SEASON AND TIME ELEMENTS IN SAND-PLAIN 
FORMATION 
THE SEASON ELEMENT 
Tue term sand-plain is here used in the generally accepted 
sense, that is, to designate the sand delta formed at the mouth 
of a glacial stream as it issued from the ice margin into the stand- 
ing waters of a glacial lake or into the sea. The bulk of such a 
plain is made up of a succession of layers of fine material slop- 
ing at an angle of 15° or more in the direction in which the cur- 
rent is moving, or away from the head of the plain. Upon these 
layers, which are known as fore-sets, rests a comparatively thin 
and nearly horizontal layer of coarse material known as top-sets. 
A third class of layers is represented by the horizontally strati- 
fied clay beds deposited in front of the constantly advancing 
fore-sets, and partly overlain by them in consequence of this 
advance. In addition there is sometimes a fourth, but much less 
extensive series of layers which have slopes just the reverse of 
those of the fore-sets. They are known as back-sets, and are 
formed, sometimes by the upward movement of the water as it 
leaves the ice and passes up and over the sand-plain,* and some- 
times by the settling of the margin of the plain in consequence 
of the melting of the ice upon which it may have partially rested. 
Wherever sand-plains have been exposed it has been found that 
the development of back-sets is insignificant, seldom amounting 
to more than twenty or thirty feet even in the largest plains. 
It is evident that if the margin of the ice were retreating during 
the construction of a sand-plain, the formation of back-sets by 
the first method would at once become a prominent feature of 
the deposition. It is doubtful if back-sets of this type have 
ever actually been noted, and back-sets of any sort are, as we 
have seen, of rare occurrence and slight development. This 
«Bull. Geol. Soc. Am., Vol. I, 197. 
452 
