Another structural feature of the schists is the breakage of the rocks along definite plane surfaces called _joints_. These usually occur in systems formed by a number of parallel joints. The joints formed as a result of stress and strain operating on the rocks during periods of mountain-making and vertical uplifts. Information as to the nature of these forces might be obtained if all the joints were carefully recorded and plotted on a map. On Mount Mansfield some of the prominent topographic features appear to be controlled by joints. Much of the north-facing cliff on the Nose is controlled by a joint trending N. 65° W., and a similar face on the Lower Lip is controlled by a joint trending N. 60° W. Along the crest of Mount Mansfield a number of joints trend about north-south. Joints of this system in the steep cliffs on either side of the crest of Mount Mansfield have been separated further by the tendency of the rocks to creep down slope under the force of gravity. These joints form the canyons or narrow passageways which are traversed by some of the trails. On Maple Ridge at about 3300 foot elevation the trail crosses a joint trending N. 50° E. which is conspicuous for its four-foot width and the extent and the straightness of the break. A number of joints belonging to this system are found along Maple Ridge. GLACIAL HISTORY OF THE AREA _Introduction_ The geologic time division previous to the present one is called the Pleistocene or the “ice age.” During this time, large continental glaciers advanced over the northern part of North America several times. The cause of the ice age is not known with certainty and whether geologic history will repeat itself is a matter of conjecture. However, it is established that these vast ice sheets covered New England and that the last ice sheet melted back from the Mansfield area about 12,000 years ago. If one stands on the crest of Mount Mansfield and looks westward over the Champlain Valley, it is difficult to visualize this entire valley completely filled with ice of the continental glacier. Yet, the evidence shows that the ice sheet was so thick that it completely covered Mount Mansfield at one time. _Evidences of glaciation_