The granite originated in the interior of the earth many million years ago as a molten mass, called _magma_. This magma moved upward through the earth’s crust by a process of melting the pre-existing rock or by forcefully pushing it aside. When it reached its present position it became cooler and minerals began to crystallize out. However, as is shown in Figure 3, it is important to understand that the surface of the land was not in its present position and that the magma actually cooled beneath a considerable thickness of other rocks. These overlying rocks, now gone, acted as an insulator and prevented the magma from cooling too quickly. If the magma had risen through these rocks and reached their upper surface, it would have formed a lava flow similar to those of present-day volcanoes and would have cooled much more rapidly. Rocks formed near the surface are characterized either by being fine-grained without visible crystals or by having a few large crystals in a fine-grained matrix; they never have a uniformly, medium-to-coarse-grained texture. Thus, the texture of the granite at Groton State Forest proves that it cooled slowly and indicates that, at the time of cooling, the granite was not at the surface. This is a reasonable postulation because studies of the regional geology indicate that a large amount of rock has been removed from this area by erosion through the long periods of geologic time. [Illustration: Figure 2. Exfoliation surface on the south side of Owlshead Mountain. Joints, of the sheeting type, are visible in the granite cliff. The decomposition of the granite by weathering in the niches has resulted in small patches of soil. The boy in the upper right gives the scale.] [Illustration: Figure 3. Sequence of events at Groton State Forest shown diagrammatically.] [Illustration: A. The Waits River Formation and other younger formations are deposited from a shallow sea during Ordovician time.] YOUNGER FORMATIONS WAITS RIVER FORMATION [Illustration: B. The sedimentary rocks are folded and metamorphosed and the granite is intruded into the older rocks and crystallizes during Devonian time.] WAITS RIVER FORMATION GRANITE [Illustration: C. Erosion removes much of the rock from the area.] GRANITE [Illustration: D. During the ice age, continental glaciers move over the land and erosion by the ice forms Owlshead Mountain and the basin for Groton Pond.] ICE GRANITE [Illustration: E. Present topography, exaggerated.] OWLSHEAD MOUNTAIN RICKER MILLS