When Molly Ockett Middle School’s Maine Environmental Science Academy (MESA) teachers asked us to offer a field trip focused on erosion at Sabattus Mountain, Rhyan Paquereau and Leigh Hayes jumped at the opportunity.
Sabattus Mountain is a state-owned property that Greater Lovell Land Trust helps to maintain.
Due to the pandemic, the multidisciplinary MESA class of 40 sixth-eighth graders is split into two groups this year for field trips. We offered the trips on different days in November, the first being sunny with no breeze and temps in the 60˚s, while the second dawned frigid and we had to do jumping jacks and run in place between parking lot demonstrations before we began hiking. Some of the students have chosen to learn remotely, thus this blog will be long as we try to explain the theme of the day.
The program began with a splashdown target, some dirt, a plug of grass, water, and a syringe.
After defining erosion as a gnawing away, one volunteer placed some soil in the center of a laminated target, filled the syringe with water, and then released the water over the dirt one drop at a time.
At first, the volunteer released the drops from about an inch above the target. Then two inches; four inches; and eventually about ten, increasing the flow each time. Other students looked on (from a distance) and noted that as the dirt became saturated and syringe was raised higher, more drops splattered outside the bull’s eye. When the dirt was replaced with a plug of grass, things changed a little bit as the roots helped keep the dirt in place by absorbing the force of the falling water. .
Drops that did splash away were brown where soil was carried away by the water, just as happens when rain beats down onto the ground and causes erosion.
Land Steward Rhyan, on the far right (with Miss Hamlin, our former intern who now works with MESA students during the day and attends college remotely at night), next explained, “Weathering is the wearing down of any material through exposure to the weather and climate within the Earth’s atmosphere. There are two basic types of weathering, biotic and abiotic. Biotic weathering occurs from living things, such as the roots of a tree getting into cracks in a rock and splitting it apart. Abiotic weathering is weathering through non-living factors such as water and wind.”
As the students took notes, Rhyan said, “If we’re thinking about erosion, there are a few observations we can make while standing at the top of the Sabattus Mountain parking lot that will let us decipher how water is impacting it. From the top of the parking lot, you’ll see a rectangular gravel lot with a gravel driveway on the right hand side that leads down to the road, and if you turn around, you’ll see the start of the trail that takes you up the mountain.
Right now nothing appears to be wrong with the parking lot, but not long ago the driveway was washed out, rutted, and difficult to drive up. Today the driveway is nice and smooth, so how were we able to fix it, and what caused the damage in the first place?
From today’s lesson topic we already know that erosion caused the damage, but how did it do it? Those few observations I mentioned hold the answer. First, standing at the top of the lot looking toward the driveway you’ll notice the parking lot itself is not level, it has a gentle grade that starts where you’re standing (the north end) and runs south toward the driveway.
You’ll also notice that the whole parking has a bowl-like, or concave, shaped to it. This means that the edges of the lot are higher than the center. The gentle grade of the lot tells us that rainwater landing on the lot is going to flow in one direction, in this case from the top of the lot down to the driveway and into the road, as opposed to pooling or flowing in multiple directions like it might on a level surface.
When rainwater lands on a large, gentle grade like this parking lot it does what is called sheeting. Sheeting is when water flows uniformly, and relatively slowly, over a wide area. Because of this, the erosion caused by sheeting water tends to be slow and happens relatively evenly.
Now that we understand that, we know it’s unlikely that sheeting would have created the deep ruts in the driveway, and more likely that it would have worn the driveway down evenly and slowly over time. The fact that the lot has a concave shape to it, in addition to the gentle grade, changes things.
We know that the gentle grade of the lot means that water is sheeting (flowing evenly, and relatively slowly) down the lot, but the concave (bowl shape) of the lot means that instead of the water continuing to flow evenly and slowly down the entire length, it instead sheets only briefly before the concave shape concentrates it in the middle of the lot.
When the water becomes concentrated it is no longer sheeting, it is channelizing. Channelizing is the opposite of sheeting. Instead of water flowing evenly and slowly over a large area, it becomes concentrated in a small area and flows rapidly. This means that channelizing water can erode a small area very quickly.
Now that we understand how sheeting and channelizing erosion work and evidence they leave behind, we know that channelizing water is what created the ditches in the driveway; and we know that the channelization is happening because of a combination of a gentle grade and a concave-shaped lot.”
