[Illustration: Figure 5. Diagrammatic cross sections illustrating the geologic history of the Calvin Coolidge State Forest Park. Vertical scale greatly exaggerated. Patterns to designate formations in cross section A are repeated in cross sections B and C.
A. Horizontal layers of early paleozoic sedimentary rocks were formed. This cross section extends beyond the limits of the Park area. Thicknesses of rock formations are approximately proportional.
B. During late Ordovician or Devonian time these sedimentary layers were folded and the rocks metamorphosed.
C. Calvin Coolidge State Forest Park as it appears today.]
A MISSISQUOI FORMATION BETHEL FORMATION OTTAUQUECHEE FORMATION PINNEY HOLLOW FORMATION B C SHELTER VILLAGE SLACK HILL
The conditions under which the Bethel formation was originally deposited are similar to those which existed during Pinney Hollow time. Shale, sandy shale and thin-bedded sandstones formed under shallow water marine conditions; however, restriction with accompanied organic accumulation did not occur. This formation is thought to be of either Cambrian or Ordovician age.
The easternmost, and therefore youngest formation to be found in the Park area is known as the _Missisquoi formation_. This formation, which is only partially represented here, is made up of dark phyllite, light blue-gray schist with conspicuous crystals of garnet and biotite, and gray quartzite. The rocks of this formation can be seen along Broad Brook which flows north along the eastern border of Coolidge State Forest Park.
This formation is considered Ordovician in age with the conditions of deposition being quite similar to those which existed during Ottauquechee time. Once again the ocean waters were restricted and organic-rich black muds accumulated in quantity on the ocean floor.
The early geologic history of Calvin Coolidge State Forest Park is known from the rock formations discussed in the previous section of this pamphlet. The Pinney Hollow, Ottauquechee, Bethel and Missisquoi formations of Cambrian and Ordovician age were deposited, essentially horizontally, from the shallow seas which covered this area 550 to 450 million years ago (See cross section A, Fig. 5). During some of this time these seas were at least partially restricted as evidenced by the Ottauquechee and Missisquoi black phyllites. Other Ordovician formations were deposited on top of these four formations, but have been completely stripped away by erosion during the millions of years since their deposition. Some of these later Ordovician sediments contain great thicknesses of volcanic rocks which probably covered the Coolidge Park area.