
GEOLOGICAL SURVEYING 311 
thoroughly dried and crumbled down, when the shape and 
nature of the larger fragments can be studied with the help 
of a lens. These will be found to be simply minute boulders, 
angular, subangular, and often striated, and quite unweathered. 
The “clay” may be still further reduced by shaking it in 
water and passing it through a sieve. By using sieves of 
various degrees of fineness, all the gritty particles above a 
certain size may be sifted out, and only an extremely fine- 
grained residue be left. The grit, examined microscopically, 
is found to resemble in every respect, save size, the small 
fragments which the student may have determined with the 
aid of his pocket-lens. They all alike consist of fresh, 
unweathered mineral matter. The residue which is not 
separated by the finest meshed sieve may be reduced by 
shaking it in water and allowing it to settle. From the 
turbid water a sediment is gradually thrown down. The 
water which still remains clouded can then be decanted and 
allowed to stand until it clears. In this way we obtain a still 
finer grained mechanical precipitate. These sediments are of 
precisely the same character as the gritty materials separated 
by the sieve—they are fresh and unweathered, being com- 
posed of what may be termed “rock-flour,’ the chief 
constituent of which is powdered quartz. It is this “rock- 
flour” that forms the major portion of the so-called boulder- 
clay—the proportion of true clay throughout the mass 
appearing to be relatively insignificant. Indeed, according 
to Professor Crosby, “till in its natural condition is often less 
than one-tenth and rarely more than one-eighth pure clay.” 
Boulder-clay is believed to be the bottom- or ground- 
moraine of massive glaciers or ice-sheets—and to have been 
formed by the crushing and grinding action of ice in, motion. 
Formed and accumulated under these conditions, we can 
readily understand why it should consist essentially of fresh, 
unweathered rock-materials. But it is beyond the purpose 
of these notes to give any particular account of this remark- 
able formation. It may, however, be of service to the 
field-observer to indicate certain points which ought to be 
noted when he begins to map in a till-covered region. First, 
- then, the configuration of the surface should be considered. 
Sometimes the ground in such a region is devoid of any 

