168 The American Geologist. September, 1903. 
sediment in proceeding from the head of the plain to the fron- 
tal margin. To the latter point only the finest sands and clay 
could be transported. There the sediment would assume a steep 
inclination in the force-set beds. The frontal pitch of the Bal- 
lardvale plain is admirably preserved, composed of thinly bed- 
ded, fine sands, with a dip of about thirty degrees to the south. 
Overlying the delta as a whole, is a thin layer of coarse sed- 
iment. As the delta building progressed and the debouching 
stream assorted its burden the plain structure grew laterally as 
well as longitudinally. The lateral expansion, or lobes, show a 
finer deposition than the center. They were out of the main 
current, and transportation to them of aught but finer sediment 
was unlikely. As the plain developed vertically, the head 
slightly in advance of the frontal portion, it became necessary 
for the stream current to ascend its previous deposition. To- 
ward the latter stages of the delta building, when the surface of 
the delta approximated the water level, and slightly exceeded 
it at the head, then the current of the feeding stream must 
have been forced to make a sharp ascent to reach the horizontal 
surface of the delta. Some deposition was constantly taking 
place at the point of ascent, the effect of which was to produce - 
the obliquely inclined series of back set beds. 
Owing to a lack of cuts and exposures, this series of beds 
is poorly shown in the Ballardvale plain. Enough can be seen 
to be satisfied that such structure exists, though its limitations 
cannot be satisfactorily determined. That they form, in them- 
selves, an inconsiderable and very minor portion of the plain 
structure is more than probable. The ratio of forward growth 
to the deposition of back set beds has been estimated to be about 
forty to one. This would give, theoretically, a width of back 
set beds in the Ballardvale plain of about one hundred feet. 
From a study of back set beds in other delta plains, the propo- 
sition that they are formed by a down sliding of material upon 
withdrawal of the ice-contact, seems untenable. There is every 
indication in texture, homogeneity of structure, and surround- 
ings, to prove that they are the constructive result of an up- 
current stream. 
Numerous cuts made by temporary, wet weather streams, 
down the northern face of the delta-plain reveal a quasi- strati- 
fication, formed bv the downward gravitation of sediments al- 
