356 FREDERICK W. SARDESON 



in which a start could have been made by those masses which were 

 carried much farther, is shown. 



It may also be argued that a series of masses could, under repeated 

 cause, lodge one behind the other. An explanation for the entrapped 

 drift- wedge, D, in the section (Fig. i) may be that the block bb was 

 set forward, and a gap behind it filled with till before the main mass 

 started. As to general application of this particular case, any mass 

 of till, resting on a lamina of clay, might be started and then rotated 

 forward in like manner; but with no stratified rocks as a distinctive 

 mark, such cases might be difficult to interpret. 



Evidence tending to prove that the glaciers have elsewhere plowed 

 up the bed-rock of the region is not rare. It has been seen in the 

 excavations for buildings, stone quarries, and road-gradings in 

 Minneapolis and St. Paul, where the underlying rock is of Ordo- 

 vician age, and is exposed along the gorge of the Mississippi. Else- 

 where it is generally concealed by the glacial drift. In these exposures, 

 blocks of limestone are often seen scattered or grouped in the bowlder- 

 clay, as if the rock had been torn up in large masses, and are so 

 exactly like the rock which exists in situ in this region as to be unmis- 

 takable. While they might have come from miles away, so far as 

 the kind of rock is concerned, they are more probably torn from the 

 sides of preglacial valleys now buried under the glacial deposits 

 in this vicinity. Of more particular importance here are certain 

 clay-shale masses which occur in the bowlder-clay. By the character 

 of the clay and of the included lenticular masses of crystalline lime- 

 stone, and by the contained fossils, the original position of these 

 clay-shale masses may be determined. One or more of the beds 

 whence they came are still in place over a great part of this area, 

 under the drift and above the limestone. The shale-masses in the 

 drift have presumably been torn up not many miles from the place 

 where they now lie. 



These drifted clay-shale masses are generally not weathered, 

 and retain their original blue color. They are often unmixed with 

 pebbles of northern drift, and in a few cases they retain their original 

 stratification, as well as their fossils. Small masses are most common 

 and least distinct. The largest are ioo feet or more in horizontal 

 diameter, and 50 feet or less thick. They generally have upturned 



