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Soil Formation: Glaciers, and the Weird Land we Farm

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This article comes from the NOFA/Massachusetts 2019 February Issue Newsletter

By Noah Courser-Kellerman

New England is a weird place to farm. We live in a mostly tree covered landscape of rolling hills, weathered mountains, deep ponds, swamps, rivers, outcroppings of bedrock, and once in a while, some really nice soil.  In many areas, soil can change from heavy, wet clay to rocky, sandy soil in a matter of a hundred feet or so. This patchwork effect is increased by the crosshatching of millions of miles of stone walls built by the first European farmers in this area as they tried to eke a living from the rocky ground. It has even been posited that the orneriness of New England’s soil is at least partly responsible for the same trait found in its farmers.

But where did this landscape come from? Why are we blessed with perched water tables, endless crops of “New England Potatoes”- field stones- and house sized boulders seemingly dropped from space in the middle of our woods and fields?

The answer is Glaciation. New England has been scoured by glacial ice several times over the past million years. 20,000 years ago, New England was covered by the Laurentide Ice sheet, an extension of the polar ice cap created by the last ice age.  It descended over the eastern half of Canada south to New York City on the coast and St. Louis inland. This enormous glacier was thousands of feet thick in places.

A glacier is like a river of ice. Snow lands on its “headwaters”, and the ice slowly moves downhill over the landscape. As thousands of feet of ice scrape over mountains and hills, stones, boulders and soil are picked up by the ice and transported, sometimes for very long distances. Rocks at the bottom of the glacier are ground to small pieces against each other. Grooves called striations can still be seen today running north to south on exposed pieces of bedrock- marks left as other stones were ground over them by the glacier. The mixed-sized jumble of rocks and sediments mixed into the ice is called Glacial Till.

slopeWhere the ice flowed over an outcrop of bedrock or a hill left by a previous glaciation, large amounts of glacial till dropped out of the ice and formed hills around them. These hills, called Drumlins are usually steeper on the northern end,  with a more gradual slope on the southern end. Choate Island in Essex is a great example of this landform. Drumlins can be good farmland, especially on their southern slopes. Like all till-based soil, however, it tends to be full of rocks, from golf ball to refrigerator size. The subsoil in drumlins is hard and compact- a result of the enormous pressure of the ice passing overhead.

The end of a glacier is a tug of war between the flow of ice and the rate of melting. If melting happens slower that the ice flows, a glacier flows onward. If melting is faster than flow, it retreats. If melt and flow are balanced, the end stays put. The analogy of glaciers as ice-rivers works, but at their ends, a better analogy might be glaciers as ice-conveyer belts, transporting everything a glacier picked up in its path and dropping it at its end. This pile of rubble is called an End Moraine. Long Island is the end moraine of the ice sheet at its southernmost limit. Around 18,000 years ago, the Laurentide Ice Sheet began to retreat north. It left behind several more end moraines, including Cape Cod, Stellwagen Bank and the end of Plum Island are all end moraines, produced when the retreat of the glaciers slowed or stopped for a period of time. Moraines are made of till, but tend to be less compacted than drumlins.

As the glacier retreated, Glacial outwash formed where the water leaving the glacier’s end deposited sediments as it drained away. Unlike till based soils, soils from outwash tend not to be rocky at all, but consist of even sized sands and silts. Kettle Ponds form where icebergs fall off of he end of a glacier and sink into recently deposited outwash sediment. As they melt, they leave huge holes in the ground that become ponds. Eskers are serpentine hills or ridges, sometimes miles long, and are the remnants of rivers flowing through cracks in the ice.  Sediment and stones freed by melting ice were carried and deposited along the course of these under-ice rivers. When the ice melted away, piles of stone and sediment were left along their path. Weymouth Massachusetts is home to North America’s tallest esker, at over 90 ft. The boulders strewn inconveniently over our fields and pastures are known as Eratics and are dropped out of the glacier during periods of rapid melting.

              

The retreat of the Laurentide Ice Sheet left many recognizable features visible in our landscape today, but the origin story of many New England soils isn’t yet complete.          

Sheep can thrive on sloping terrain, Sea level has changed significantly over past 11,000 yearsThe immense weight of thousands of feet of ice over the course of thousands of years had pushed the tectonic plate downward. Even though global sea level was much lower than it is today because so much water remained locked in ice, the sea rushed in as the glacier retreated. Relative to the land, sea level in New England around 15,000 years ago was close to 200 feet higher than it is today!

The combination of a shallow sea over much of coastal New England and the massive influx of water and sediment from the melting glaciers deposited deep layers of silt and clay in many areas. My farm in Essex is an example of this. Basically, it is an ancient mudflat. All of the flat areas of the farm are blanketed in a heavy, poorly drained silt loam, with blue clay subsoil that I used to make pottery with as a child. Drumlins on the edges of my farm are a more typically New England glacial till soil of fine sandy loam with a good helping of rocks.

As the tectonic plate rebounded, sea level rapidly fell relative to the land, reaching its lowest level around 11,000 years ago. At this time, sea level was around 150 feet below its current level! George’s Bank was a huge island. Fishermen today still occasionally catch mastodon teeth and the flint spear points of the people who hunted them hundreds of miles from what is now the shoreline. The sea level has risen slowly for the past 10,000 years as more ice has melted, and is accelerating with the effects of climate change.

New England’s soils are brand new, geologically speaking. Soil formation is a continual process, as the rock particles deposited by the glacier weather and release the nutrients they contain, are broken into smaller pieces, or re-form into clay and other minerals. Granite is the most abundant rock in our region. Soils derived from it tend to be relatively poor in nutrients except for potassium, which is part of the reason most of our soils need to be limed periodically.

As you pick rocks next spring, take a moment to wonder where they came from- what outcropping hundreds of miles away was ground down and became part of the land you grow food on? New England is a weird place to farm, yes, but it is certainly interesting. The impossibility of huge fields devoted to monoculture is, I think, a blessing in disguise.  A drumlin’s south slope covered in a blanket of well drained fine sandy loam might be flat enough for a vegetable field.  The more steeply sloped upper part might be a good place for an orchard with good air drainage. A herd of sheep could be well fed on the part of the drumlin dotted with erratic boulders the size of cars. The list goes on, and possibilities abound for our patchwork landscape to become a patchwork of diversified organic farms. Out of constraints and limitations come innovation, diversity and creativity. 

If you enjoyed this article be sure to sign up for the full e-mini series here where you’ll be the first to receive the next individual installation of this monthly “Soil Science- From the Textbook to the Practical” through the end of 2019.  Or go back and read the first article in this series here.

You can also learn more from Noah Courser-Kellerman at his upcoming NOFA/Mass educational event in April titled “Soil Science for Gardeners”.  Watch the NOFA/Mass upcoming events page for details coming soon.

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