Tale of Two Bedrocks

R.Cheek

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Visit Bartholomew’s Cobble and Rocky Woods Reservation.

Tom Wessels is an ecologist and founding director of the Conservation Biology Program at Antoch Univeristy New England. He is the author of numerous books, including a forthcoming field guide to reading the forested landscape to be published by Countryman Press.

This article originally appeared in the Fall 2009 issue of Special Places, The Trustees' member magazine. To subscribe, join The Trustees today.

 

Some of my earliest memories of being out in the woods were with my dad, who loved to take the family for short hikes at Bartholomew’s Cobble, just over the border from our cottage in Salisbury, CT. I remember well the Cobble’s impressive marble outcroppings right next to the Housatonic River, its stately trees, and its rich flora.

by Tom Wessels

Trillium and Dutchman’s breeches were two of the first wildflowers I learned to identify on these outings. Only later did I learn that the Cobble, due to the moist, nutrient-rich soil derived from its marble bedrock, held more species of vascular plants than just about anywhere in the entire Northeast.

Of all the bedrock types in New England, there are two that generate soils at the extremes in terms of nutrients – marble and granite. Western Massachusetts and Vermont are amply underlain by marble derived from the building of the Taconic Mountain Range more than 450 million years ago – the Berkshires and the Green Mountains being the ancient relics of that once imposing range. Eastern Massachusetts and New Hampshire, in contrast, host a lot of granite, remnants of the Acadian Mountains formed almost 100 million years after the Taconics.

Within ten miles of my current home in southeastern Vermont, I can see both types of bedrock in action at a granite bald called Black Mountain, and a nearby, marble-infused talus slope (a jumble of boulders that cleaved off and created a rocky apron at the base of a cliff). The plant communities at each site not only look different; they strongly differ in their feel and smells.

My favorite time to visit the talus slope is in late April, when its emerald carpet of wild leeks, Dutchman’s breeches, trout lily, and trillium creates a verdant display weeks before the canopy of bitternut hickory, white ash, and sugar maple leafs out. A month later when the canopy is full, only the trillium remains. The others, known as vernal ephemerals, have died back and been replaced by later developing species like blue cohosh, maidenhair fern, and baneberry.

Of all the plant communities I visit in my regional wanderings, none matches the slope’s biodiversity. The marble is rich in calcium, which allows the soil to hold other important nutrients such as magnesium, potassium, and phosphorus. Together they create an incredibly nutrient-rich soil. Not only that, but the soil that forms from the weathering talus is very fine-grained and holds moisture like a sponge, while the talus allows for good drainage. The talus also provides structure that creates a huge array of microenvironments, allowing specialized plants to coexist on different portions of the boulders that jut from the slope.

Just eight miles to the southwest stands Black Mountain. Late June is my favorite time to visit this dramatic bald, when the mountain laurel that graces its flanks is in full bloom. At dusk during a good year, the copious blossoms make it seem as if it has snowed. But as one ascends toward the summit of Black Mountain, the laurel fails as the environment grows ever more dry and acidic. Unlike marble, granite weathers very slowly, creating coarse, thin, extremely dry soils with a consistency like Grape-Nuts® cereal. Granite also releases hydrogen ions as it weathers, creating very acidic, nutrient-deprived soil. The summit of Black Mountain is the hottest and driest site in the entire state of Vermont, home to a species-poor community dominated by just three well-spaced, woody plants: pitch pine, bear oak, and low-bush blueberry.

These three plants are familiar to those who roam wherever the granitic roots of the Acadian Mountains lie exposed, including eastern Massachusetts. At Rocky Woods Reservation in Medfield, pitch pine and bear oak perched on granite represent one of the most fire-adapted plant communities in New England. Along with the low-bush blueberry, they thrive in dry, acidic sites where frequent fires remove competitors. The bear oak and blueberry have underground roots and stems that can withstand temperatures as high as 1,000 degrees for up to a minute. They will burn to the ground in a fire, but in the following season will vigorously sprout back and increase their reach. The pitch pine has very thick bark to protect its cambial tissues. If a severe fire does kill its trunk, unlike other regional conifers it can stump sprout and grow back.

We tend to think of New England as a single region sharing a similar climate. Yet where marble is exposed we find verdant, moist forests rich in wildflowers; the parched, stunted forests on granite, meanwhile, look far more like the desert woodlands of southern Utah. This striking difference is simply due to the nature of these two bedrocks, which were formed long before any forests graced this planet and exert powerful forces over the plant communities they support.