Devens Disc Golf
Why Devens?
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| Figure 1. (Wood 2016) Basket #12 General |
The Disc Golf Courses at Devens are unique. Two separate courses wind their way through thick forest, rocky terrain and steep elevations changes. The Hill, on the west and south sides of Shepley’s Hill, is the original course, and as its name implies is dominated by numerous and often very large (up to 90 ft) elevation changes. The General is the newer course, lies on the north and east sides of Shepley’s Hill, is longer and straighter, and is not as extreme in its elevation changes (though it is still very hilly). These courses are free to play, lie about an hour from much of the Boston Area, and have a local disc golf shop (The Hawks Nest) which is the hub for the local disc golf community. The locals take strong ownership in the course, both in upkeep and in the largest weekly league in Massachusetts. It is all of these factors that has made Devens my home course and it is the inspiration for this blog. Nowhere else that I have played so beautifully and organically merges geology with disc golf. The geology shapes the course and the course gives the disc golfer an avenue to experience the geology.
The Terranes of Eastern Massachusetts
This first entry is going to try to unravel the geologic history of the Devens area. If you want to focus on what rocks are at Devens and how they affect the course, then wait for the next blog entry. This section is about how this part of Massachusetts came to be. The geologic history of the Devens area is quite complicated and is still being figured out by geologists, but I am going to try to give you the basic idea of what we think we know at the current time.
First we need to introduce the main players in the tectonic history of eastern Massachusetts as we will be using these names a lot both in this entry and in other, including other courses.
Laurentia – this is the core of the current North American Continent, If we go back to the Early Paleozoic (500 MYA (million years ago)) the margin of Laurentia was around the Berkshires in Western Massachusetts.
Gondwana- Name of the supercontinent that consisted of the cores of current continents of Africa, South America, Antarctica and the India subcontinent.
Avalonia – (or Avalon Terrain) a portion of the margin of the Gondwanan continent (Possibly an island arc like Japan) in Late Proterozoic and Cambrian time. It is now located in Eastern New England, the Maritime Provinces and Scotland.
Ganderia- Another portion of the margin of the Gondwana continent but separate from Avalonia. It is now thought that this subcontinent was split into two pieces, often referred to as its leading edge and trailing edge. Ganderia is present in New England and the Maritime Provinces. The Devens area is likely on the trailing edge segment of Ganderia.
The Merrimack Belt (terrane)- A belt of meta-sediments and igneous intrusions thought to be a part of, or deposited on the trailing edge of the Ganderia subcontinent. The rocks in and around Devens are from this belt, which trends from southern Maine to the Connecticut coast.
The Nashoba terrane- a group of highly metamorphosed and deformed metamorphic and igneous rocks that lie between the Merrimack terrain (Ganderia) and the Avalon Terrane (Avalonia). Only present in Massachusetts.
Figure 2. From Pollock et al. 2009. This map shows the location of the various terranes that now make up Eastern Massachusetts In Proterozoic- Ordovician time.
Geotectonics of the Devens region.
Ganderia was formed off the coast of Gondwana, along the margins of Gondowana, possibly as island arcs on the margins of these continents. The time of its formation is not clear but is likely in Late Proterozoic time (before 550 MYA). By Late Cambrian to Early Ordivician time (515 MYA to 485 MYA) both Ganderia and Avalonia separated from Gondwana as a new spreading center formed between them and the main Gondwana landmass. As this rift expanded Ganderia and Avalonia were separated from Gondwana and moved towards Laurantia. During this time of separation and transit rifting also occurred within the Ganderia landmass, splitting it into two pieces so that when Ganderia finally approached Laurantia it likely came in two large chunks. When the leading edge of Ganderia eventually slammed into the Laurentian coast the Taconic orogeny occurred (Ordivician Time 485 MYA to 440 MYA) building mountains along the margin of Laurentia, forming new volcanic arcs, and deforming the edge of Laurentia and this portion of Ganderia. Evidence of this event is found father to the west in Massachusetts. After the Taconic Orogeny finished, sedimentary basins formed just off the margin of the now expanded Laurentia, taking sediment from the both Laurentia and from the trailing edge of Ganderia which was now approaching the Laurentian Continent. It is in this environment that it is thought that the sediments that would become the Merrimack Belt were first emplaced. Some of these sediments can be seen in other parts of the Merrimack belt such as at Coggshall Disc Golf course in Fitchburg. By Early Silurian time (440MYA- 420 MYA) the trailing edge of Ganderia slammed into Laurentia in the Salinic Orogeny. At this point deposition of sediments sifted farther eastward continuing till early Devonian time (415 -400 MYA). It is at this time that the sediments that would become the main components of the Merrimack Terrane in the Devens area were likely deposited. Next, The Avalon terrain slammed into Laurentia /Ganderia to the East and pushed the Nashoba terrane underneath the Merrimack terrane. This event is called the Acadian Orogeny (415- 375 MYA) and it is this subduction of the Nashoba terrane that triggered the emplacement of the igneous plutons in the Merrimack, including the Ayer Granite. Compression from this event likely formed many of the deformational features we see in Merrimack Belt rocks like folds, shear zones and fractures. Much later, in the Permian, Gondwana itself crashed against Laurantia in the Alleghanian Orogeny (300- 250 MYA) , this event and subsequent rifting in the Mesozoic do not seem to have had as much impact as the earlier events in the Devens Area but are important in other parts of New England. Basically Eastern Massachusetts consists of a series of terranes that subsequently collided with and were accreated to the edge of the North American continent. Devens sits in one of these terranes whose origin if much more Aftrican or South American than North American and has been heavily deformed and altered by these numerous collisions.
