Fig. 250.—A wave-cut chasm under control by joints, coast of Maine (after Tarr).
Coves, sea arches, and stacks.—It is the headland which is most exposed to the work of the waves, since with change of wind direction it is exposed upon more than a single face. The study of headlands which have been cut by waves shows that the joints within the rock play a large rôle in the shaping of shore features. The attack of the waves under the direction of these planes of ready separation opens out indentations of the shore ([Fig. 250]) or forms sea caves which, as they extend to the top of the cliff by the process of sapping, yield the coves which are so common a feature upon our rock-bound shores ([Fig. 259], [p. 238]). With continuation of this process, the caves formed on opposite sides of the headland may be united to form a sea arch ([Fig. 251]).
Fig. 251.—The sea arch known as the Grand Arch upon one of the Apostle Islands in Lake Superior (after a photograph by the Detroit Photographic Company).
A later stage in this selective wave carving under the control of joints is reached when the bridge above the arch has fallen in, leaving a detached rock island with precipitous walls. Such an offshore island of rock with precipitous sides is known as a stack ([Fig. 252]), or sometimes as a “chimney”, though this latter term is best restricted to other and similar forms which are the product of selective weathering ([p. 300]).
Fig. 252.—Stack near the shore of Lake Superior.
Whenever the rock is less firmly consolidated, and so does not stand upon such steep planes, the stack is apt to have a more conical form, and may not be preceded in its formation by the development of the sea arch ([Fig. 260], [p. 239]). In the reverse case, or where the rock possesses an unusual tenacity, the stack may be largely undermined and stand supported like a table upon thick legs or pillars of rock ([Fig. 253]). In [Fig. 254] is seen a group of stacks upon the coast of California, which show with clearness the control of the joints in their formation, but unlike the marble of the South American example the forms are not rounded, but retain their sharp angles.
