234 



EARTH FEATURES AND THEIR MEANING 



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 contin- 

 uation of this process, the caves 

 formed on opposite sides of the 

 headland may be united to form 

 a sea arch (Fig. 251). 



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 



FIG. 251. The sea arch known as the 

 Grand Arch upon one of the Apostle 

 Islands in Lake Superior (after a pho- 

 tograph by the Detroit Photographic 

 Company). 



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). 



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 forma- 

 tion 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 



FIG. 252. Stack near the shore of Lake 

 Superior. 