He continued, “So how did we fix it? Erosion, like many natural forces, is very difficult to totally stop; but we can control it to a certain extent. One option is to stop the water from channelizing, since sheeting water erodes relatively slowly and over an even area.
We could have done this by bringing in lots of new gravel and removing the concave shape of the lot, but this was an expensive option so we decided to take a different route. Instead of trying to eliminate the channelization we decided to manipulate it by strategically building channels of our own, and you can see them if you look around.
Standing in the same spot that we started in, you’ll notice that a wide, deep trench called a swale rings the perimeter of the parking lot. This creates an existing channel for the water sheeting off the mountainside to follow and run off into the woods, instead of running on the lot.
Inevitably rainwater will land on the lot no matter how nice of a perimeter swale we have, so you’ll notice another smaller swale built just above the driveway that sends any water that is channelized by the concave shape of the lot off into the woods before it can channelize on the driveway.
The last repair we made was building a small structure called a waterbar at the start of the trail up the mountain. A waterbar is a small channel reinforced with stone that gets water off a steep section of trail before it can channelize.”
By looking at a topographic map of Lovell and locating Sabattus Mountain, we showed students the shape of the mountain so they could make inferences about how it was shaped over time.
Topographic maps allow us to do this because they have what are called contour lines. Contour lines are the squiggly black lines that run parallel to each other all over the map that show us the hills, valleys, and mountains (also called “relief”) of the landscape. Each contour line represents a set amount of change in elevation above sea level that is most often measured in feet. What this means is that on a map with 10-foot contour lines, each line represents 10 feet of vertical elevation gain if you’re going uphill and 10 feet of elevation loss if you’re going downhill. The smaller the number, the more contour lines you will see on the map and a more accurate picture you will have of the hills and valleys of a landscape. Even though the contour lines represent a set amount of gain for each line, you’ll notice that some are much closer together than others, and understanding what this means is how we read the map. Contour lines that are close together indicate a steep grade, while contour lines that are spaced far apart indicate a gentle grade.
When we look at the contour lines on Sabattus Mountain, we notice that from above it is almost teardrop shaped. The contour lines on one side of the mountain are relatively far apart and indicate a gentle grade, while the contour lines on the opposite side are incredibly close together, indicating a very steep grade that eventually turns into a near vertical cliff. Now if you look at some of the other mountains and hills on the map you’ll notice that almost all of them share this shape to a certain degree.
Why is that? The answer is simple: glaciation. Approximately 13,000 years ago when the Laurentide Ice Sheet was migrating across the Northeastern US it covered all of Maine in a blanket of ice that was a mile thick in many places. This meant that most mountaintops you see today would have been completely covered by a thick sheet of ice. And since we know glaciers move, and we know that mountains effectively do not, what results when a glacier meets a mountain is a case of an unstoppable force meeting an immovable object. The migrating ice can’t push the mountain out of the way, so it has no choice but to go over it.
As the glacial ice moves over the mountain a great increase in pressure occurs, and with this increase in pressure comes a rise in temperature right where the mountain and the bottom layer of ice meet. This temperature increase melts the bottom layer of ice, and the resulting water is pushed over the mountain under the glacier by the force of the glacier’s movement. Once the water reaches the opposite side of the mountain it seeps into existing cracks in the bedrock that the mountain is made of, and because the pressure on the opposite side of the mountain is lower, the water refreezes in the bedrock cracks.
When water freezes it expands, and this expansion breaks off some chunks of bedrock that are then embedded in the migrating glacier and can be carried many miles away. The action of refreezing meltwater breaking off chunks of rock and those rocks then being carried away is called plucking.
Over time this plucking action is what creates the contours we see today, and looking at it on a map can tell us which direction the glacier was migrating. The glacier approached from the “smooth” side that has gentler contours. As it moved up this smooth side and over the summit of the mountain, the melted water seeped into cracks on the other side, plucking rocks that were then carried away, leaving a “rough” face on the opposite side of the mountain.
In geology, a mountain with this shape that was formed by a migrating glacier plucking rocks from one face is called a Roche Moutonnee (pronounced “roosh-mont-on-ay”), a term coined by a French mountaineer. Roche moutonnees can be formed from bedrock outcroppings of all sizes, so keep an eye out for them the next time you’re out in the woods or looking at a topographic map. Most mountains in the Northeast are Roche moutonnees, but some are not and take on different shapes.
At last we began hiking and from the get go, there were signs of erosion all along the path.
