448 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. .ilay 28,
in section ; for when a gully has cut through the accumulation in a
radial direction, it shows what is drawn in fig. 5, straight-sloping
Fig. 5. — Radial Section of a Fan,
parallel layers ; but if a section has been made by the main-valley
stream across a portion of the fan, we see the lines of accumulation
to be in curves, as shown in the diagram, fig. 6. These curves must
Fig. 6. — Section -of a Fan on a clwrcl.
be, from theory, hyperbolas. The lateral changes of position of the
depositing stream, and the partial growth of each layer, are denoted
by false-bedding.
It may be remarked that the only difference of form between an
alluvial fan such as this and the fan talus sketched in fig. 2 is in the
degree of slope.
An unusual case of a fan was observed at Murgi, in .S"ubra, where
the side-stream entered the main valley as a waterfall with a height
of about 200 feet. Where this fall was, the fan had its apex ; and
from that point (on which, of course, the debris, as well as the water
itself, fell) it spread just like any other.
The slope of the alluvial fans varies. I have noted the inclination
of some fans near Thonde, in Zanskar : four in succession had an
angle of 6°, 4°, 6°, and 8° ; again, opposite to Leh, the angles 3°,
4°, and 6° occur. It is the case that the smaller fans have the
greater slope, and the larger ones the more gentle slope; again, those
have a gentle slope that belong to, and lie in front of, ravines
that reach far back into the mountains. This must be because
such ravines have streams which, draining a larger area, or
coming more from snow and glaciers, carry a greater volume of
water ; and with that greater amount, and consequent greater car-
rying-power, of the water, the detritus will not remain on a slope of
the higher degree, but will only be deposited when it can form for
itself a cone or fan of the slope corresponding to the circumstances
of the character of material and transporting-power.
In the Nubra valley, from which I have taken my example, the
fans commonly are distinct and separate, with a space of main-river
alluvium between each ; but it is perhaps as often the case that they
have been formed so near to each other as to meet in the course of
their growth and to touch each other ; and in such cases the layers of
each must have slightly overlapped in the forming.
On the left bank of the Indus, opposite Leh, there is a great ex-