Forensics in Charlestown Forest

Clues left behind by glaciers

By Mike Freeman / ecoRI News contributor

Pitch pines in the Francis Carter Preserve in Charlestown, R.I.
 (Tom Mooney/TNC)

Surrounded by pitch pines, The Nature Conservancy’s Tim Mooney looked across the Pawcatuck River’s south bank in the Francis Carter Preserve. Pointing at the forest 10 feet closer to sea level than the pines, he noted the different tree composition across the river.

“That’s a floodplain forest,” he said. “Pin oaks mixed with red maple. When the glacier retreated, a meltwater river likely piled sand here before the moraine, which these pines tolerate, while the lower plain became swampy and friendlier to the forest community across from us.”

Later, Mooney pointed to shade-tolerant hemlocks growing along one of the Charlestown Moraine’s north-facing nooks. Along a 2-mile line he showed a series of vernal pools strung along the same side. Atop the moraine he noted ghostly oak trunks lingering from spongy moth outbreaks. The moraine, he said, provides much of southern Rhode Island’s meek incline along with the well-drained soils favorable to its prominent oak forests.

Though the Wisconsin ice sheet departed around 15,000 years ago, vestiges are especially visible in southern Rhode Island, not far from where the ice pushed up Long Island, Block Island, Martha’s Vineyard, and Nantucket before retreating. When the glaciers left, returning plant communities sorted themselves according to the soils and landscape features they found.

Today’s forests are young twice over. While most species can live for centuries, Europeans cleared woodlands prodigiously. Rhode Island forest cover went from 95% at first contact to 24% by the mid-1800s. As farms moved west, trees reclaimed pastures, and the comparatively young woods we see now are the remedial product.

In its geological way, the ecosystem itself is younger still. The entire biosphere, including us, is only here because of vanished ice.

“There have been many ice ages,” said Thomas Boving, a University of Rhode Island geologist. “We’re living now in an interglacial period.”

Forests do what they can in these periods before glaciers gather their breath to push south again.

“Whatever forest was here disappeared,” Boving said. “Glaciers act like bulldozers, obliterating everything before them and plowing it forward.”

The Northern Hemisphere, then, is a glacial Etch-a-Sketch, with the ice leaving outlines for plants and animals to recolonize after wiping it clean. Southern Rhode Island has many public places to see how life has adapted to this latest makeover.

“Species thrive as much in conditions they can tolerate as those they prefer,” said Fern Graves, a state Department of Environmental Management forester. “Rhode Island is a mix of glacial soils, with respective limitations. Some are well-drained tills, such as on a moraine, and some are wet, where the ice ground everything off a granite ledge, leaving poor drainage. Different plants tolerate different conditions better.”

Charlestown’s Buckeye Brook Road is a great place to see how such conditions warp and woof through Washington County, providing tree neighborhoods as distinct as Providence’s old ethnic enclaves. Staying on the road, you walk west atop the well-drained moraine. Oak-hickory forests handle this dry, rocky terrain well and dominate. Mountain laurel thrives here, too, not minding the droughty, acidic soil.

As you descend, you come to where the ice scraped the water table, forming Poquiant Brook. Atlantic white cedars and red maples take this wetter biome over, along with hydrophile shrubs like sweet pepperbush. If you walk the Vin Gormley trail you’ll find a cedar swamp.

“Cedars and rhododendron love poorly drained mineral soils,” Graves said.

Further east, just off Route 138, the Great Swamp has subtle patchworks of these glacial-legacy soil differences, with dry oak stands and red maple swamps making a quilt of the place, and Audubon’s Fisherville Brook Preserve showcases several forest glacial adaptations. Kames are soft mounds like giant ski moguls left where ice blocks calved off. Eskers are snaky ridges formed by sub-glacial river deposits. Both comprise Fisherville’s lumpy landscape. Most of New England’s vernal pools, too, formed where a discarded ice block depressed the ground enough to hold water seasonally, providing amphibians a fishless nursery for juveniles.

On Fisherville’s yellow trail, you can stand next to a vernal pool with red maples all around, while bitternut and mockernut hickories favor a small, drier esker as white pines and various oaks dominate the well-drained kames.

As young as they may be from both axes and ice, then, any woodland still feels primeval, as fine a place as any to scrub your mind a bit in deep time. Ice comes and goes in New England like tides. Walking our woods, picking out features left by glacial wastage 15,000 years ago, makes you feel both eternal and hyper-ephemeral, an oddly quelling juxtaposition in the fret of the current moment. We’re lucky to have such places, and studying how life adapts to periodic obliteration is an instructional way to gather our own breath.