A good place to start the study of geology is with the surficial geology maps of the area under study. Aside from differences in buildings and roads due to different dates of the maps, the major difference is that instead of using colors to show house omissions (pink), woods (green), and open area (white), the geologic map uses colors to illustrate the different materials of the surface of the land. An understanding of the five different types of material and why and how they occur is important to this study. The majority of the Wintergreen Brook watershed above the Nature Center is composed of glacial till. Also present are bedrock, of two distinct types (sandstone and trap rock), ice-contact stratified drift, artificial fill, and outwash sands and gravel. The last type of material is not shown on the quadrangle maps, but is shown on the Water Authority report on Lake Wintergreen.
The first step back in geological time is to the period 15-20,000 years ago when this area, like all of New England was buried under a one mile thick sheet of ice. As the ice advanced from the north it scoured the surface of the land, removing softer material, pushing it on ahead and changing the shape of harder material. The large boulder which makes Judges’ Cave is an erratic or rock that was brought there by the glacier. Just north of there are striations made on exposed bedrock by glaciers.
The fact that West Rock is still much higher than the land near the brook indicates that there must have been different materials in each place when the glacier pushed through. It exposed the harder bedrock of West Rock (now covered in some places by a thin soil made by the soil building processes of water, weather and plants since the glacier melted). In the lower areas the glacier left the various types of sediments that are indicated on the map. Glacial till, indicated by green color on the map, is an unsorted compact sediment containing many sizes of particles. In an exposed area of till, students could see how many different kinds of rocks they can find and see that they are not like the two types of exposed bedrock in this area.
The glacially placed material indicated by pink on the geologic map is ice contact stratified drift. (Drift is an old word applied to glacially deposited materials. It derives from the earlier belief that these materials were deposited during Noah’s flood. The theory’s changed, the name remains.) This material is different from the till in that the different sizes of material, sand, gravel, and larger stones occur in layers roughly sorted by size. This occurred because, as the name implies, these sediments were deposited in contact with the glacier and the alternate freezing and thawing as the glacier retreated over the years produced these layered deposits.
The only location of ice-contact stratified drift upstream of the Nature Center is just west of the bend in Wintergreen Avenue just below the lake. This area is covered with vegetation so that the stratified nature of the deposits is not visible. Several slides of other areas will illustrate this type of deposits.
The outwash sands and gravel occur along the brook above the lake and the artificial fill occurs to create the dam which holds Lake Wintergreen, around the entrance to the Nature Center, and under the parkway.
One of the most interesting places, geologically, in this area, is the place where the water leaves the lake. It is near the middle of the east side of the lake. To the south of the exit the water is held in by a natural bedrock dike of trap rock. To the north of the exit the water is held in by the man-made dike (dam) made from blocks of the trap rock and earth. The land falls away quickly from the lake here, making for a beautiful waterfall as the water becomes a brook again. In the cut of the falls, the red sandstone is visible, in layers, with the glacial till on top of it. This is below the trap rock which forms the dike.
Seeing this leads us back in time to find out how these two different types of bedrock came to be in such close association here.
