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28 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
delta across whose surface the stream swings in the process of discharging its load of gravel, 
sand and clay. 
“While the stream is aggrading its delta, it swings from side to side through the are 
whose trace is the free margin or shoreline of the deposit and whose center is the mouth of the 
glacial stream. Take the stream at a moment when it lies at one side (say the left) of its 
delta contiguous to the ice front. Its burden of gravel and coarse sand enters into the con- 
struction of the delta proper. Over the bottom of the lake or bay the clays carried out in 
suspension are constantly coming to rest at distances from the delta margin determined by the 
presence and velocity of the currents and the time taken for the particles to fall through the 
water. For some distance over the bottom in the path of the stream-made current, the finer 
particles of sand which have not at once been drawn by gravity down on the delta talus will 
come to rest, forming a deposit of very fine sand extending outward from that part of the base 
of the delta. Around the remaining portion of the area confronting the delta base, clays will 
deposit as elsewhere over the floor of the water body. In the course of a few days or weeks 
or months, dependent on velocity, load, and the area of its delta fan, the stream will have 
moved laterally across its delta to the opposite side. The fine sands will now have been 
deposited over the entire area in front of the delta base while clays will have been deposited 
on that side where sand was previously going down. Still later, the stream will have swung 
back to the left of the delta and sands will be depositing along that portion of the basin floor, 
while clays are deposited over all the area on the right. The stream thus swings to the left 
and right of its delta, strewing fine sand over the bottom in advance of the delta. These 
changes will continue so long as the stream is building up its delta and the water body is 
unfilled with sediment. There will thus be built up on the floor of the basin an alternation 
of layers of clay and fine sand, whose stratification seen in a cross-section drawn transverse 
to the axis of the delta will be that shown in figure 22, in which the black line represents the 
sand layers, the white banding, the clays. 
“Where the stream halts, the sand layer will be thicker than where the stream has 
moved steadily along in its lateral motion. At the extreme right and left, where the stream 
has halted and turned back on its course, the sand bands should be thicker than in the middle 
of its shifts. 
“Sand partings will ordinarily be thinner than the clay partings for the reason that the 
fine sand is depositing over the basin only beneath the laterally shifting, stream-made current, 
while clays are making everywhere else in the longer time during which the stream fails to 
cover the much larger segment of the are traversed by its swings. The thickness of clay 
layers and sand layers will be greater the slower the rate of lateral swinging of the stream; 
the sand layers will thicken toward the delta, the clay layers will thicken away from it; and 
at a distance beyond which the fine sand is carried in suspension, the deposit of clay will be 
from this cause alone continuous. The rate of lateral shifting will increase directly as the 
load carried by the stream since the excess of detritus left on the delta plain over that carried 
to its edge fills up the bed and causes the current to slide off on to the part not so much built 
up or to give off distributaries which will naturally start out from the side toward which the 
stream is shifting. Thus increase in load and marginal discharge will not give rise to a pro- 
portionate increase in thickness of the prodelta sand layers for the reason that the stream 
will not deposit sand for so long a time over a given space, because its cycles of swinging will 
be more rapid. : : 
“Delta streams tend to break up into minor streams or an interlacing of streams, so that 
there will frequently be many lines of prodelta sand deposition, introducing minor bands of 
sand and clay. The breaking out and shutting off of a distributary which ends independently 
on the delta edge will give rise to lenticular partings of sand over the prodelta floor.” Woop- 
worTH, 1905, p. 181-183. ; 
