CRITERIA FOR AQUEO-GLACIAL SEDIMENTATION. 37 
1905, p. 184, 185; David and Priestley, 1908, p. 305, 306). If the wind action 
came at regularly recurring periods there should be regularly recurring deposits 
of wind-blown material in the clay layers. Loess deposits often occur adjacent 
to glacial deposits (Huntington, 1914, p. 575-577). 
> If vegetation flourished near the ice front, dead leaves, twigs, seeds, and 
spores would be washed by streams emptying into the basin of deposition, and 
settle to the bottom. On account of the freezing of such streams during the 
winter, such material would not be expected often in the winter components of 
banding. Emerson (1898, p. 706-710) has found such vegetation in the coarse 
components of the Connecticut River clays, and Coleman (1902, p. 71-79) 
found similar vegetation in the coarse layers near Toronto. None of this 
organic matter was found by them in the winter components, although such 
material might be found in the winter components exceptionally.' 
2. FACTORS CAUSING IRREGULARITIES IN BANDING. 
The conditions of regular banding have now been discussed briefly. It 
is seen that to have regular banding at all, the disturbing elements must be 
as nearly absent as possible. In a visit to almost any glacial clay deposit it 
is evident at once that although regularity of interval between the layers is 
the rule, there may be interruptions of various kinds in the regularity. 
A. Even in locations where the disturbing factors are at a minimum the 
grounding of icebergs may be a common occurrence. The ice in coming to rest 
drags over the bottom, destroys some of the upper layers and contorts many 
more lower down. Contorted zones produced in this way are the most com- 
mon irregularities in otherwise regularly banded clays. Glaciated rock-frag- 
ments are very often found mixed in such contorted zones and a majority of 
such fragments are found on the top of the contorted layers sometimes in a 
till deposit. In some cases the contorted zones show evidence of actual ice 
advance. The contortions may be completely cut off on top and the con- 
torted zone itself may have folds eight or ten feet or more from the tops to the 
bottoms of the arches, with a thin layer of till at the top. 
The deformation of layers, intercalated between horizontal layers of the 
clays, may be due in many cases to shearing produced by ice, well above the 
1It might be noted here that the extremely thin coaly laminae in parts of the Ecca shale, which 
lies conformably on the Dwyka tillite of South Africa, might have originated in the same manner as the 
peaty layers in the glacial clays. 
