Why are there boulders at Meigs Point?
Quick Description. At Hammonasset Beach State Park the
landscape and surficial materials (soils etc.) are all relatively
young, geologically speaking. Although there are lots of stones of
various sizes, there is no bedrock (ledge) exposed in the park.
Instead we mostly find sandy beaches with some gravel and muddy
marshes. An abundance of large stones, however, are found at Meigs
Point. This Earthcache focuses on the surficial geology at Meigs
Point. The interested cacher may access information ahead of time
by reading “The Geology of Hammonasset State Park”,
written by Nancy McHone of the State Geological and Natural History
Survey of Connecticut at the following web site:
http://www.ct.g
ov/dep/cwp/view.asp?a=2716&q=325134&depNav_GID=1650
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.
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Seasonal Fees: Hammonasset Beach State Park charges an entrance
fee, at least during the season and possibly all year on
weekends. Check the CT DEP Parks’ web site for
additional information at
www.CT.gov/
dep/cwp/view.asp?a=2716&q=325210&depNAV_GID=1650
In the summer it gets crowded, especially on weekends. It is
recommended this Earthcache be done on a nice day in the fall or in
the spring.
Introduction. About 25,000 years ago, Connecticut
was in the grips of the last Ice Age and was buried under a
continent-sized glacier that was about a mile thick (the glacial
ice was about 5 miles thick in Canada). At its greatest
extent, the southern end of the ice stretched to the south-shore of
Long Island. About 20,000 years ago the global climate began
warming and the ice gradually started melting. It is a
characteristic of glacial ice, when it is thick enough, to flow
from the direction of greater thickness toward its margin where it
thins and disappears completely. In eastern Connecticut, the
ice flowed generally toward the south-southeast.
As glacial ice flows it scrapes across the land and erodes the
soil and rock on which it rides. Glacial ice carries (transports)
the eroded debris, both large and small in size, along with
it. Water also erodes soil and rock, which it
transports. Waterpower, however, is limited by its
velocity. A very high velocity is necessary to move large
particles. And as flowing water starts to lose velocity, it
leaves behind (that is, it deposits) the coarsest particles, say
the gravel and boulders. As it loses more velocity water
deposits sand. Mud only settles out of water after the water
velocity ceases. In this way, water sorts its deposits by
size. Glaciers do not work that way. Most of the debris
that the glacier carries is deposited when the ice melts, leaving
on the ground an unsorted mixture of mud, sand and gravel, with or
without boulders. This process is similar to the way road
sand is left by the side of the road when sand-laden snow, plowed
to the side of the road during a storm, melts. The debris
left by glaciers is called glacial till, or just plain till for
short.
At the end (terminus) of the glacier, melting occurs at the same
rate as (or faster than) flow can replenish the ice. There,
piles of debris accumulate. We refer to the debris
accumulation as a moraine. There also, streams of melt water
flow off the top and sides of the glacier and also from cracks and
holes (tunnels) in the glacier. These streams erode much of
the debris and carry-off the sand and gravel and mud. The
boulders are, however, too large for the power of the water to move
and they are left behind. A terminal moraine may be
recognized by linear boulder fields that accumulate at the end of
the glacier.
ACTIVITY ONE. Find your way to above location
(N41o14.934”,
W072o32.647”). This is the only place along
the beach at Hammonasset that large rounded boulders are found.
Hence, they cannot be a product of beach formation. The
boulders extend eastward along the beachfront and then into the
water (i.e. they are submerged) for a half-mile or more. There are
no high velocity streams in the area today. It is hard to
imagine that the boulders mark the bed of a former river because
there are no banks to constrain such a river. More likely the
boulders mark the edge of the glacier at some time during its
melt-back history. At the end of the Ice Age climate was warm
enough that the southern parts of the glacier melted faster than
glacial flow could replenish the ice. Hence, the ice margin
migrated northward from Long Island. Then the climate must
have cooled for several years and the ice front remained stationary
for several years at Meigs point. The boulders were brought
by the southward movement of the ice and were left when the ice
melted. If you look, you can find several boulders where the
underlying material is exposed.
1. Describe the material on which the boulders rest.
You can see this material just off the observation platform if you
crawl around on the seaward side of the boulders. Do you know
the geologic name for this material?
2. Find the benchmark (a circular brass disc about
3” in diameter that is cemented to one of the
boulders). It is where the GPS reading was taken so it should
be within 10-15 feet of where you take your reading. The date
when the benchmark was established is stamped onto the brass
disc. What is that date?
ACTIVITY TWO. Find a trail that will take you to
N41?15.292”, W072?32.322”. (Do
not cross the marsh: in addition to disturbing the
habitat, you will subject yourself to ticks and mosquitoes and
maybe irate park rangers). Notice the soil in the grassy
areas over which you will walk to find the trail. It is
composed of cobbles as well as loamy material. It is glacial
till. It is the same material you saw under the boulders on
the point. The trail leading to the GPS coordinates
follows a rocky elevated area into the marsh. It is another
terminal moraine.
3. This moraine is just like the moraine at the previous
location. Do you think it is older, younger or the same age
as the terminal moraine at the previous location? We think it
is younger. Explain the logic for our choice or, refute our
interpretation. This question requires some thought.
From the platform at this location notice the moraine boulders
just to the east (see picture above). The marsh seems to be
engulfing them. Develop another logic to explain the age of
the marsh relative to the age of the moraine. Are the
boulders just sinking into the marsh or is the marsh developing
around the boulders?
Conclusion. We hope you can see that the landscape
around us was not just plunked down, but rather has a history of
development. With a little thought we can interpret parts of
that history. That is part of what makes earth science so
interesting.
How do people log this Earthcache? People should
try to answer the Earthcache activity questions 1 to 3 and submit
an image of themselves atop one of the moraine boulders at Meigs
Point.
Difficulty: 1
Terrain: 1: Short hikes of less than 0.5 mi (one
way) are over mostly level ground.
Type of and: State Park
Earthcache category: Glacial feature.