Otter Brook- Keene NH
Intro
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Figrue 1: (Wood 2016) #9 basket looking back at outcrops
of Rangley schist.
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The drive to Keene, New Hampshire from Boston is diverse and scenic. One starts the drive amongst brownstones and triple deckers underlain by the late Precambrian puddingstone and shales of the Avalon terrain, African in Origin. As you travel northwest, away from the city along route 2 the scene turns more pastoral, open fields and rolling deciduous forests that sit atop tortured stone, the remnants of ancient mountain building events from the great collisions that built Pangea. The hills get larger and the bedrock more prominent as you pass from the Nashoba Terrane, through the Merrimack belt and into the central Maine Terrane. The landscape becomes more forested and wild, interrupted every ten miles or so by quaint New England towns. One passes Mount Monadnock near the boarder of New Hampshire, standing tall, resistant to the wind, rain and even the glaciers of a millennia ago. Finally after following deep gorges cut deep by streams you enter Keene and Otter Brook State Park. To travel from Boston to Keene region is to cross continents, to enter the hinge that once was the edge of North America. A lot of what makes Otter Brook unique, its rugged, sometimes chaotic topography, its piles of stones and its ever sifting character ties back to its geology, it is a place of transition, from continent to basin, from Africa to North America, from till to bedrock, from exposed to buried, from primal nature to man-made, it is a place of flux frozen in time.
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| Figure 2 (USGS 2016, modified by Wood 2016) Google Earth map of Major Geotectonic Terranes from Boston to Keene. Greens are outcrops of the Avalon Terrane; Orange is outcrops of Nashoba Terran; ,Periwinkle is outcrops of the Merrimack belt or Central Main Terrane; Dark Blue is outcrops of the Bronson Hills Volcanic Belt; Yellow is outcrops in the Hartford/Springfield Basin. |
Tectonic setting
Rising as a set of hills in western New Hampshire and central Massachusetts is the Bronson Hills Volcanic Belt. It forms a strip of older, igneous, volcanic and metamorphic rocks that were the core of a volcanic chain that formed off the coast of North America in Ordivician time (around 480 MYA) when the Iapetus ocean started to close. Sediments from this volcanic chain was deposited to the east in the open ocean, forming vast piles of sediment that became the Rangley formation, which the bedrock foundation of the Otter Brook area. This depositional basin of the Rangley formation closed during the Acadian orogeny (420 -370 MYA) as at least two subcontinents collided with North America (Avalonia and Ganderia). This event was long, complex and still being studied and understood. The sediments caught between these sub-continents including the Rangley formation were profoundly changed by this event. It compressed them, realigned its mineral grains, contorted and in some cases obliterated its original sedimentary bending and it heated them to temperatures so high that they partially melted. The rocks of the Rangley formation were pushed them eastward against the hard unyielding core of the Bronson Hills Belt which acted like a giant backstop, forcing them upward, folding the sediments over themselves to make tight anticlines and synclines and pushing the Rangley in the vicinity of Otter Brook, vertical, with younger rocks to the east and older to the west.
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| Figure 3 (Thompson 1985, Modified by Wood 2016). Geologic Map of the Monadnock Quadrange, Otter Brook section. Notice That Otter Brook DGC is located in the Rangley Formation stretching across the boarder between the Lower and undifferentiated Rangley. |
At Otter Brook it is difficult to see the original nature of the rocks, but glimpses are still there. The grittiness of the schists comes from sandstone layers in the original sediments, and subtle differences in the color and composition of the rock reflect differences in the composition of the original sediments. The deformation due to the Acadian orogeny on the other hand is ever-present, foliation is nearly vertical and folds and other convolutions of the layering and foliation of the rocks are seen in many outcrops. The Rangley Formation at Otter Brook represents the ancient margin of the North American continent 420 million years ago. Now that margin has become a suture in the continent as it has been ploughed back, smashed and baked into heart of the mountains of New England
Roadcut
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Figure 4: (Wood 2016). North side of roadcut (My car for scale). Notice
red ferriogeous schist at far end of the oucrop, grey schist in middle
and migmatiteat close end of outcrop. This is the bedrock that
underlies the large ridge which holes 1-3 and 7-9 play on.
