TorociKArnv i'uo.m wavk kuosion 215 



clay or break tlie supporting pillars by tbcir mere inomeiitum. On 

 Devils island tbe caverns number two score or more, and a continuous se- 

 ries of them are presented for a distance of a mile. Their chief interest lies 

 in their fantastic architecture. Caverns also occur on Prescpie isle, Sand 

 island, and others of the Apostle group. They are also found on the 

 mainhind, especially along the shores of Chippewa point. 



CO VES 



On exposed sandstone cliffs, coves are formed by erosion along joint 

 l)lanes. Master joints with a northeast-southwest trend are common in 

 the sandstone. These joints stand head on to the most violent storms, 

 those which come from the northeast. The waves gradually open these 

 joint planes until a cove is finally made. A typical cove is a narrow 

 canyon-like chasm, not exceeding 10 feet in width. Its height, which 

 depends on that of the cliff, may be 30 or 40 feet. The cove extends 

 back, following the course of the joint for a distance of several hundred 

 feet, its walls graduall}^ contracting as they are removed from the action 

 of waves. The floor of the cove slopes upward gradually until it reaches 

 the top of the cliff. Such a floor is an inclined plane, up which the 

 water is forced with great violence, bearing with it coarse shingle and 

 boulders, which are finally deposited on top of the cliff. It is common 

 to find a mass of boulders at the shore end of these coves. The best 

 example of this feature is Stella cove, on the east shore of Presque isle. 



CLIFFS 



The loftiest cliffs of the region are those composed of glacial drift, 

 chiefly boulder-clay. An impressive example of this type is seen on 

 the north end of Oak island, where the cliff reaches the height of nearly 

 200 feet. The rate of wear on exposed clay cliffs is rapid, in spite of the 

 cliff beaches at their base. None of these cliffs api)roach the perpen- 

 dicular, but they maintain a high angle of slope, frequently exceeding 

 40 degrees. The sandstone clifis are less common ; they reach a maxi- 

 mum height of 60 feet. As a rule, these cliffs are perpendicular, their 

 recession being due to undermining of the base by wave action. Less 

 than one-fifth of the shoreline is bordered by sandstone. 



BENCHES 



At the base of cliffs, benches are found. Those at the foot of sand- 

 stone cliffs are narrow, rarely exceeding a width of 10 rods, and aver- 

 aging much less than this. From the foot of the clay cliffs a bench 

 extends for a distance, in some cases, of several hundred rods, as is well 

 exhibited on the east shore of Madeline island. These benches become 