The students became erosion sleuths and at one point we had them line up on either side of the trail where channelization was especially evident.
Standing beside the water channel, they each contributed a sentence describing how they imagined rain to flow downhill. There were rocks and roots to move around or bounce off of, leaves to gather . . .
and finally a waterbar, that if it works correctly, directs the flow off to the side where the water can spread out on the adjacent forest floor rather than cause further erosion along the trail.
From there, we continued uphill until we reached this spot. It’s a transition spot, where the natural communities switch from mostly deciduous trees below to conifers above.
A couple of older students in each group impressed us with their tree knowledge. Not only could they name the two most prominent species by common name: American Beech and Northern Red Oak, but they could also offer the scientific name: Fagus grandifolia and Quercus rubra respectively.
By the sight of the tree stumps and their knowledge of the Maine woods, they knew that the mountain had been logged years ago. Logging removes trees that protect the ground. from soil erosion. The tree roots hold the soil together and the canopy protects the soil from the hard falling rain we seem to experience more frequently of late. Logging also results in the loss of dead leaves, bark, and branches on the forest floor, which play an important. role in protecting the forest from erosion.
As we looked up the trail into those conifers, we could also see bare granite and lots more roots where human activities have also greatly accelerated erosion. As the students experienced all along the route, each hiker’s step churns up mud and sets the stage for more erosion.
A short distance before we reached the summit, we paused beside a granite outcrop to discuss life on a rock. It’s fascinating to think about the fact that the glaciers had sanded Sabattus Mountain down to bare bedrock. So how did plants come to grow on the rock? How do the plants get water? And how do they secure nutrients.?
Given those formidable factors, the only group of plants. that can overcome them is crustose lichens, which look like they’ve been painted onto rocks or trees.
Lichens are not true plants, but rather a symbiotic association between a fungus and green and/or blue-green algae(cyanobacteria). And in recent years we’ve learned that yeast is also involved. Fungal filaments wrap around a microscopic algal body. Through photosynthesis, the algae and/or bacteria produce carbohydrate energy to fuel themselves and the fungi, while the fungi grow around the algae to create a structure that absorbs and holds nutrients and water. Yeast, it is believed, likely helps ward off predators and repel microbes.
The crustose lichens colonize the exposed granite by their microscopic wind-dispersed structures called soredia. Once a soredium contacts the granite, it anchors itself. It takes at least ten years before a crustose lichen is visible to us!
And fifty years before their tissues have covered enough granite for colonization of the next stage, foliose lichens that look foliage-like. Foliose lichens grow atop crustose and consume them much as a predator might.
Only the central part of a foliose lichen is attached to the rock. It’s wavy undersides capture and trap fine materials carried over the rock by wind and rain. Soil starts to form, slowly increasing its water-holding capacity and availability of nutrients.
Pincushion mosses, those light green mosses that look like . . . pin cushions, form the next layer upon the rock. They grow atop foliose lichens and killing off the former in the process. It takes more than one hundred years before a pincushion community appears on granite, so treat them with respect. They are especially fragile and don’t securely anchor to the rock.
As detritus (decayed leaves and sticks) gets captured and more soil forms, fruticose lichens such as reindeer lichen, may begin to grow. With each stage, the water-holding capacity, nutrient availability, and soil formation increase—allowing for the invasion of the next stage.
Eventually flowers and. grasses displace the fruticose lichens. and for the first time the outcrop is stitched with roots. Shrubs and acid-tolerant trees, such as this white pine sapling, can then germinate. Their roots will force their way into even the tiniest cracks in the rock and then they’ll exert tremendous pressure as they grow, widening cracks and breaking the rock.
We know you never thought we’d get there, but at last we reached the summit, where we’re greeted by signs created by Lovell Box Company/Western Maine Slabworks owner and Greater Lovell Land Trust board member Brent Legere.
As everyone took in the view, Rhyan talked about continental glaciers that are huge and cover large areas, move very slowly, and destroy relief by grinding down land features like ridges, hills, and valleys.
Glaciers are able to move for a combination of reasons, but perhaps the largest factor influencing the movement of glaciers is the direction in which the glacier slopes. Think of a glacier like a pancake on a griddle. When you pour pancake batter onto the griddle it spreads out slowly and becomes flat, but you can still note a definite high point in the center of the pancake. Because the batter is a liquid, if you pour more batter onto the pancake it doesn’t get taller, it expands its reach and becomes larger. Because glacial ice forms from compacted snow it has a unique structure and behaves much like pancake batter on a griddle. As more and more snow falls onto the glacier’s origin (high point) the edges of the glacier are able to expand, resulting in migration.