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One of the most unique opportunities that Otter Brook presents to the geologist/disc golfer is not even on the course itself, but next to it. Just west of the turnoff for the park is a large roadcut that slices through a high ridge that much of the disc golf course is built on. Being only a few hundred feet away from some of the holes, looking at the roadcut is an excellent way of seeing what is going on underneath the ground at Otter Brook. The easiest way to divide the rocks you see in the cut is by color, distinct areas of red, grey and white can be seen. The reddish rock is most prevalent on the east side of the cut. It is a ferruginous schist, having begun as marine siltstone and shale this rock has undergoing a high degree of metamorphism, changing its original mud into platy minerals like mica and chlorite. It has a strong vertical foliation expressed by orientation of the mica grains. The reddish hue is due to iron content in the rock which “rusts” and turns red when exposed to air. The grey rock is most striking in the center of the cut as it forms what looks like a shear grey wall, it again is a strongly foliated schist, but with a lower iron content, hence a grey rather than reddish appearance. It has a similar origin to the ferruginous schist and in some places these two rocks cannot be distinguished from each other. In places this schist is interrupted by small pods of white rock, near these pods the foliation is often disturbed and deformed. The white rock is very different from the schists, it shows no clear foliation but rather is made of interlocking crystals of feldspar, quartz and biotite mica. This rock is migmatite and is formed when a rock is heated to a point that some (but not all) of it melt. When the melted portion recrystallizes, it forms migmatitie. Notice that the foliation of the grey schist is much more convoluted near the migmatite. Keep these rock in mind as you go down and play the course. You will see them in bedrock outcrops and in loose talus. Otter Brook gives you a unique opportunity to see a cross section of the rocks underneath the course.
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Figure 5: (Wood 2016). At Roadcut, Grey schist, forming a nearly vertical wall.
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Figure 6: (Wood 2016) White migmatite surrounded by grey schist. Notice the
more convoluted foliation of the schist around the migmatite.
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Glacial geology
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| Figure 7: (wood 2016) Hole #14, grounded covered by a thick glacial till with numerous erratics. |
Like most of New England, Pleistocene glaciation had a major impact of the Otter Brook area. The entire course is covered by a layer of glacial till that makes outcrops of bedrock few and far between. The till appears to be thinner and partially removed on the front nine of the course, east of the ridge. The topography is often chaotic, with steep slopes containing bedrock outcrops and flatter areas that have between 5-10 feet of glacial till that is often held in place by tree roots. Small streams cut this till in places leaving earthy mounds. On the back 9 of the course the till appears thicker and more constant. I could find no bedrock outcrops on the back 9 and the terrain is more gentle and hummocky. The cobbles and boulders in the till are different to the bedrock rock types exposed in the roadcut and the front 9 of the course. Most prevalent is a biotite granite which has a different texture and mineralogy from the migmatite in the bedrock,it has smaller crystal grains and more numerous but much smaller mica flakes. It tends to occur as more rounded boulders and might be the Fitzwilliam Granite that outcrops farther to the north and west, carried here by glaciers. Gneisses and schists of exotic origin are also found as erratics in the till, in some places mixed in the Rangley red and grey schists making identifying the origin of many of these rocks difficult.
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Figure 8: (Wood 2016). Glacial erratic composded of fine grained biotite mica granite.
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A geologic walkthrough of the course.
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| Figure 9: (Wood 2016) Otter Broojk |
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| Figure 10:(Wood 2016) Glacial erratic containing large xenolith of metamorpohic rock in granite. Notice striped pattern in the metamorphic rock that contrasts to the massive texture of the granite. |
Hole #1 - You start the course by climbing rapidly up the east side of the large ridge that bisects the course. This area is still covered with glacial till and most of the erratics here do not match the bedrock. A large erratic about halfway up the fairway contains a xenolith, a piece of metamorphic rock that is surrounded by granite. The xenolith is likely a piece of the sedimentary or metamorphic rock that the granite intruded, it fell into the molten magma but still retains some of its original foliation and mineralogy.
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Figure 11: (Wood 2016) Tee #3. Tee shot drops down
a steep hill of migmatite.
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Holes 2-3 - Both these holes play down the large ridge. The hillsides are steep and contain a lot of boulders, some from till but some also from the underlying bedrock. White pieces of migmatite can been seen on the slopes of the ridge here, and it is the hardness of the migmatite relative to the schist that likely makes this ridge such a prominent feature.
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Figure 12: (Wood 2016) Behind Hole #5 tee, grey schist
showing strong steeply dipping foliation.
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Holes 4-6 - These holes are relatively (compared to the rest of holes on the front 9) flat. The bedrock here is the grey schist and can be seen in outcrop on hole six and in places to the left of 4. The till is heavily eroded here by streams coming down from the north, making the terrain very bumpy. Rocks on the ground are a mixture of talus from the ridge and boulders washed out of the till. Locals have made stacks of stone cairns on these holes. You can see great examples of all the rocks here on these holes. Yoda’s Swamp on hole 6 us a marshy area caused by a small steam backing up behind a small ridge that separates it from hole 4.
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Figure 13: (Wood 2016). Looking towards hole #5 basket. Cobble of quartzite in
foreground. Numerous rocks on ground surface from partially removed glacial till
deposit. Basket sits on small hill of glacial till.
