The Mystic Moraine at Bluff Point

Quick Description. Bluff Point State Park and Coastal Preserve is a great place to spend a morning or afternoon or the entire day because there is so much to see and do. It is an undeveloped state park with a bumpy access road and limited facilities (rest facilities are found at the parking area and at the end of the trail near the bluff). There are hiking and biking trails galore, none very strenuous and some relatively smooth allowing for family biking and access for those with ambulatory difficulties. It is a long hike however…just over a mile one-way. At the end of the trail you will find a variety of beach, marsh, and upland habitats. Bird watching is always popular here because over 200 species have been seen at this site. There is a spectacular bluff-top vista of Long Island Sound, Fisher Island, and the beautiful crescent-shaped Bluff Point-Bushy Point beach.

This EarthCache explores the geology of Bluff Point. Two companion EarthCaches explore the beach dynamics at the Bluff Point-Bushy Point Beach.

Bluff Point State Park and Coastal Preserve, N41^o20.156’, - 072^o02.023’

Groton, CT PARKING LOT

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.

To get to the parking lot at Bluff Point State Park: Coming from I-95:  Take exit 88.  Turn south (left if coming from I-95 north; right if coming from I-95 south) onto SR 117 South.  Turn right at the end onto Route 1 South.  Take a left at the first light onto Depot Road.  The drive into the park is under the rail-road tracks after the residential area. Park entrance is at the left side of the parking area at the end of a rather bumpy drive.

Introduction

Global climate during the last few million years has been controlled largely by the amount of solar radiation that reaches the northern hemisphere. The amount of incoming northern hemisphere solar radiation is modulated by several astronomical factors: the eccentricity (out-of-roundness) of the earths orbit about the sun, degree of tilt of the earth’s rotational axis relative to the sun (plane of the ecliptic), and the earth’s precession. These factors affect the amount of heat the northern hemisphere receives and that affects the global temperature. The temperature of Earth’s atmosphere has varied cyclically between cold ice age and warm greenhouse climate.

20,000 years ago Connecticut was in the midst of an ice age. Glaciers up to a mile thick covered the state. About 19,000 or 18,000 years ago the climate warmed and the ice began melting. It glacier melted in the south first and gradually the ice front melted northward. About 15,500+ years ago Connecticut was finally ice free.

Ice is not a strong material. When glacial ice is thicker than several hundred meters it deforms under its own weight and it slowly flows. It flows from areas where the ice is thicker to areas where the ice is thinner. In New England, the glaciers flowed south to southeast, away from the thicker ice accumulations in Canada. If snow and ice accumulate faster than it melts the glacier flows outward and the ice front advances (southward in Connecticut). If melting is predominant, the ice front melts northward. Because it takes longer for the thicker ice in the north to melt, the glacier continues to flow southward even while the ice front melts and retreats northward.

Topographic map (C.I.=20’) of Bluff Point State Park and Coastal Preserve showing Pleistocene glacial and modern beach deposits. Mystic Moraine deposits are labeled mem on map. t = glacial till, Cp = sand and gravel deposited by glacial meltwater streams, b = modern beach, sm = modern salt marsh, w = water, lblack dashed-line is the inferred ice margin where moraine deposits (boulders) are missing. After Stone and others, 2005.

Bluff Point is at the seaward end of a headland between the Poquonock River estuary and the Mumford Cove estuary (see map). The morphology of the headland is drumlin-like (elongate and somewhat streamlined) and was probably shaped by subglacial erosion of the local bedrock. You will be able to see this most clearly if you visit the park when trees are free of their leaves. It is not mapped as a drumlin on the state map (Stone and others, 2005) because it lacks a thick deposit of compact glacial till; bedrock crops out in places, especially in the southern third of the headland.

Activity 1. Take the western most trail past the beach (formerly the Groton Town Beach) to the following point: N41^o 19.618’, -072^o 02.035’. (You can find this without a GPS by locating three large boulders on the east side of the trail after the town beach.) Along the way, note that east of the trail the hillside slopes downward toward the trail. There are, however, several westerly directed spurs coming off the hill. A couple of the spurs create slight rises along the otherwise flat trail. Notice also the relative abundance of boulders.

