The river began around 550 million years ago as the Taconic Orogeny compressed, folded and heated sediments of a previous era as it thrust them into mountains. Now only the hard, erosion-resistant metamorphic rocks of the mountain core remain, exposed as the Piedmont of eastern North America. Weathering and erosion stripped away the softer mountain rock. Rivers carried the sediments to the sea.  One of these rivers became the lower Susquehanna.

Later the rise of the Appalachian Mountains to the north and west created a drainage basin that extended through Pennsylvania and into south-central New York. Rock weathering and erosion wore down these mountains throughout the Mesozoic Era, the age of dinosaurs, and the Cenozoic, the age of mammals and birds.  Now, just as it has for about the last 400 million years, the Susquehanna continues to carry ground-down remnants of mountains to the sea.

A series of continental glaciations in the Pleistocene Epoch gave the Susquehanna Valley its most recent sculpting. Although the glaciers terminated a bit north of the lower Susquehanna, their immense weight dimpled the land, and their melting sent massive floods that scoured the valley into the broad canyon we see today.

About 10,000 years ago, Native Americans came to the valley to hunt, fish, and work their farms. European colonists continued forest clearing for larger farms, towns, and factories. The river became an artery for commerce.  Railroads and canals followed its valley.  Local limestone was “burned” for plaster and mortar to build cities. During the 1930s, its rapids were dammed so the Susquehanna’s power could turn the motors of commerce and light the night far beyond the valley.

Whenever I visit the valley, I am awed by the resilience of nature in the face of centuries of human impact. Shenk’s Ferry Wildflower Preserve is a good illustration. In the 1700s Shenk’s Ferry had its eastern terminal on flat land just upriver from Grubb Run. That land is now crowded with trilliums each spring, just as it was before the ferry. In the late 1800s and early 1900s, Grubb Run saw an explosion (literally) of industry and commerce. Three railroads left their mark on the preserve, one at river level, one near the upper edge of the preserve, and one that left only the remains of a pylon for a bridge that was never built. A few houses and small factories nestled in a flat area at the upper end of Grubb Run. In 1907 a dynamite plant exploded, killing 11 people. The community was abandoned. Forest and a naturally diverse plant community reclaimed the land.

Trilliums make the steep sides of Grubb Run special in the spring. Typically the flowers are white and flat in profile like Trillium erectum. In the 50 or so years that I’ve enjoyed them, I’ve heard these plants called T. erectum ‘Alba’, T. erectum, T. flexipes, or a hybrid swarm of T. erectum and T. flexipes. My preference is just to call them the Shenk’s Ferry trillium, even though the type is also found in other glens on the north side of the river.

Some time ago Dr. Richard Lighty, then Director of the Mt. Cuba Center, observed that along the Susquehanna north of Harrisburg, Trillium erectum typically had red flowers. A bit south of Harrisburg, petals blended streaks of white with streaks of red. At Shenk’s Ferry flowers were generally white with peach-colored ovaries but rarely plants with red-streaked petals stood out in the population. To him, this suggested that, probably due to the ebb and flow of glacial climates, red T. erectum from the north moved south where they met T. flexipes moving north from populations to the south and west. Hybridization produced the Shenk’s Ferry form while the southwest side of the river remains more like T. flexipes.

Brian Carson showed Dot Plyler and me a population of T. erectum in southwestern Quebec that suggests another possibility. In that population, the T. erectum had frequent yellowish, white, and bicolored forms as well as the expected red ones. Continental glaciation would surely have destroyed them in Quebec, but they could move south as the climate became colder. The lower Susquehanna may have served as a refuge for such color variation that, as Dr. Lighty suggested, was diluted by hybridization with other species moving north and east as glacial climates eased.

Trillium is only one of many genera at Shenk’s Ferry. Once the dirt road drops to river level, it runs along steep slopes on the left and a well-vegetated railroad embankment on the right.  A bit before trillium time, the slopes are alive with Virginia bluebells (Mertensia virginica) and Virginia saxifrage (Saxifraga virginiensis). The blue marsh violet (Viola cucullata) grows below some seeps. Dutchman’s britches (Dicentra cucullaria) brightly greet visitors from both the slopes and the railroad bank. Squirrel corn (Dicentra canadensis) is there too, though less common.

There is very limited parking across the Grubb Run Bridge, but a jeep road continues uphill. Trilliums cascade down the road cut where they can be photographed in some comfort at eye level. As you go higher, the roadsides level somewhat, and the rattlesnake fern (Botrychium virginianum) grows among the trilliums.

