CRITERIA FOR AQUEO-GLACIAL SEDIMENTATION. 4] 
H. Tidal scour would efface seasonal banding. Where the water of the 
sea is deep enough and the tidal action on the bottom very mild, seasonal band- 
ing might be found in marine deposits. 
I. Seiches in lakes may have an influence on banding. A piling up of 
the water at one end of a lake by the wind might increase the depth several 
feet and in this way affect the sedimentation to some extent. At the opposite 
end of the lake the water would be slightly lower, during this time, so that sedi- 
mentation might be slightly affected there. Seiches due to other than wind 
effects cannot be considered here. (See Darwin, 1898, chapter 2). 
J. Earthquakes are known to stir up bottom mud. Irregularities would 
be produced in this manner. (See Shaler, 1888, p. 408-419). 
K. In Alaska, Tarr (1909) noted that the glacial streams sometimes rise 
suddenly without apparent cause. He suggested the breaking of a glacial dam. 
The falling of large ice blocks in the subglacial tunnel might very well dam the 
torrent temporarily. When the water overcame such a dam the torrent would 
rise suddenly as observed. The action of earthquakes would precipitate or 
cause the falling of ice blocks, although blocks would fall and do fall without 
such a cause. I observed a large fallen block in the tunnel of the Roseg glacier 
in Switzerland, in 1913. 
L. Apparent lack of banding may be observed in glacial clay deposits. 
Is there a lack of banding in such deposits? In much of the slate underlying 
the tillite at Squantum Head no banding is apparent on the weathered or fresh 
specimens. Microscopic sections, however, reveal just as regular banding as 
can be found in any of the slate at Squantum. The reason for the invisibility 
of the banding to the naked eye is the fact that all the sediment is so fine and 
the difference between the texture of the coarse and fine layer is so small that it 
cannot be seen. Whether this explanation would account for some apparently 
unbanded Pleistocene clays remains to be proved. 
When clays are over-ridden by advancing glaciers, the banding is often 
so crushed and contorted that it may not be apparent. The slate under the 
tillite at Squantum Head has been crushed in just this way, and this may con- 
tribute to the invisibility of the bands, but it is not thought to be the main 
cause. 
| 
| 
| 
1 
ql 