Left. View from trail, looking east, of bouldery hill-slope. Notice that hill slopes toward the south instead of the normal westward slopes on this trail. Boulders are slightly more concentrated is this area than in other areas of the hill. This is interpreted as a pile of debris draped across a drumlin-like hill by processes at the end of the glacier during a period of its melt-back. This is an end-moraine, deposited as the ice front melted northward.

Right. Some of the boulders on the hill-side are quite large. This one is a little farther along the trail and is 7-10 feet long.

At the way-point I hope you notice that the hillside slopes in a different direction and that there are more boulders at the way-point than either north or south of this area

Question #1. If the hill slopes toward the trail where you are, move a little farther along the trail until you see the hill slope illustrated above. Toward what geographic direction does the hillside illustrated above slope?

If you walk directly over the to the east side of the hill (or better, on your way back to the parking lot, take the eastern trail) you will find another area of boulders. Yet a third area of boulders is found at the western tip of the Bushy Point Beach (see map). Concentrations of boulders in this area are interpreted to be deposits left at the southern edge of the ice during a period when the ice retreat momentarily slowed or stopped. Such deposits are called end moraines. This is part of the Mystic Moraine. Additional bouldery areas can be found at numerous places eastward from Bluff Point all the way to the Borough of Stonington. The moraine formed some time around 18,500 years ago when the rate of meltback of the glacier ice was exactly balanced by the rate of forward flow of the ice sheet.

Continue your walk to Bluff Point. When you are almost there you can look out to the southwest and see the salt-marsh behind the Bluff Point-Bushy Point Beach. In the distance you can see Bushy Point

Activity 2. Find your way to the following point: N41^o18.887’, -072^o02.178’

This should place you at the top of the bluff (cliffs) at Bluff Point. There is usually a nice breeze here. Walk around and find a park bench and enjoy the view. From this point one can, on a clear day, see Fisher Island to the southeast and, on a very clear day, the eastern part of Long Island to the south and southwest. One can see the Connecticut shoreline as far west as the headland behind Ocean Beach in New London. All of Long Island is composed of glacial moraine and outwash sediments. Two large moraine deposits on Long Island mark the farthest south that the last ice sheet extended. The northerly moraine is referred to as the Harbor Hill Moraine. Fisher Island is part of that moraine system. It formed about 19,000 years ago (Stone and others, 2005).

The cliffs at Bluff Point are formed of granitic bedrock. There are a couple of places where you can safely walk down and look at the bed rock. It is pink and gray banded- (layered) rock. The pinkish colored mineral is potassium feldspar and it is responsible for much of the banded appearance of the rock. The gray layers contain biotite mica and quartz. The white mineral is sodium feldspar. In places you may be able to fine some garnet, a purplish colored mineral. Look also for small black magnetite.

Left. Granite bedrock exposed on bluffs (cliffs) near the end of Bluff Point. Note the fractures are spaced about 12 inches apart. For scale, grass blades are 2-3 inches long.

Right. Large fragments of granite at the base of bluffs near end of Bluff Point have a size (thickness) that is similar to the distance between fractures on the outcrop.

The outcrop is well fractured. Fractures in the outcrop are spaced several inches (12 or more) apart from one another. Can you imagine what size rock would fall off the outcrop during heavy surf?

Question #2. Look at the base of the cliff. What are the sizes of the rocks found there? Do you think there is any relation between the fracture spacing in the outcrop and the size of the rocks at the base of the cliff?

Question #3. Walk toward the beach and note the how the size of rocks at the base of the cliff changes. Describe that change. Do you think that any of the beach sand is derived from erosion of the granite bluffs? Why or why not?

How do people log this Earthcache? People will need to provide answers to the questions 1, 2, and 3 and submit an image showing the size of the largest boulders at location one (Activity 1). Have one of your children or a companion in the picture for scale.

Difficulty: 1

Terrain: 1 Site is accessed after a long walk on mostly flat terrain. The walk is on a trail along which one could drive a sedan. Indeed it was an automobile road before the 1938 hurricane destroyed a beach cottage community. Walking on soft beach sand can be tiresome.

Type of land: State Park

Earthcache category: Glacial feature, coastal feature.