The Glaciers Went Here! Birch Mtn Rd Glastonbury EarthCache

Quick Description:  When glaciers traveled across New England in the last Ice Age, they left their mark!  This EarthCache explores some of the evidence left behind that tells us where they came from and how they affected the landform in this area.

Location:  Glastonbury,Connecticut N 41^o 42.8657’, -072^o28.1802’ (parking area)

Date listed:

Waymark Code:

Listed by:  CTGEOSURVEY

Purpose: This EarthCache is created by the Connecticut Geological and Natural History Survey of the Department of Environmental Protection.  It is one in a series of EarthCache sites designed to promote an understanding of the geological and biological wealth of the State of Connecticut.

Directions:  From Route 2: take exit 8 to route 94 east.  Follow Hebron Ave (rte. 94) eastward for 6.5 miles.  Turn left (north) onto Birch Mountain Road.  Follow Birch Mountain Road 0.5 (one-half) mile north to a gate,on the left, for a utility company’s right-of-way (ROW) access road under the power lines.  Park near the gate to start the Earth Cache. 

Long Description: The following directions will guide you to your first destination at coordinates N. 41^o 43.0398’, -072^o 28.4693’.  Pass around the gate and follow the dirt roadwest.  As the road winds (it will cross the newly relocated Shenipsit Trail at marker #56), an outcrop will come into view.  Continue to follow the road as itloops around and to the top of the outcrop.  The road will continue west while a trail (the old Shenipsit Trail) continues north into the woods .  Follow the trail north about 250 yards (it runs parallel to a stone wall in the distance to the west which you may not be able to see when the trees are vegetated). The trail will lead you to the coordinates above where it intersects with the blue-blazed (relocated) Shenipsit Trail. Arriving at your location large boulders should be in plain sight (Figure 1, the Shenipsit Trail passes between the boulders). Walk around them and take a good look. They appear to be composed of white, black, and clear minerals,

Figure 1.  Two largeboulders.  Image was taken to the east of the boulders along the Shenipsit Trail (notice blue blaze on tree in middle).  Boulders are composed of light gray granitic gneiss.

probably feldspar, biotite, and quartz respectively.  The largest of the boulders (about 8 feet tall) is quite angular.  This would lead us to believe that it has not traveled very far.  As rocks are transported by gravity, water, or a glacier they become rounded.  This is because as they travel, the edges are chipped away from contact with other rocks.  After you are done examining the rocks, look around at the surrounding landscape.  You are standing at the highest elevation around; i.e. there are no higher areas near-by from where these rocks could have rolled. Certainly a torrent of water necessary to move the boulders would not flow at the top of a hill or ridge. The rest of the EarthCache will enable you to determine where the boulders came from and how far they may have traveled.

To find coordinates N. 41^o42.9448’, -072^o28.5108’, follow the trail back from where you came.  As you are walking look for other boulders much smaller in size.  The trail passes between two boulders directly opposite one another just before coming to the ROW.  The larger of the two, which is on your left, is the one of interest (see Figure 2).  Inspect the rock and the area on the trail in front of it.  You should notice that the rock is very similar in composition to the larger boulders we have already seen.  At the base of the rock we can see that the bedrock on which the boulder sits and the boulder are of different lithology.  The bedrock is dark gray colored schist of the Littleton Formation.  It is a medium-grained mica schist with millimeter size garnets and 2.5 cm long staurolite

As you come out of the woods at the utility company ROW, you will have reached the spot of the next location.  After enjoying the view, inspect the rock on which you are standing.  Here you will find it easy to see a series of parallel grooves in the schist bedrock (Figure 3).  As you look around more you see that these lines are quite extensive.  They cover the entire surface of the top of the bedrock.  They can also be found on outcrops in the woods and down the hill in the ROW. 

Figure 3.  Glacial grooves and striations on surface of Littleton Schist at edge of ROW.  Foliation (feet of geologist are on top of weathered foliation plane) cuts across the rock at almost 60^o to the orientation of the grooves.  Glacial groove orientation is S. 38^oE  (142^o) at this location.

Notice that the grooves are at an angle to the layering (schistosity foliation) in the gray Littleton Schist. What could have created these?  Grooves could be etched into the rock from off road vehicles (and some are present near-by).  To be certain of their origin you may want to walk into the woods again along the trails to see similar grooves (if you follow the Shenipsit Trail north about a mile, many outcrops containing glacial grooves and striations will be crossed).  They were not made by machines.  These are called glacial striations.  They are formed when rocks and other debris frozen into the base of the glacier gouged into the bedrock as they were being dragged along the bottom of the glacier and ground up. 

