Geology of Chatfield Hollow
Rock Types Found:
Metamorphic: Monson Gneiss - Light to dark, medium- to coarse-grained
All of the bedrock (rock below the surface) found in the park
is a hard, light-colored granite-type rock called the Monson
Gneiss. Originally it was igneous rock formed at the edge of
the North American continent as an ancient ocean was being
forced down under the North American continent. Africa and
Europe/Asia were moving toward North America when
Pangaea formed around 450 million years ago. As the ocean
floor plate was pushed under the continental plate, some of it
melted and forced its way toward the surface. The rock that
formed was once the molten material in the magma chamber
which didn’t make it to the surface and slowly cooled to form granite.
Later, as the ancient ocean continued to close, either a microcontinent or Africa collided with North America and created great
pressure and high temperatures during this collision. The rock then
changed the granite into gneiss, which is a metamorphic rock deep
underground. Millions of years of erosion of the overlying rocks
have now exposed this rock, the Monson Gneiss, at the surface.
Gneiss is a metamorphic rock, which means it formed at great depth where pressures and
temperatures were much higher than here on the surface. As the rock reaches the surface due to
erosion of surface rocks, the pressure is released and 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 that moved a longer distance become
more rounded as their corners and edges are
broken or ground off.
Rounded rocks found in the forest were deposited 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,
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.
Connecticut is a state with long, narrow valleys. These valleys are 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.