The two types of bedrock in the Wintergreen Brook-West Rock area (the same as under most of New Haven and the central lowlands of Connecticut) are the sedimentary sandstone called Arkose and igneous diabase or trap rock. To understand their formation and association here we need to go back to events that began about 210 million years ago.
It is important to realize that at the time of the glaciers this area wasn’t so different from its appearance now. The shape of the land was close to its current shape and the oceans and continents would be recognizable with the main difference being that the sea level was much lower when so much of the fixed quantity of water on earth was locked up in the glaciers. This means that dry land extended nearly to the edge of the continental shelf off New England. Although any life would be very limited under a mile of ice, between glaciations and at other places in North America human beings were around and plants and animals would look familiar.
When we take the major step back to the Triassic period over 200 million years ago the world would look very different. The continents were joined into one mass, Pangaea II, 230 million years ago and by 190 million years ago they split into two land masses, Laurasia and Gondwanaland. Over this period of time the land that is today North America was astride the equator. Two hundred and ten million years ago the equator went through the present mouth of the St. Lawrence River and crossed the other edge of the future continent around Oregon. Twenty million years later this land mass had moved north so that the equator went through northern Florida and the north of Mexico.
There weren’t any flowering plants but there were forests of Gymnosperms and ferns and the first dinosaurs and primitive mammals were just appearing. Just before this story starts all of what will be Connecticut was made of fairly continuous metamorphic rock, now about 600 million years old.
Two hundred and ten million years ago a large block of the metamorphic rock, extending from the eastern edge of the Connecticut valley to west of the Hudson River began to subside to form a large graben or trough. It is believed that this occurred in response to the opening up of the Atlantic Ocean as Pangaea II split up. The land on either side of the graben rose while the land in the center continued to sink. In the end the vertical displacement between the top of the metamorphic rocks to the east and west and the bottom of the graben was over three miles (see illustration).
Over the next ten million years erosion of the high land rocks to the east and west brought sediment into the graben. There the sediment built up to form the New Haven Arkose which consists of pinkish, gray, brown and reddish arkosic sandstone and conglomerate with interbedded layers of reddish siltstone. Exposures of this stone along Wintergreen Brook in the Nature Center range from dark purplish red to lighter brick red. Just above the Lake the bedrock is gray sandstone. Just as happens today, the flow of water over distance sorts out particles by size so that larger boulders stayed near the outside edge of the graben while finer particles were carried further.
About 200 million years ago there were some episodes of volcanic activity. In this area the magma was trapped between the layers of the sandstone or pushed its way into vertical cracks. Trapped between the sandstone, the magma cooled very slowly and formed the igneous rock called diabase which makes up West Rock.
Several million years later movement within the earth caused an uplifting in the center of the graben which was accompanied by a subsidence of the edges. This created cracks and fissures in the sandstone and diabase layers as well as tilting of the layers toward the east in what had been the east (present day directions) of the graben and toward the west at the other side. This tilt is evident at the outlet of Lake Wintergreen and also along the brook just south of the Nature Center entrance. There was at least one more period of volcanic activities which intruded magma through a vertical crack in the West Rock sill to form the buttress dike at the south end of Lake Wintergreen.
While this was happening dinosaurs were at their zenith. The tracks which are preserved at Dinosaur State Park in Rocky Hill were formed as these large animals walked on layers of drying mud and sand in former lakes in the graben. During this time there were flying reptiles, small mammals and the first birds appeared.
Erosion, a constant and ever present force, continued through the Jurassic and Cretaceous periods and wore down the center of the uplifted area so that the deposits of sandstone and diabase from the Triassic period are completely worn away from the western highlands of Connecticut and New York east of the Hudson River. In the West Rock area the erosion was able to wear down more of the sandstone than of the diabase which led to the general positions of ridges and valleys which the glaciers covered but probably didn't change very much as far as general location.
Following the 20 million year period of intense and dramatic large scale movements in the earth of this area, there was a much longer period, until the Great Ice Age which began two or three million years ago, with less dramatic movements and the continuous work of erosion shaping the land. During this period North America drifted slowly north until it was in its present relationship with the equator. Dinosaurs became extinct, flowering plants appeared and diversified and the animal world evolved toward what we know today.