210 



PHYSICAL GEOLOGY 



Fig. 196. — Perce Rock, Gaspe, Canada. (Photo. S. Powers.) 



sea level and the roof more than 50 feet above. Sea caves are ex- 

 cellent indicators of ancient sea levels (p. 214). 



Sea caves occasionally extend inland and open to the surface of the 

 ground, sometimes behind headlands one hundred or more feet in 

 height, and at considerable distances from the shore. During quiet 

 weather these openings appear on the surface as deep wells, but during 

 storms the water is sent through them with great force, sometimes 

 throwing spray high into the air, and they are consequently known 



as blowholes, spouting 

 horns, etc. Blow- 

 holes are sometimes 

 formed simply by the 

 landward extension 

 of sea caves whose 

 bottoms, as well as 

 roofs, usually have a 

 strong upward in- 

 clination inland. 

 They are also formed 

 when, in the land- 

 ward cutting of a 

 cave, a vertical joint 

 is encountered which, 

 when enlarged by hydrostatic pressure and the compression and 

 expansion of air, is drilled to the surface. 



Arches are not uncommon features on some coasts. They are 

 formed (1) by the uniting of two caves on opposite sides of a head- 



[97. A sea arch. When the roof falls the point 

 will become an island. (De Martonne.) 



