB. It has carved deep, narrow channels called the Susquehanna Deeps much studied by geologists during the dam building at Holtwood in the early 1900s. There are six Deeps located in the Susquehanna Gorge that runs from Turkey Hill to Holtwood, about ten miles of canyon-like narrow terraces and steep stream-cut hills.  

Bluestem Goldenrod, Solidago casesia

White Turtlehead, Chelona glabra

We compared the plant communities just above the scour line to the open bedrock terrace. Turtlehead and Goldenrod enliven  edge communities and all there is deep green and flowery finery. Three steep steps down to the bedrock zone, however, we walk out into grassland interspersed with tough perennials. There are no woody shrubs or trees except on slightly higher outcrops that offer crevices for deep rooting, but even these trees are stunted, twisted, gnarled from repeated flooding.

Late Boneset, Eupatorium serotinum

Kenz explores the scour line that marks the transition to the next terrace.

Slender False Foxglove, Agalnis tenufloia

We walked way out on the bedrock terrace as far as the spits of rock would allow. Great Egrets lifted from the rocks beyond the wide channel. They are passing through on fall migration now, though a few may stay as long as the fishing is good and ice doesn't seal over the tadpole and frog ponds. We were out there for an hour, late in the day and as the warm air aloft cooled with the setting sun hundreds of migrating Turkey Vultures spun round and round on sinking air to find night roosts along the river. Some trees were heavy with them while others simply congregated on the warm bedrock. Taking a cue from rock-hopping vultures, I laid down on the schist and felt the warmth radiating up. Not a bad place to stay the night.

Descending Turkey Vultures.

As day turned to dusk we clambered back to the edge of the terrace and climbed through a darkening woods to find the trail. We were happy to find a large clump of Violet-toothed Polypore on a fallen tree and watched as daylight-purple rims of this bracket fungi transformed into glowing bioluminescence edges in the twilight woods. 

Violet-toothed Polypore, Trichaptum biforme

Looking back at the bedrock terrace from a high point it was easy to see how the river has incised channels along fractured zones of bedrock. These broad channels contain fish, often trapped all winter until high water comes again to flush them out. These elongated channel ponds will soon serve as important winter feeding grounds for Otter, Herons, and Eagles. They also important to geologists who come here to interpret the tectonic and climatic history of this river. In a larger sense, the bedrock terrace interprets the formation of the Eastern U.S. and Mid-Atlantic. See Notes below. 

Amos investigating Crayfish along the edge of an incised channel pond along a fault.

On the way back to the car we watched a Red Bat fly figure-8 loops and listened to a Barred Owl holler from the cliffs of a river hill above. Soon there will be a full moon and we may come back and walk River Road under moonlight and look out over the terraces under its light. 

Notes:

Reusser, Luke, et. al. "Late Pleistocene Bedrock Channel Incision of the Lower Susquehanna River at Holtwood Gorge, Pennsylvania."  Sourced from: Researchgate.net

https://www.researchgate.net/publication/237696997_Late_Pleistocene_Bedrock_Channel_Incision_of_the_Lower_Susquehanna_River_Holtwood_Gorge_Pennsylvania