Many thousands of years ago a huge portion of North America was covered by a continental glacier called the Laurentide Ice Sheet, which covered most of Maine in an ice sheet a mile thick in most places. This continental glacier ground down the landscape, and left behind numerous geologic formations that we can still observe today.
He then pointed out Mount Tom in Fryeburg, which exhibits the same form as the mountain we stood upon: Roche Mountonee, and reminds the students that the glacier approached from the “smooth” side that has gentler contours. As it moved up this smooth side and over the summit of the mountain the melted water seeped into cracks on the other side, plucking rocks that were then carried away, leaving a “rough” face on the opposite side of the mountain.
Before lunch we walked along the ridge to the western edge, and gained a better sense of the plucked off side of Sabattus. Then some students looked for familiar landforms to locate their homes before breaking for lunch right there.
Being much cooler and windier for the second field trip, those students chose to dine in a more protected spot at the T in the trail.
After lunch we hiked along the ridge line past a quartz outcrop to continue on the loop.
And then we snuck off trail for a few minutes to examine an erratic boulder.
By now it should be clear that one thing glaciers are notarious for is removing material from the Earth’s surface, but what happens to all that debris? As the glacier melts, or retreats, the debris is deposited along the landscape in a wide variety of ways depending on the size of the debris.
The most prevalent glacial deposits found in Maine are glacial erratics. Erratics are stones of all sizes that are picked up by glacial ice, moved across the landscape by the migrating glacier, and deposited far from their bedrock origin as the glacier melts and retreats. The name erratic comes from the fact that these stones are often made of a different type of rock than the bedrock they are found on and because they are dropped erratically all over the landscape.
Most erratics in Maine originate from the plucked rocks of Roche moutonnees, which are carried off by glaciers and then dropped off somewhere else. The largest erratic in Maine, Daggett Rock in Phillips, was plucked from the rough face of Saddleback Mountain and carried 12 miles to where it now sits and measures 80ft long by 30ft wide!
Erratics can be pretty easy to spot, as they are usually quite large, often slightly rounded from riding around in glacial ice, and will be detached from the surrounding bedrock. As mentioned before, they are often made of a different type of rock from the bedrock they are found on, but this is not always the case where a single bedrock type is widely distributed. For example, this erratic that sits on top of Sabattus Mountain appears to be made of granite, which comprises most of the bedrock in the area. It is, however, still detached from the bedrock it sits on, has a slightly rounded shape, and appears as if it were randomly placed on the landscape.
This being a class that suffers from Nature Distraction Disorder much the way so many of us do at Greater Lovell Land Trust (meaning we are easily and wonderfully distracted by any thing in nature that begs to be noticed), some of the students noticed that the erratic has long been a porcupine den for so full was it of comma-shaped scat.
And because it was there before us, we had to perform a magic trick with the rock tripe and toad skin lichens, which were brown to begin. Lichens are cryptobiotic, meaning they have the ability to cease all metabolic activity and molecular reactions through complete desiccation—in fact, they might as well be dead and do appear so because they become stiff and dry. Pour water over them, however, and within minutes they begin photosynthesis, thus turning green.
At last we began the well-traveled descent back through the conifers where the mosses made some of us feel like we were walking into an enchanted forest.
For those students on the first field trip, ice wasn’t an issue, but for the second group, we had to pick our way carefully down.
Soon, everyone realized the true extent of erosion as the trail was carved out of the land. Leigh asked the students how old they were in 2011. Most were a few years old.
In August 2011, a major rain event that was downgraded from Hurricane Irene to a tropical storm by the time it reached Maine, pummeled the state with high winds and heavy rain. The trails on Sabattus took a beating as the students could clearly see.
We measured the distance between where the trail should have been and where it actually is.
It’s about a 17-inch difference.
Over the years, GLLT staff and volunteers have created water bars to try to alleviate some of the damage. Some of the students counted over 20 waterbars, most on the descent.
Eventually we passed from the conifers back into the deciduous forest and on down the trail to the parking lot.
Once again, it was our immense pleasure to travel the trail with so many students who have an appreciation and understanding of the natural world. We loved spending two mornings with MESA on the Mountain.