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Figure 14 (Wood 2016). Yoda’s swamp, caused by backing up of small stream by
a small ridge directly behind camera. Notice boulders from glacial till
in foreground and road proximity to road in background.
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Figure 15: (Wood 2016) Hole #7 tee.
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Hole 7- You drive from an elevated tee (likely underlain by more resistant schist) to a flat fairway with a large number of stone cairns. From this point you climb quickly back up the ridge. The ridge is made of migmatite in this location, clear from the white rock fragments littering the slope. Take time to look at pieces of the migmatite, in places it has feldspar and mica crystals well over an inch long.
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Figure 16: (Wood 2016) Rock Cairns on fairway of #7.
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Figure 17: (Wood 2016) Migmatite rock fragment. Note large feldspar crystal just below disc.
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Figure 18: (2016) #7 fairway looking up to basket. Note white rocks on
steep slope to basket, these are fragments of resistant migmatite.
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Figure 19: (Wood 2016) Small fragment of migmatite with large
crystal of biotite mica (black crystal directly above disc.
Notice reddish coloration around mica grain due tro presence of iron.
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Hole 8- Hole 8 takes you to the top of the ridge. The first part of the hole is underlain by more grey schist and then another area of migmatite underlies the pin and outcrops behind and to the right of the hole, making an impressive peak. Again look for large crystals in the migmatite
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| Figure 20: (Wood 2016) Migmatite forming a bedrock ridge behind basket of hole #8. |
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Figure 21 (Wood 2016) Outcrop of grey or rusty schist to left of #9.
Notice steeply dipping folialtion in schist.
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Hole 9- Now you go down the other side of the ridge and into the lower Rangley. The white migmatite disappears and is replaces by a steeply dipping and softer ferruginous schist. This schist is best seen to the left of the fairway and on the path to the 10th tee. These are the last bedrock outcrops on the course that I could find. As the ferruginous schist is less resistant to erosion it does not pop out of the glacial till like the migmatitie does.
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Figure 22: (Wood 2016) Hole #10, glacial erratics litter the
stream that crosses the fairway.
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Hole #10 By hole #10 you are now down the back side of the ridge an in an area covered by a thick blanket of glacial till. As you cross the stream notice the large number of boulders in its bed, these are erratics in the till which the stream does not have the energy to carry away. These erratics tend to be granitic and were likely from locations to the north of west.
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Figure 23 (Wood 2016). Hole #13 basket. Large glacial erratics
in till that dominate back 9 of course.
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Holes 11-15- The rest of the course sits on the glacial till. It feels like a completely different course from the front 9. Considerable earth moving was done around hole 12, as you can see old culverts and the pin sits on a small hill pushed up by a backhoe or some other construction device. Hole 13 contains lots of large erratics in front of the basket, while hole 15 plays down the old road that likely led back to the earth moving area at hole 12.
Hole 16- This odd hole takes advantage of the chaotic terrain that can form on glacial till. The tee shot winds through innumerable trees to the top of the large hummock. The basket sits blind in an odd depression. There are a couple of possible explanations for this depression. It could be man-made, a place where till was excavated as the dam was built or it could be a depression formed by the melting of a piece of ice in the till. Just as rocks are deposited in till, large chunks of ice can be deposited as well. When the ice melts that land collapses above them forming a depression. When large enough they fill with water and are called kettle ponds. This one is too small and shallow to form a pond but might be formed through the same process.
Holes 17-18 - The last holes of the course wind back down to Otter Brook, meandering through numerous trees and more thick glacial till until reaching the mud and silt of the Otter Brook floodplain.
Well I hope you enjoyed this look at Otter Brook. Next up the Quarries in Barre Vt. Rock piles, granite, giant pits of stone and natural air conditioners.
References
Keene Disc Golf Club http://www.keenediscgolf.com/
Robinson, Peter and Goldsmith, Richard. “Stratigraphy of the Merrimack Belt, Central Massachusets.” U.S. Geological Survey Professional Paper 1366 E-J. Washington D.C.: 1991. Electronic URL http://pubs.usgs.gov/pp/1366e-j/report.pdf
Skehan, James W. Roadside Geology of Massachusetts. Missoula: Mountain Press Publishing Company, 2006. Print
Skehan, James W. Roadside Geology of Connecticut and Rhode Island. Missoula: Mountain Press Publishing Company, 2008. Print
Thompson, Peter J. “Stratigraphy, Structure and Metamorphism in the Monadnock Quadrangle, New Hampshire.” Contribution #58 Department of Geology and Geography University of Massachusetts, Amherst MA. 1985.
Van Diver, Bradford B. Roadside Geology of Vermont and New Hamoshire. Missoula: Mountain Press Publishing Company,1987. Print
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