Figure 3: from Sorota 2013. Cross sections showing the area of the Merrimack terrane (MT) during Silurian to Devonian time. In Silurian time (A) sediments are derived from both Laurentia and Ganderia (block to right of picture). In Devonian time (B) Avalonia collides with Laurantia, subducting under the Merrimack Terrane, triggering formation of igneous intrusions (in red).
Near the boundary of two terranes
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| Figure 1 - Map of geologic terranes in Devens area, Modified from the Massachusetts State Geolgical Map |
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The Ayer Granite
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| Figure 2 (Wood 2016) Block of Ayer Granite Showing large feldspar crystals. Note that long axis of crystals all trend in same direction. |
The Ayer Granite consists mainly of feldspar, quartz and biotite mica, it has distinctive large feldspar crystals 2 or more inches long. At Devens, the Ayer granite is heavily foliated, which means that it has a preferred direction of alignment of its crystal grains and its compositional variations. The foliation likely formed in Devonian time as part of the Acadian orogeny. This collisional event compressed the Ayer Granite in the NW to SE direction, causing folding and faulting while elongating it to the NE-SW. Think of this as taking a block of play dough and squeezing it together between your hands, with one hand to the NW and the other to the SE. It shortens in the direction you push it, but spreads out and elongates in the perpendicular direction to your compression.
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Figure 3- (Wood 2016). Basket, Hole 12 General
bedrock outcrop of Ayer Granite, With bands
of varying crystal size (courser on top right and
finer on bottom left).
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The Ayer granite can been seen everywhere at Devens. The best way to see the crystal structure of the granite is in broken off pieces littering the ground. In many places rectangular 2-3 inch long feldspar crystal can be seen. At hole # 4 on the Hill there is a rock near the tee (people often sit on it due to its flat top also caused by the foliation) which shows the aligned orientation of feldspar crystals very clearly. The granite outcrop below the basket of #12 General consists of both fine grained and courser grained portions of Ayer Granite. Most outcrops of Ayer Granite are elongated in the NE-SW direction, parallel to the foliation. Many have a smooth arching surface (Hill, holes 2, 14, 15, 18 tee, general 12, 9) and are likely the tops of folds in the granite. Other outcrops have sharp angular tops and a steep NE trending SW dipping foliation can be seen (Hill, holes 4, 3, 8). On the far eastern side of the area (General 1,-4) there are localized shear zones in the granite that are expressed as series of steep steps in the topography.
Figure 4- (Steve Wood 2016) Hole #4 Hill, near tee. Block of foliated Ayer Granite. Note nearly horizontal fractures and horizontal orientation of feldspar grains.
Figure 5- Hole 18 tee general (Steve Wood 2016). Smooth and rounded top to bedrock outcrop of Ayer Granite in front of tee. Note more fractured nature of outcrops to left of fairway.
Figure 6- Hole #4 Hill (Steve Wood 2016) Bedrock Outcrop of Ayer granite showing steep NE trending and NW dipping foliation.
