7] Sand-Boulders in the Drift in Central Missouri. 921 
“the broken ice-carriers crushed in the jams of many an ice-pack. 
The presence of the sand-boulders forced upon their edges, and 
often broken, bears evidence of the violent action of the wind 
and waves upon the ice. These floes were stranded or sunken 
_in waters which derived at least a part of their mud from the 
forth, The boulders were generally rounded before their strand- 
ing, for often the clay between them does not show any sand 
derived from the removal of their angularities; yet broken frag- 
_Ments may be seen (as at B in sections). Owing to their porosity 
_ the amount of ice necessary to lift the masses would not be many 
_ times their volume (except at jams they are rarely less than sev- 
_ tal feet, at least, apart), as their buoyancy would be greatly in- 
_treased by the frozen water within them; and we know that the 
Waters of the Arctic seas freeze to a depth of seven to eight feet 
during the long winter, and fresh water somewhat deeper. The 
: unbroken boulders could not have been pushed along, nor could 
they have been formed so as to have been transported upon the 
back or within a glacier. These deposits were probably made 
ina shallow arm of the sea (cut off by the Ozark ridge rising a 
ad hundred feet higher); yet the waters may have been either 
sega or even fresher,—as in the Gulf of Obi to-day,—owing 
i! 
the amount of fresh water pouring into it; and not in a glacial 
take held back by an ice-dam south of a vast, extensive lake. 
x » We know that the Mississippi Valley has undergone 
‘onsiderable oscillation and warping since the Tertiary period. 
; a In conclusion, the presence of sand-boulders of northern 
~ S10, as well as the erratics, deposited in a somewhat stratified, 
i ted clay, is a demonstration that the Drift of Central 
Souri Is of subaqueous origin. 
ited E. Andrews has observed similar sand- and gravel-boulders in the 
; à which the lake tunnel was driven at Chicago. Some of these con- 
wo sea weet Other pockets are fain at Homewood CT 
from panes elsewhere. However, the Drift in Northern Illinois aifiers 
i Ssouri, and contains less free sand, and an absence of the abundance 
TE Sand-boulders seen at the latter place. 
