Joints are more conspicuous of the two types, and typically belong to a general system so that at a given location they tend to be parallel. On top of Owlshead, for example, the most prominent joints trend N.25°W. (read: North twenty-five degrees to the west) with dips[2] that are vertical or dipping steeply to the southwest. Another set of joints trends N.10°E. with dips that are vertical or dipping steeply to the southwest. Joints represent the breakage of the rock due to stress and strain. Some joints result from tensional forces set up within the rock itself by contraction due to cooling of the originally hot solidified rock. Other joints result from larger-scale forces within the earth’s crust which cause earthquakes and general movement of land masses. An exhaustive study of all the rocks in a large area would be required to determine conclusively the origin of the joints on Owlshead. In addition to the nearly vertical joints, a third set of nearly horizontal joints may be observed on cliffs. These joints are called _sheeting_ and apparently are related to the depth from a former topographic surface which existed at the time the sheeting originated. The vertical joints and sheeting are important qualities of a rock to be considered in choosing a rock for commercial quarrying. Not only do these factors effect the ease of quarrying, but they also determine the amount of waste material which would have to be removed and discarded because of poor size and shape. Exfoliation is the term for breakage due to the disintegration caused by decomposition of the rock on surfaces exposed to the weather. It is characterized by the scaling off of concentric shells of altered rock to produce a convex surface. Rocks showing exfoliation surfaces are not common at Groton. One of the best developed exfoliation surfaces, illustrated in Figure 2, occurs at the base of the cliffs on the south side of Owlshead Mountain. Once joints have formed, they are enlarged by weathering. In particular, rocks are pushed apart by a “frost wedging.” When water freezes it expands by about one-tenth of its volume. If it is confined it may exert a pressure of as much as 138 tons per square foot. In this manner, huge blocks may be pushed apart. If they are at the edge of a cliff, or part of the cliff itself, they may eventually break off and fall to the slope below. The accumulation of broken rock at the base of a cliff is called _talus_. _Origin and age of the granite_