Fractures
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| Figure 8- (EPA and Fleming Incorperated 2013) Map of fractures present on Shepley's Hill |
The Ayer Granite has been intensely fractured. These fractures are numerous, in multiple directions and likely formed in a wide variety of deformational and unloading events. Much of the rugged blocky nature of many of the bedrock outcrops at Devens are due to individual fractures. There are two zones of more intense fracturing, the Disc Golf Fracture Zone and the Nona-Shep Fracture Zone. The Disc Golf Fracture Zone cuts right through the heart of the hill course (holes 2,3,5,7,8,9,15,18) and is expressed as a series of parallel N-S trending ridges and valleys with secondary NW-SE trending fractures. Hole 8 travels up a fracture controlled valley, the army built a road through a fracture valley which serves as the fairway for holes 3 and 5. On 18 you tee off the top of a fold crest with fractures isolating the outcrop. The Nona-Shep zone lies at the far corner of the General (Holes 5-7) and consists of a series of NW-SE trending fractures. Hole 5 drops down into the fracture zone in the step like fashion common on the east side of the hill. On hole 7 you come back across the fracture zone. These fracture zones and many of the individual fractures were likely formed in during one or another of the major mountain building or rifting events through Massachusetts long and complicated tectonic history. Some other fractures are likely more recent (last 10,000 years), particularly fractures that roughly parallel the land surface. These fractures might be caused by the removal of pressure on the outcrops when either ice (from Pleistocene ice sheets) or sediment cover was removed, causing the rock to expand and crack. These fractures are mostly seen along side-slopes and on hill crests and tend to dip a little more steeply than the terrain.
Figure 10- (Steve Wood 2016) Hole #8 Hill . Fracture controlled valley, looking back from basket towards tee.
Glacial Impact
The bedrock hill of The Devens courses most likely existed before Pleistocene glaciation but was heavily altered by the ice sheet’s presence. When the ice sheets did cover the area they came from the North-Northwest, moving to the south-southwest. The ice sheets removed much of the soil cover on the hill and likely took some bedrock boulders away to form glacial erratics. The larger boulder in front of the 14th tee (Hill) is likely a glacial erratic. The north and west facing slopes of the hill (Holes 10-12 of the hill, 11-15 on the general) are much smoother than its south and east facing slopes (18,1-4 on the hill, 1-4 on the general). This is due to the ice sheets scouring the northwest side of the hill, removing soil but keeping the bedrock intact (the foliation trend of the rock likely also encouraged this scouring action). After cresting the top of the hill, the ice sheets likely plucked large pieces of exposed bedrock, pushing these boulders downslope or removing them entirely. This process created a more craggy appearance to the outcrops and groundcover on this side of the hill (Holes 1-3 the Hill, 2 and 1-4 General).
Figure 11- Hole # 12 General (Steve Wood 2016)- Gentler Slopes and smoother terrain of the North Side of Shepley’s Hill
Figure 12 - Hole #1 Hill (Steve Wood 2016). Steeper and more rugged terrain on North Side of Shepley’s Hill.
References and Links
Devens Disc Golf Course Review Page - http://www.dgcoursereview.com/course.php?id=1186
The Hawks Nest - http://www.hawksnestdg.com/
Devens League Website - http://devensdg.blogspot.com/
Gannett Fleming Inc. and US EPA Region 1. Shepley’s Hill Bedrock Investigation. Environmental Proctection Agency: 2012. Electronic URL https://www3.epa.gov/region1/superfund/sites/devens/507276.pdf
Kopera, Joseph P. Preliminary Bedrock Geologic Map of the Ayer Quadrangle, Massachusetts [map]. 1:24000. Office of the Massachusetts State Geologist. University of Massachusetts. Electronic URL ftp://128.119.45.20/pub/stategeologist/for_Ray/ayer_prelim_final.pdf
Pollock, Jeffrey C, Hibbard, James P, and Sylvester Paul J. “Early Ordovician Rifting of Avolonia and Birth of the Rheic Ocean: U-Pb Detrital zircon constraints from Newfoundland.” Journal of the Geological Society. May 2009.
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
Sorota, Kristin Joy. Age and Origin of Merrimack Terrane, Southeastern New England: A Detrital Zircon U-Pb Geochronology Study. M.S. Thesis. Boston College. 2013. Online. Persistent link: http://hdl.handle.net/2345/3043
Zen, E-an, Gildsmith, Richard, Ratcliffe, N.M., Robinson, Peter, Stanley R.S., Hatch, N.L., Shride, A.F.,Weed, E.G.A., and Wones, D.R. Bedrock Geologic Map Of Massachusetts[map]. 1:250,000. U.S. Geological Survey. 1983
Very cool.
ReplyDeleteVery nice, up-to-date summary!
ReplyDeleteVery nice, up-to-date summary!
ReplyDeletenice info! thanks!
ReplyDeleteSteve are you still doing this? Not a golfer but I use the courses to teach natural history. I am near the Barre Falls course and would love to visit it with you if interested. Joe
ReplyDeleteHow can we play this with best beginners golf putters anyone's let me know?
ReplyDelete