The gated main tail is on the north side of Grubb Run. It runs upstream, ducks under a power line from Safe Harbor Dam, and terminates at the masonry overpass of a Conrail line.

The white trout lily (Erythronium albidum) grows on the steep and muddy bank to the left of the trail – unfortunately about two-thirds of the way up. More accessibly, Phlox divaricata lines some sections of the trail and the white violet (Viola striata) is common. On the right, sharp eyes may spot a few leaves of the putty root orchid (Aplectrum hyemale).

Beyond the power line, trilliums are less common. But there may be a few showy orchid (Galearis spectabilis) at the trail’s edge, protected from trampling by a ring of stones.  The trail ends at the railroad overpass. The flat stones by the stream are a cool place for snacks, and the slopes offer a chance for a leg-stretching scramble. Scouring rush (Equisetum hyemale) grows in a seep above the stream and hepatica grows above the boulder field.

For explorers, ill-defined paths run east, roughly perpendicular to the main trail. The ruined settlement and dynamite factory lay along their tracery. Further east, the land rises as a trail runs along and well above a stem stream of Grubb Run. Here the geology of basement rock changes to a shale that weathers into relatively acidic soil. The plants change too. With great luck and perfect timing, you can find Obolaria virginica, a saprophytic member of the Gentian family. Dwarf ginseng (Panax trifolius) is common there, though much easier to see along the trail from the River Road parking at Tucquan Glen further east.

Plant communities on the south side of the river seem subtly different from those on the north, possibly due to more intense and long-lasting development. At the intersection of Rt. 372 and McCalls Ferry Road, Lock 12 shows how river transport bypassed the Susquehanna rapids. A bit upstream, McCalls Ferry Road passes stone foundations being reclaimed by nature. It leads past an excellent roadside display of bloodroot (Sanguinaria canadensis) and Dutchman’s britches. There are trilliums there, too, but they look subtly different from the Shenk’s Ferry trillium. At Holtwood Dam they are more typically like Trillium flexipes.

Just beyond the dam a pawpaw (Asimina triloba) grove shelters a spread of toothwort (Cardamine concatenata) where later blue cohosh (Caulophyllum thalictroides) appears with a scattering of trillium and mayapple (Podophyllum peltatum). Dam building and the road gave openings for troublesome aliens like Tatarian honeysuckle (Lonicera tatarica) and a few Ailanthus altissima.

Our native pawpaw is the most useful and interesting tree at this pull-off. It belongs to the custard apple family (Annonaceae), and its fruit lives up to its family’s common name. The pulp is edible with the texture of custard, but its fruit is filled with large seeds. The glacial climate of the lower Susquehanna was similar to areas well north of pawpaw’s present range. As glaciers advanced, pawpaw had to move south, and the mammals of the late Pleistocene did the moving. As the climate warmed, the mammals returned and they inadvertently carried pawpaw seeds within them to repopulate the valley.

Upriver from the dam, rock weathering produces acidic soil. Rhododendrons spill down the slope of a tributary stream. Ferns and mosses fill open spaces where maianthemum runs much as it did when Native Americans were resident. 

If you continue on McCalls Ferry Road to its Rt. 74 intersection and turn north toward York you will intersect Rt. 425. A right turn will take you past a panorama of farms until Rt. 425 plunges down to the shores of Lake Clark, the Safe Harbor Dam impoundment. On the right, Indian Steps Road is marked by a totem pole. Down that road, the life of the native people of the Susquehanna Valley is remembered at Indian Steps, a unique museum of their culture. The museum holds an extensive collection of arrowheads and stone hammers all decoratively embedded in its walls. On the second floor, early photographs are displayed that show the development of valley industrialization. Across the road, a hiking trail runs through typical eastern hardwood forest to a tributary stream that jumps from a ledge in its eagerness to join the river.

Past Indian Steps Road, Rt. 425 enters Otter Creek Historic Area with preserved mills, a picnic area, a boat launch, and great views across the lake toward Shenk’s Ferry Glen. Past the creek, the road rises sharply, but there is a good trillium display on the left. The Urey Overlook parking is on the right at the crest of the hill. The overlook is well worth the half-mile (0.8 km) walk. The broad river spreads before you. Across the river Shenk’s Ferry Wildlife Preserve and other downriver glens are covered in a bank of green, evidence that nature and man’s activities continue the braid of history that began so long ago in the ancient Susquehanna Valley.