Boulder Hopping At Glacial Park Ledyard CT EarthCache

BOULDER-HOPPING AT GLACIAL PARK

LEDYARD, CT

A unique concentration of large and small boulders is found at Glacial Park in the west central portion of Ledyard, CT. These boulders are draped over the top of a couple of hills and in the intervening ravine. The boulder field is about 100 yards wide and a little less than a mile in length (see Fig. 1). A discontinuous line of boulders, much less concentrated,

Figure 1. Map shows the distribution of surface deposits at and surrounding Glacial Park in Ledyard, CT. hlem = Hammonasset-Ledyard end moraine; t = till; gr and led = glacial stream and lake deposits, some in contact with the end of the glacier and some in contact with left-over blocks of ice in front of the glacier, sw = swamp deposits, black dashed line = interpreted ice margin. (After Stone and others, 2005.)

stretches from Hammonasset Beach State Park₁, eastward to Ledyard and into western Rhode Island. This area is part of what geologists refer to as a recessional moraine. This EarthCache involves some strenuous climbing over and around large boulders and will be very difficult for people with ambulatory difficulties. Likewise, small children will need help and should be carefully monitored.

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

Location: N. 41^o26.450’, -072^o 02.966’

How to get there: From I-95, exit at RT12 North. From I-395 take exit 79A and take RT 2A to RT 12 south. Take Rte. 12 to RT. 214 . Head east on rte 214 about 2 miles to Avery Hill Extension (first right). Turn right and take Avery Hill Extension about one half mile to Whalehead Road (at end of Avery Hill Ext.). Turn left onto Whalehead Road and go east about one quarter mile to Glacier Park, on your left (see sign above). A power line crosses road at entrance to the parking lot.

Introduction. Approximately 25,000 years ago Connecticut was in the grips of the last Ice Age (called the Wisconsin Glacial Event). The climate was much colder! At that time, glacial ice extended from the northern reaches of Canada, where it was as much as 5 miles thick, southward to the southern shores of Long Island. The thickness of the ice was about a mile or more over much of Connecticut during the coldest period. Ice of that thickness flows from areas where it is thicker toward areas where it is thinner. In Connecticut the ice flowed generally toward the south-southeast, as indicated by striations (glacial scratches or gouges) in the ledge and the orientation of drumlins (glacial hills).

As a glacier flows it erodes the soil as well as the underlying bedrock. Sand and rocks frozen into the bottom of the glacier abrade the underlying bedrock. This creates not only scratches and gouges in the rock but also enormous amounts of debris: rock, sand and mud (ground-up rock). Some of the rock fragments may be very large. All this debris is transported to the end of the glacier where it is dumped forming a deposit we call an end- or terminal moraine.

Melting occurs at the end (terminus) of the glacier. If the rate of melting is the same as the rate of glacial flow, the terminus stays relatively stationary. If the rate of melting is faster or slower than flow can replenish the ice, the terminus of the ice either advances or recedes.  In general, an end moraine accumulates during times when the ice-front is relatively stationary. Melt-water streams, flowing from the top and sides of the glacier and from cracks and holes (tunnels) in the glacier, carry-off much of the sand and gravel and mud.  The boulders are, however, too large for the water to move and they are left behind.  A terminal moraine is recognized, in part, by linear boulder fields that accumulate at the end of the glacier.

The greatest extent of the Wisconsin Glacier is mapped by the location of the terminal moraines that formed at its southern end. When the climate began to warm, the southern part of the glacier melted faster than the ice flow could replenish. This caused the end of the glacier to migrate northward. While the end of the glacier melted northward, the glacier itself continued to flow southward, constantly bringing debris with it. Several times during the warming a succession of colder years resulted in the ice margin remaining relatively stationary because the glacier continued to flow southward as fast as it melted back. When this occurred small discontinuous terminal-moraines formed. They are referred to as recessional moraines. One of those recessional moraines is the subject of this EarthCache.

Activity 1. The boulder field may be accessed N. 41^o 26.565’, -072^o02.952’

after a short walk over a trail that is relatively easy

to navigate. Once you get to the above location the ease of walking ends because the boulders are closely spaced and lie on steep valley-walls.

The boulder field fills a shallow valley or ravine and extends eastward and westward over the adjacent hills. The boulders come in all sizes up to several tens of feet in diameter. One of the ways you will log this cache is to measure the three largest boulders you find: you can start looking now. The boulders are all sub-angular to sub-rounded; that is glacial

processes have rounded off their edges. Not many end-moraine deposits can boast this concentration of boulders. Indeed, compare the boulder concentration here with those at

Figure 2. Several views of the boulders around the Activity 1 location. Note the blue blazes on trees shown in the image on the right: this is part of the trail. Part of this EarthCache is quite literally a boulder-hopping game. Left image shows northern edge of boulder field where boulder concentration diminishes. Center and right images show concentration of boulders. Note there is little or no soil between the boulders.

either Hammonasset₁or Bluff Point₂; the boulders at Meigs Point (Hammonasset) and at Bluff Point are more scattered.