Striations indicate the last direction the glacier was moving:  the ice moved parallel to the striation.  They only indicate the last ice flow direction because a change in direction during the retreat of a glacier may have obliterated earlier directional indicators. It turns out that these particular striations and grooves record average ice movement direction during the ice age.

Figure 4A.                                Figure 4B.

Figure 4A. 

Figure 4B.  Chatter mark (at point of pen) that may be lunate shaped indicating movement was toward the SE (top).  Cresentic gouges (convex side points in direction from which the glacier came) indicating SE ice movement are also reported from the area.

As the striations are linear features, the ice theoretically could have been moving in either direction.  A closer look (Figure 4) at some of the striations located closest to the power lines, one may notice that it appears as though some grooves contain “chatter marks” (see definition in caption for Figure 4), some of which have been chiseled out into half-moon shapes (lunate) whose convex side points in the direction toward which the glacier moved.  Toward what direction would the ice have moved?  Movement from the northwest is consistent with data found in other areas nearby.

Now we have all the information we need to infer the origin of the large boulders we encountered at the beginning of this exercise.  The ice was last moving from the northwest.  The bedrock map of the Marlborough Quadrangle (Figure 5) shows the Glastonbury Gneiss underlies a large area to the northwest of our location.  The Glastonbury gneiss is a gray medium-grained gneiss with quartz and feldspar.  This would explain the mineral composition we noted when first examining the boulders.  The closest mapped outcrops of contact between the gneiss and the schist are ~1/2 km away.  A rock that has been transported by a glacier and is resting on bedrock of different lithology is known as a glacial erratic.  We now know that the boulders are really glacial erratics that have been transported at least 1/2 km from their source to the northwest.

Figure 5.  Geologic map of a portion of the Marlborough Quadrangle showing Birch Mountain Road.  The pink area on the map is underlain by Glastonbury Gneiss;  the areal extent of the Littleton Schist is shown in yellow;  two of the black dots show the GPS-locations of our observations and the third shows the parking area on Birch Mountain Road.

How to log this EarthCache:

1.What is the weight of the largest boulder illustrated in Figure 1? To answer this estimate the volume of the boulder by estimating its average dimensions (length, width, and height). It is an irregular shape so this may present a challenge. For instance the boulder is about 8 feet high at its highest, but the average height may only be about 6'...or 5 1/2 feet or more or less. Then multiply the average length times the average width times the average height. A rule of thumb that stone masons use is that a stone wall 2' x2' x3' weight about a ton (a ton is 2000 pounds). So, dividing the volume by 12 should give you the weight of the stone in tons.

2. Accompany your answer with a picture. First lay your GPS along one of the striations so that its antenna points in the direction toward which the glacier moved. Take a picture showing your GPS unit and the striation. Determine the bearing of that direction. A measurement is preferred but if you cannot do that with your instrument chose from the following: E (090°), ESE (112.5°), SE (135°), SSE (157.5°), S (180°), SSW (202.5°), SW (225°), WSW (247.5°), or W (270°).

3. Optional: Submit a picture of you or a member of your party doing this EarthCache if you would like us to post it on our Geology Webpage at the Connecticut Department of Environmental Protection. We are particularly interested in developing activities that families can do together, and want to show that EarthCaches are a fun way to get outdoors.

Difficulty: 1

Terrain difficulty: 3

References:

Bell, M., 1997, The Face of Connecticut, Connecticut State Geological and Natural History Survey, p. 124-125.

Bennett, M.R., and Glasser, N. F., 2003, Glacial Geology, Ice Sheets, and Landforms, 109-115.

Snyder, G.L., 1970, Bedrock Geologic and Magnetic Maps of the Marlborough Quadrangle East-Central Connecticut (1:24,000), U.S. Geological Survey Geologic Quadrangle Map GQ-791.

Stone, J.R., Schafer, J.P., London, E.H., DiGiacomo-Cohen, M.L., Lewis, R.S. and Thompson, W.B., 2005, Quaternary Geologic Map of Connecticut and Long Island Sound Basin (1:125,000), United States Geological Survey Science Investigation Map #2784.

This EarthCache was written by Earl Manning in 2008 while he was an undergraduate student at Central Connecticut State University and working part time for the State Geologic and Natural History Survey of Connecticut.  He enrolled in the Graduate School at the University of Oklahoma in the Fall of 2009.

The Kongscut Land Trust graciously gave permission to publish this EarthCache on land under their stewardship.