now be lifted the old sea level would be indicated by the 

 undermined sea cliff with the flat terrace in front. These 

 wave cut notches are not uncommon along the Maine coast 

 where ever resistant rocks face the sea. Sometimes they 

 occur one above the other like a series of giant stairs; the 

 record of each pause in the retreat of the ocean. On Mount 

 Desert Island these levels extend from 20 feet above the pres- 

 ent sea level to near to the mountain tops at 1500 feet. 



Rocks vary greatly in their resistance to wave attack. 

 Sometimes the character of the rock itself enables it to with- 

 stand the buffets of the sea. Again the bedding plains or 

 joint structure may cause rapid erosion in some places and 

 slow erosion in others. This difference in the rate of ero- 

 sion induces irregularities in the typical form of the sea cliff 

 and terrace. Isolated masses of rock are sometimes left 

 standing on the terrace. These may reach the size of an is- 

 land like Boneventure and Perce Rock on the Gauspe' coast. 

 More often they are small rock stacks which will soon yield 

 under continued attacks of the sea, unless preserved by up- 

 lift. On the seaward side of Day Mountain on Mount 

 Desert Island are several well developed rock stacks iden- 

 tical with those forming today on the shore several hundred 

 feet below. 



Along our Maine coast our granites are often cut by dikes 

 of basalt. This latter rock is less resistant than the sur- 

 rounding granite and so yields to erosion. A chasm or 

 purgatory is formed often scores of feet in depth with walls 

 of solid granite. The surging of the waves in and out of the 

 chasm give rise to the phenomena known as sprouting 

 horns, blow holes and thunder caves. Lifted above^ sea 

 level these chasms furnish another index of elevation. 



The last episode in our geological history was the re- 

 treat of the great ice-sheet. This left the rock surface of 

 our state covered with debris from the melting ice. Within 

 reach of the waves this cover was soon washed off leaving 

 bare rock. The upper limit of this washed surface shows 

 the highest sea level since the glacial period. 



The sea is not only destructive but also does a certain 

 amount of constructive work. The fragments from the des- 

 truction of the land are worked over by the waves and cur- 

 rents. The larger pieces are left as pebbles near the shore 

 line, the finer sand is carried farther out where motion is 

 less, while the finest material or clay is deposited in quiet 

 water. If these marine deposits are now found on land 



91 