The boulder composition at Glacial Park is interesting because of its monotony; 99% of the boulders are composed of medium-grained granitic gneiss and gneissic granite. They are pinkish-orange and orangish-gray in color. There are minor variations in the exact mineral compositions of the boulders, which provide a clue to their pedigree. All were plucked-up by the glacier from the outcrops over which the glacier flowed (see Figure 3).

Figure 3. Geologic map (Rodgers, 1985) of Glacier Park (red spot) and area to north. Different geologic formations that could be source areas for boulders are shown. Zw = Waterford Gneiss; Zwm = Marmacoke Formation, Zsh = Hope Valley Alaskite Gneiss, Zp = Plainfield formation, Zsph = Potter Hill Granite Gneiss. Note different scale of this map.

Areas of granitic outcrop north of the park are the likely source of the boulders. These areas are underlain by the Potter Hill Granite Gneiss and the Hope Valley Alaskite Gneiss (Zsph and Zsh). These two formations differ on the types and percentages of their feldspars, a trait that is discernable to a trained geologists. Otherwise they look alike. It is reported (Goldsmith, 1985) that these formations have widely spaced fractures that would lead to the production of large boulders. Some of the rock formations (Zw, Zwm, and Zp) exposed north of Glacial Park are not granitic and do not normally form boulders. When they do the boulders usually get crushed and broken into smaller fragments. Nonetheless small boulders of gray and dark gray gneiss and amphibolite gneiss are found in the park, particularly on the hillside to the east of the ravine.

Activity 2. If you follow the blue-blazed trail over the boulders you will see numerous rocks with sharp, recently broken sides appearing as if they have just been split apart. Geologically speaking, they have only recently been split. The process is ongoing. Rocks, of course, have

Figure 4. A. Incipient fractures in boulder near location for Activity 1. Notice small plant (near top of pen) that has grown in this tiny fracture (pen = 5.5”). B., C. Clearly these fractures have been enlarged. Perhaps a sapling/small tree grew hundreds of years ago in the fracture illustrated in B. A small tree is currently growing in the fracture shown in C. It is possible that this fracture has been widened by the growth of the tree.

natural fractures and incipient fractures. Once a fracture starts a couple of common natural processes help it along, progressively widening the fracture.

One process is caused by the expansion of ice when water freezes: ice occupies about 9% greater volume than the water from which it froze. Expansion during ice formation can exert significant pressure on the sides of the fractureif water in the fracture does not have room to expand upon freezing. Perhaps you have unwittingly performed and experiment verifying that observation by allowing liquid in a bottle or jar to freeze in your freezer or outside during a cold winter night. The frozen substance is more than capable of breaking the container in which it is stored if it does not have room to expand upon freezing. Fractures in rocks are widened and lengthened by freezing water.

Once a fracture is established, seeds may get blown into them and germinate. As the plant grows it enlarges its roots and is likewise able to widen and lengthen a fracture (see Figure 4). If you look carefully during your walk along the blue-blazed trail, you will be able to spot numerous examples of the effect of plant growth on the enlargement of fractures.

An additional point of interest: N.41^o26.883’, -072^o03.001’

Around the corner is another feature

of Glacial Park: a kettle. It is located on Avery Hill Extension. A kettle is a large depression (hole in the ground) in a sand and gravel deposit. Kettles are formed by melting of left over blocks of ice that were surrounded and possibly buried by deposition of sand and gravel from melt water streams.

References.

Rodgers, John, 1985, Bedrock Geological Map of Connecticut. State Geological and

Natural History Survey of Connecticut, Nat’l. Resource Atlas Series, 1:125,000, 2 sheets.

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). U.S. Geol. Surv. Sci. Invest. Map # 2784.

How do people log this Earthcache?

Answer the following questions:

1. What is the size of the three largest boulders seen along the trail? To document this provide a picture of you or a member of your party next to the largest boulder seen.

2.Goldsmith pointed out to his readers that there was little sand or mud matrix surrounding the boulders at Glacial Park. Can you imagine a setting during the formation of the Ledyard moraine where most of the end-moraine has abundant sand and mud matrix but Glacial Park in Ledyard does not?

Difficulty: 2.5

Terrain: 3.5

Type of land: Town Park

Earthcache category: Glacial geomorphology.