Deven’s Disc
Golf Part 2
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 |
The disc golf courses
at Devens are situated on a bedrock hill of Ayer Granite that lies close to the
border of the Merrimack and Nashoba terranes. If you approach Devens from the east on RT 2 you enter the Nashoba
terrain just after its intersection with I-95. As you climb the hill just past the I-495
Interchange you pass over the Clinton Newbury Fault zone and into the Merrimack
Terraine. Rather than one sharply
defined fault, the fault zone is a series of faults and more ductile shear
zones that penetrates into both the Nahoba and Merrmack terranes. The Devens area is actually considered to be
in the fault zone and and deformation of the Ayer Granite at Devens is at least
partialy a result of movement along this fault. Farther west, outcrops usually
consist of the Oakdale formation, metasediments of the Merrimack Terrane.
.
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.
 |
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
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