What
happened to the rocks in Chatfield Hollow State Park?
Background:
All
of the bedrock (ledge) found in the park is a hard, light-colored granitic-type
rock called the Monson Gneiss. Originally it was a granitic igneous intrusion
formed at the edge of the North American continent as the Iapetos Ocean plate,
which was roughly in the area now occupied by the Atlantic Ocean, was being
forced down under the North American continent. Africa and Eurasia were moving
toward North America. As the ocean floor plate was subducted under the
continental plate, some of it melted and forced its way toward the surface. The
rock we now see as the Monson Gneiss was the molten material in the magma
chamber, which didn’t make it to the surface and slowly cooled to form a
granitic pluton. Later, as the Iapetos Ocean continued to close, either a
microcontinent or Africa collided with North America. The pressure and high
temperatures produced during this collision and subsequent folding of the rocks
caused the metamorphism of the former granitic pluton. Millions of years of
erosion of the overlying rocks has now exposed this rock, the Monson Gneiss, at
the surface.
Figure
1.
The
Monson Gneiss is light-colored, and looks the same everywhere in the park. In
some places it is covered with blue-gray lichens.
Gneiss is a metamorphic rock, which means it formed at great depth where pressures and temperatures were much higher than here on the surface. When the pressure is released as the rock reaches the surface due to erosion of overlying rocks, the rock tends to break in many places. Tree roots widened these fractures, as did water freezing in them and expanding. Glacial ice flowing over the rocks moved some of them. Those rocks moved only a short distance are still angular, just as they broke originally, while those moved a longer distance became more rounded as their corners and edges were broken or ground off.
Figure 2.
Some of the angular rocks plucked
off by the glacier can be seen near the center of this picture.
Objectives:
In
this park visit you will examine the rocks near the entrance to the park and
learn:
1. The identity of the Monson Gneiss
2.
What
glacially plucked rocks look like.
3.
How some
caves formed in Connecticut.
Procedure:
1. Near the park entrance, to the left of the road as you
enter, there is a hill which is all exposed bedrock. Examine the rock making up
that hill. In your own words, describe what that rock look likes, including its
color, the shape and size of its grains, how the grains are arranged, whether or
not there are breaks in the rock of the hill. This is the Monson Gneiss.
2. Look at the rocks on the south end of the hill (the end
closest to the park entrance). There
are many loose rocks here. Are they the same kind of rock as the rest of the
hill? These are some of the rocks that were plucked off of the outcrop by
glacial ice as it moved south.
3. Sketch the shape of a few representative loose rocks.
4. If you walk around to the other side of the hill, following
the trail, you will find several caves among the rocks. Examine them closely
(you don't need a flashlight, they are shallow). How do you think these formed?
5. Young people enjoy exploring and playing in caves. Can you
think of another situation where someone might have found these caves useful?
Try
to come back to the park on your own time to enjoy the rest of the Chimney Trail
and the other trails. There is also a nice beach for swimming in the summer on
the pond. Fishing is also popular here.
For
a detailed description of the geology of the park, follow the Chimney Trail,
blazed in green. Begin near the Nature Trail, on the south side of the road.
Almost immediately the trail encounters a large outcrop of Monson Gneiss. The
trail climbs over the south end of the outcrop then runs along its eastern side
before crossing the road. Notice all of the fractures in the rock.
Figure
3.
Cave
in large outcrop at the beginning of Chimney Trail, in Monson Gneiss
The
Chimney Trail has large amounts of exposed bedrock (also called ledge),
all of it named Monson Gneiss (pronounced "nice"). If you look closely
at this rock, to become familiar with its appearance, you can then try to find
similar loose boulders of other rocks in the park. These loose, rounded rocks
were moved here by glacial ice between 24,000 and 16,000 years ago. Many of
these rounded rocks, called glacial erratics, are also Monson Gneiss, as the ice
usually didn't move rocks very far. But you may find a few other rock types.
Follow
the Chimney Trail on across the road, through the woods and over the bridge. The
next large outcrop also contains some small caves, including one the trail goes
through.
Figure
4.
More
Monson Gneiss where the Chimney Trail goes through a rock fall cave.
In
the area of Figure 4 you can see some banding in the rocks, caused by the
concentration of different minerals into layers during metamorphism. Because
some of the minerals are harder than others, differential weathering has caused
these harder layers to stick out of the rock more than the softer layers. There
is also a pegmatite vein here, a narrow band of course-grained lighter colored
rock which formed when some of the minerals in the rock melted during
metamorphism and was injected into a weak zone in the rock. Each mineral has its
own melting temperature, and those with lower melting temperatures, such as
quartz and feldspar, sometimes melt during high temperature metamorphism.
After leaving the rocky outcrop, the trail traverses
wooded areas. Sometimes it crosses long, narrow valleys. These valleys trend
north-south because as the collision between North America and the European and
African
continents
took place, the land was folded like an accordion into north-south hills and
valleys. This has made travel in Connecticut much easier in a north-south
direction, but less so in an east-west direction, which requires constant ups
and downs.
Figure
5. A
typical north-south valley.
Rounded
rocks found in the forest, especially away from outcrops, were deposited here as
the glacial ice melted around 17,000 years ago. Rocks moved by water or ice lose
their sharp edges and corners from abrasion and become rounded. These rocks
which the glaciers moved are called erratics. Most of the erratics in the park
are Monson Gneiss, but 
Figure
6. The rock
in the
center of the picture, with the coin on it, is Portland Arkose.
sometimes you may find a rock that looks different. At one point along the Chimney trail a reddish-brown rock containing small pebbles can be found. It is a piece of the Portland Arkose that is found to the northeast of Chatfield Hollow, in Durham and farther north. Portland Arkose was quarried in Portland for many years and shipped all around the United States, even to California, to be used as the building stone called "brownstone". The piece seen in the picture is a little hard to identify because of its moss and lichen covering, but a careful examination will reveal the small pebbles it contains. Arkose is a type of sandstone, a sedimentary rock made by the cementing together of grains of sediments.