It’s an unseasonably warm winter afternoon, and Julie Brigham-Grette is taking me on a geological journey through the past 20,000 years.

Right now, we’re turning down a squelchy, pitted dirt road in the back fields of Sunderland. Minutes ago, Brigham-Grette told me that a local Hadley farmer banned her from his fields — she’d driven back there for a geology student field trip and buried her wheels in the mud. The farmer had to haul her out.

“I sent him a thank you card, I even sent a UMass hat,” she said. “But he still won’t let me come back.”

The back of her hybrid hatchback is stuffed with large maps and diagrams that block the rear view. Up front, it’s just the two of us and our muddy hiking boots. We pass a small road sign.

“Not maintained in winter,” Brigham-Grette reads aloud. “Well,” she adds, “we’re not going to get stuck.”

Very reassuring.

Brigham-Grette, a rail-thin geologist with straight blond hair and a flashbulb smile, first arrived as a professor at UMass 30 years ago. She specializes in paleoclimate, or ancient shifts in climate, and what those changes tell us about our climate today. Her work has taken her from the Arctic Circle in Alaska and Siberia to right here in our own backyard. In the process, she’s racked up an impressive body of knowledge about the geologic past of the ground below us.

Today she’s showing me how that history meters everything about life in the Valley, from the agriculture to the economy to the drinking water.

Mercifully, we — and the car — survive our mud road traverse unscathed. Arriving on a forested ridge, we pull over and jump out by a barbed-wire fence that overlooks a deep rent in the landscape: an open-pit mine for sand and gravel.

I stand on the gravelly road verge and look down. On the other side, angled beds of sand and gravel tower some 50 feet over the flat, gouged out bottom of the pit. The beds are tilted about 30 degrees, with flat layers of the same material over the top of them.

These beds, Brigham-Grette tells me, mark the shoreline of ancient Lake Hitchcock, a massive body of water that filled the Pioneer Valley in Massachusetts at the end of the last ice age, from about 16,000 to 12,000 years ago. The lake’s influence, and that of the ice that formed it, are still key to understanding our landscape today and how we use its resources.

“It’s a pretty young landscape, geologically,” she tells me.

When the glaciers arrived in our region, they dragged along piles of gravel, sand and rock entrained in the ice. Once they began to melt, those sediments got dumped at the southernmost edge of the glacier, becoming a dam in parts of the Connecticut Valley.

Behind the dam, trapped meltwater created a lake. About 200 miles long and up to 150 feet deep at its greatest extent, the lake survived for 4,000 years, although not all at the same time. Basins of the lake drained from south to north.

“You could ice skate across Lake Hitchcock from Amherst to Northampton in winter time,” Brigham-Grette says.

The tilted beds of gravel we’re now looking at, like other deposits in the valley, are an ancient delta, a wedge of sediment discharged into Lake Hitchcock by rivers of entering meltwater. When the lake drained, the empty landscape of flat deltas and lakebed muds was the precursor to our Connecticut River. Most of us live on the floor of what was Lake Hitchcock.

“Today, the Bradley airport is on top of the Farmington delta, and the Chicopee Airforce Base is on top of the Chicopee delta,” says Brigham-Grette. “Nice and flat.”

The gravel now creates a lucrative industry for crushed road bed material, cement, and other construction. This particular gravel pit is now exhausted, but others are still active, like a nearby Sunderland pit owned by Delta Sand and Gravel. In 2017, the non-metallic mineral mining industry represented $1.8 million in wages for Hampshire County workers, according to the Massachusetts Executive Office of Labor and Workforce Development.

Over the course of the Pioneer Valley’s human history, the lake also created other industries. Far from the disturbance of entering rivers, in the lake’s center, layers of clay quietly accumulated as fine muds drifted out to the lake bottom.

The gray clays were mined over 100 years ago to fire into red bricks that make up many of the older mill buildings of Amherst and Northampton.

The lake’s geologic leavings are important for more than just gravel and sand mining, though, Brigham-Grette tells me. The huge package of sand and gravel filters rainwater that seeps through it, helping purify municipal water supplies, like Sunderland’s drinking water.

We hop back in the car to drive down to see what else relies on that crystal clear water. At the base of the hill marking the end of the Sunderland Delta, we park at a small lot and hike into the trees. Brigham-Grette points out little “mud volcanoes” of sandy water spurting out into a pool where the hill starts. The water that infiltrated the delta sands and gravels emerges here as artesian springs clean enough to drink from.

Clean enough, too, for the Sunderland Hatchery operated by MassWildlife. In a series of man-made reservoirs delved into the soil just downhill of where we stand, protected by towering pine trees, 600,000 brown, brook, rainbow and tiger trout drift and sway, many as long as my arm.

