28 ON THE FRESH- WATER GLACIAL DRIFT 



force of the wave is greater in northern waters than in southern, being more dense. 

 The wind is more powerful, and storms are more frequent. Slides occur there in 

 the same manner as upon Lakes Erie and Michigan, carrying down standing trees 

 and houses. On the waters of Bad river, Ashland county, Wisconsin, slides fre- 

 quently happen at the high bluif banks, precisely like those in the bends of the Cuy- 

 ahoo-a river in Ohio. In the blue, the red, and the purple clay beds there are occa- 

 sionally inclosed patches of sand and gravel, not stratified ; but this is not common. 

 The sand and gravel are generally in layers between beds of clay, but tapering 

 out in distances of no great length. There are also bunches of clay inclosed in 

 the strata of sand, though such cases are rare. 



Boulders Moved hy Ice. 



Around the borders of small northern lakes, it is not unusual to see a line of 

 boulders compactly arranged at the water level. They are usually too large to be 

 moved by the action of waves, and are pushed up along the shore, so as to present 

 from the water the appearance of a rude wall or fence composed of rounded rocks. 



More than fifty years since, President Dwight, of Yale College, described the 

 movement of boulders towards the shore in a small lake in Salisbury, Connecticut. 

 I once examined the place in company with the late Professor Averill, of Union 

 College, who lived in the vicinity. He had often observed them near the shore, 

 and near the surface of the water, and was satisfied of their motion towards the 

 land. 



Where the top of the boulder was within a foot of the surface, there was a dis- 

 tinct groove behind it, its direction being in a right line for the land. In front 

 of it the mud was pushed up, showing that it had been forced in that direction. We 

 concluded that the ice in winter was equal to one foot in thickness, and generally 

 more. 



As in all bodies of water, there is here some fluctuation of level. Ice a foot 

 thick or more would envelop the upper part of the boulders in shallow water, and 

 extend down around them below the general thickness. The increase in bulk by 

 freezing must take place from the centre outwardly, and thus create a slow but 

 powerful motion at the edges towards the shore. 



Lines of boulders may be seen on the banks of the St. Lawrence, between Mon- 

 treal and Quebec, pushed against the shore in part by the expansion of fixed ice, 

 and in part by masses of floating ice. There is in Iowa a small lake, which is 

 belted by so strong a line of boulders, that it was at first supposed to be an arti- 

 ficial fence or wall of stone, and which has thence taken the name of the " Walled 

 Lake." 



On the shores of MiUe Lac, which is at the source of the Eum river, in Min- 

 nesota, and which is about twenty mUes in diameter, there are very heavy lines of 

 large boulders, rising five and six feet above water-level. There are also several 

 small islands in this lake, at difl'erent distances from the shore, composed entirely 

 of large boulders, generally more than two feet in diameter, which have accumu- 

 lated in the sanje way with those on the shore. One of these has a height of twelve 



