In addition to the usual emphasis on ecology, this January’s Forest Exploration introduced an additional focus: geology. Alongside Claire O’Neill, EwA naturalist Tom Eid shared his knowledge of geology and natural history. As some of you may know, the word “fell” itself is a reference to the reservation’s geology; a fell is a high, rocky, and barren landscape. During this month’s walk, we examined dikes in rocks and revisited a topic some of us might remember from grade school; igneous, metamorphic, and sedimentary rocks as well as their relative placements at the Fells. We also explored some of the lingering visible flora and fauna including conifers, hemlock and pine adelgids, and the unique plant communities of rocky outcroppings (just one example of the interplay of geology and ecology!)
“Seeing the Fells through a naturalistic lens instead of a purely recreational one fundamentally changed my perception of the space. I began to pay attention to places differently, noting not only individual species but also the relationships between these species and their environments and interspecies interactions. Observing and recording these kinds of details over time can show us the effects of climate change on biodiversity and inform how we protect that biodiversity.”
Mina’s interest in natural history took root during a New England field studies course at Lesley. This course involved learning about ecology and plant identification at local parks and urban wilds. One of these sites was the Middlesex Fells...
Once a month, EwA leads a biodiversity walk in the Fells. It’s a free-form activity of ours which only purpose is to explore what Nature wonders are in season: what is flowering, fruiting, passing by, living here?
We invite our guests to write about the walk. Here are highlights of what Mina and we enjoyed this time!
EwA’s Forest Explorations consistently have great attendance, but I’ll admit I partially expected the bitter chill to be a deterrent for some. I couldn’t have been more wrong! In defiance of the past Saturday’s biting cold weather, all attendees showed up bundled in heavy winter boots and gloves. I’m always impressed by the enthusiasm and dedication of our group and Saturday’s full turnout further reminded me how much we all value the chance to be together in nature.
Thinking in the Long Term
As Tom pointed out early on in our walk, geology involves thinking on very different timescales from our usually ecological mindsets. I often find myself thinking of ecology in terms of the seasons, but the interplay of geology and ecology doesn’t follow the same seasonal or annual rhythms as the flora and fauna we often investigate at EwA. To offer a point of reference: many rocks at the Fells are easily over 500 million years old!
The first geological phenomenon we discussed was fault lines. I had often wondered myself about the subtle valleys and linear indentations at the Fells. These small valleys are created by tectonic plate activity and are the result of faults (fractures or zones of fractures between two blocks of rock). In New England, these faults are the result of old tectonic activity and have been filled in, making them less obvious than the crevices and depressions one might find out West.
Needle Ice: The Grace Notes of Nature
Needle Ice © Claire O’Neill
Though the intended focus of the walk was geological formations, another type of formation quickly caught the eye of many of our attendees. Inch high columns of needle ice sprawled out like miniature cityscapes at our feet. These structures, called needle ice, have been a recent topic of interest at EwA’s virtual nature hour. Needle ice presents as vertical conglomerations of needle-like ice splinters and is the result of groundwater rising to the surface via capillary action and then freezing. We unknowingly picked a very fortuitous day for the formations as these “grace notes of nature,” as Tom refers to them, only form at the right conditions and often melt quickly. In the same way that grace notes ornament music, needle ice structures truly seem like a decorative flourish on an otherwise rather bare forest floor.
A Rock Within a Rock
Our second geological stop was a dike embedded in hornfels rock directly in the center of our trail. Dikes are the result of magma filling a fracture in a rock. In this case, the embedded rock was igneous in nature while hornfels are metamorphic. At Claire’s suggestion, several of us knelt down to touch the rock. The rocks even felt different; the hornfel was slightly more porous and grainy than its smooth, inset companion.
Dike at the center: the rock was smooth to the touch | © Mina Burton
A shield lichen (maybe a bottlebrush – Parmelia squarrosa) clinging to the north-facing side of a pine | © Claire O’Neill
After pausing to examine the dike, our group took a short break from geology to discuss pines. Pines benefit greatly from their highly adapted leaf shape; needles offer less resistance to potentially damaging snow and are more resistant to water loss. The Pinaceae family includes such genera as pines, spruces, firs, hemlocks, larches, and true cedars. The larch stands out from its pine siblings in that it is the only conifer that loses all its needles and is therefore not a true evergreen.
A Less Unwelcome Adelgid
Many of the pines at the Fells are white pines (Pinus strobus). We noticed pine bark adelgids (Pineus strobi) on one of these pines on our walk. Although the word “adelgid” might spark fear in tree-lovers (especially those familiar with the invasive Hemlock Woolly Adelgid), this specific adelgid is actually native to the area. What you’re seeing in the picture below is immature females overwintering on the white pine’s bark. This native sap feeder does not cause damage to healthy established trees though it can occasionally heavily infest small or highly stressed trees.
A Change of Scene
As we gained elevation, the flora around us shifted. White pines gave way to shorter pitch pines (Pinus rigida) and dense clumps of common juniper (Juniperus communis) cropped up underfoot. The open, rocky landscape of this new type of special community, known as a rocky outcropping, supports different species than those seen in the lower forests at the Fells.
The rocks in this special community differ from those in the lower Fells as well. This area marks the highest point on Whip Hill at 256 feet and features quartz sandstone and mudstone. Both are sedimentary rocks unlike the volcanic and igneous rocks of Wamoset Hill or the metamorphic rocks of Saddleback Hill. The rocks appear almost rusty in patches and contain red, brown, and even almost purplish hues.
Tom referred to one particular expanse of rock in this area as one of the strangest rock formations in the Fells. This rock is unusually composed of a rhyolite dike, quartz, feldspar, and very small fluorite crystals. Its unique composition makes the rock a “geologist’s dream come true!”
As we meandered down from Whip Hill’s summit, we encountered three forest types in the following order; hardwood, pine, and lastly hemlock. This change took place over the course of an impressively short elevation drop of only 90 feet, making me aware of just how much geological details like rock composition as well as elevation can affect plant communities.
The Real Adelgid Threat
Once back down to 150 feet in elevation, our group passed through an Eastern hemlock (Tsuga canadensis) grove. These hemlocks are under attack in the Northeast from the Hemlock woolly adelgid (Adelges tsugae). Unlike the pine bark adelgid, this adelgid is non-native. In East Asia, the adelgid is not considered a serious pest because it has natural predators and parasitoids and because host plants have developed a resistance to the insect.
The woolly adelgid feeds off the tissues of hemlock trees and population build-up over time can lead to the decline and death of entire groves of hemlock trees. The adelgid cannot fly but is spread by wind and via animal movement. Luckily, the grove we were in appeared to be in relatively good shape and hopefully will stay as such.
Thanks to all who braved the cold to join us this month and to Tom for leading the geology portion of the walk! I’m consistently impressed by how much we can learn about nature even when much of its flora and fauna is dormant or overwintering out of sight. You can check out the rest of our records from Saturday here!
February is the coldest month, yet the forest has a lot to offer to the avert eye. 📅 Join us next month to explore the natural joys of the Fells.
Jan, 17th 2020 | by Mina Burton
Editing & layout by Claire O’Neill. The photos of species in this article are for the most the visual observations that the EwA team recorded when scouting for and during the January Forest Explorations event (© Mina Burton, © Claire O’Neill, and © Thomas Eid). Click any species photo and see the corresponding record in EwA’s iNaturalist Fells project (and its author). Photos are licensed under Attribution-NonCommercial (CC BY-NC).
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