These trout will be set free in the Connecticut River to help support a recreational fishing industry that contributes $200 million to New England’s economy a year, according to a 2015 study in the journal Ethics, Policy & Environment.

Ancient Lake Hitchcock reasserts its importance even once the fish are in the river. The layers of clay laid down 14,000 years ago at the lake bottom now make up the banks and bottom of the Connecticut River itself.

These layers are riddled by millimeter-diameter holes dug by bloodworms, the larvae of an insect called chironomid midges which make their homes in the safety of the clays. In spring when faster water rushes through the river, parts of the clay wash away, releasing bloodworms into the water as an important food source for fish. In 2004, biologist Ed Klekowski and his team, colleagues of Brigham-Grette at UMass, determined that one type of bloodworm in the river was a unique species never before described. The layers of Lake Hitchcock in the Connecticut River may contain as many as 3,000 bloodworms per square meter.

In spring, the worms pupate into adult midges for a mating flight. These, too, are captured as food by trout, catfish, and sturgeon, as well as by birds like swallows and flycatchers.

Whenever the Connecticut River has flooded, washed-out ancient lake muds have been poured across the landscape, enriching the region’s agricultural soils with nutrients first dragged here by glaciers tens of thousands of years ago.

The layers of clay hold secrets not just for biologists, but also climate scientists like Brigham-Grette.

Because Lake Hitchcock was seasonally covered by lake ice, finer particulates like mud and fine sand washed into the lake in winter. In the lake cores Brigham-Grette and her colleagues have extracted, summer mud layers are therefore thicker and sandier, while winter layers look thin, dark, and clay-rich.

Scientists can count back the years in these annual layers to learn how long the lake lasted. Moreover, they can extract secrets about the entire Earth of the past from the seasonal layers, which are known as varves.

“These thick-thin, thick-thin layers are like a barcode,” Brigham-Grette says. “That bar code was influenced by past climate change.”

One of Brigham-Grette’s graduate students, Tammy Rittenour, noticed the varves showed a regular cycle of varying thickness. In a project with Brigham-Grette and Penn State climatologist Michael Mann, Rittenour was able to show that as early as 16,000 years ago, the atmospheric phenomenon known as El Niño (or at least El Niño-like) was already affecting the world’s climate. This cyclic shift in the Pacific’s ocean and air currents every 5 to 7 years creates weather changes worldwide, including wetter years in New England and droughts in parts of Africa.

The findings, published in the prestigious journal Science in 2000, helped propel Rittenour to her job today as a professor and paleoclimatologist at Utah State University.

Brigham-Grette says the Valley’s glacial history may hold still more secrets. Glaciers are so heavy that they push the landscape downward. When they melt, the land gradually rebounds, a process that eventually caused Lake Hitchcock to break through its dam and drain in several stages.

Brigham-Grette is now studying human-induced climate warming of the Arctic and how this warming is impacting the retreat of glacial systems in Svalbard and Greenland. Learning more about rates of rebound in the Valley and rates of glacial retreat could help us understand how fast places like Greenland and Antarctica will melt with global warming, which will affect rates of sea level rise.

We end our journey on top of a ridge by the Connecticut River between Amherst and Sunderland. Standing 45 feet high and crested by trees, the ridge seems to come out of nowhere.

With a hand shovel, Brigham-Grette digs down to display sand uniform enough for a child’s sandbox. She explains this land form as an echo dune. Throughout the Connecticut Valley, ridges like this one, often underlying patches of trees in the middle of farm fields, betray where sand dunes were created after Lake Hitchcock drained, leaving sandy, infertile soil that can’t be farmed.

When Lake Hitchcock drained, the cold, dry, windy weather blew the sands of the lake bottom into the dunes, she explains.

“It was wicked cold, like central Alaska,” says Brigham-Grette, her adjective betraying her New England roots. “There was tundra everywhere. Today people drive by these clumps of trees in the Valley and don’t realize the trees are standing on ancient sand dunes.”

Evidence has shown that as far back as 11,500 years ago, Native Americans likely camped on top of the dunes in Sunderland and elsewhere, perhaps because they were well-drained and dry.

Native Americans also likely knew Lake Hitchcock far more personally than we ever can. The native legend of Ktsi Amiskw, a giant beaver, seems to tell how the lake drained. Greedy Ktsi Amiskw hoarded all the area’s waters to himself in a great pond, angering the deity Hobmock. The deity chased the beaver, breaking his back and turning him into Mount Sugarloaf, and releasing the waters of the pond.

Those waters, the legend tells, flowed to the people once more — as they still flow to us today, a melding of history and humanity atop the particles they leave behind.

“It’s these sediments,” Brigham-Grette concludes, “that give us the heritage we have.”

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