DRIFT DIVISION. 
163 
horizontal plane nearly, as far as the eye can reach, like a shore where the banks above had 
been washed away by the waves, and left the boulders and blocks that had been imbedded 
on the shore. A prominent example of this kind may be seen on and from the trail, from 
Fort Renville at the foot of Lac qui Parle, to Lac Travers, on the terrace of the High prairie. 
Where the prairie itself is on the same level as the line of boulders, it is so thickly strewed 
with boulders and erratic blocks, that a person may walk on them for considerable distances 
without stepping on the ground. Boulders are now being deposited in a manner perfectly 
similar in appearance to these belts and flat areas, around the shores of that lake, by means 
of ice. The ice encloses the boulders and blocks in shoal water, and in the spring, the swell 
of the lake, by the melting snows and the rains, float off the masses, and the wind drives the 
ice on the shores and on the marshes around the lake, where it melts, and deposits its freight 
of rocks, gravel and pebbles. This is a prominent feature on almost all the lakes far to the 
north, where they are surrounded by drift deposits, or where loose rocks had been by any 
means left in shoal water. They are piled up along the shore in great numbers.* 
Boulders and blocks are very common in many parts of the country, collected in groups, 
where, in a small area, are hundreds or thousands, and sometimes hundreds of thousands 
of tons of them; while all around, few or none are observed. Col. Abert, Chief of the 
Topographical Bureau at Washington, first called my attention to localities of these trains of 
erratic blocks in the United States in 1835. He had seen them thus grouped on the prairies 
in the central parts of Illinois, and I have since seen similar facts on the northern prairies 
in Iowa ;t on the Coteau de Prairie ;X on the terraces two hundred or three hundred feet 
above Lake Erie, in Ohio; on Long island, and in some of the valleys of the Highlands. 
They have been long known in Europe. They often form a striking feature of the country. 
Boulders and Erratic Blocks. 
The term boulder is applied to rounded masses of rocks, that are supposed to have been 
worn to their rounded forms by attrition. The term is, by common usage, applied to masses 
from six inches diameter, to the largest sizes ; but many of the large rounded masses called 
boulders, have received their forms by the atmospheric causes producing disintegration, by 
which the edges of angular blocks crumble more rapidly than the other portions. I have seen 
this effect produced so frequently, that there is not the slightest doubt, that many of the 
rounded masses described as boulders, owe their forms to this cause.^ It is not doubted, 
* These facts in relation to an ancient coast line, but subsequent to the period of the drift deposits, should have been men¬ 
tioned under the quaternary deposits ; but there are many more facts somewhat analogous to these, that properly belong to the 
drift for discussion. 
t Featherstonhaugh’s Geological Report, 1836, p. 141; Mather’s MSS. Geological Report to the Secretary of War, 1836. 
t Mather’s MSS. Geological Report to the Secretary of War, 1836. 
^ This change may be seen well illustrated in Foster (R. I.), on the road from Sterling (Conn.) to Providence (R. I.), in a 
range of trappean rock a few rods east of the principal branch of the Moosup river. (Vide Mather on Diluvial Deposits, 1835, 
