704 
PROFESSOR PRESTWICH ON THE ORIGIN 
The question involves the consideration of the angle of repose of detrital matter 
under different conditions. If the detritus consisted only of angular or sub-angular 
debris, the angle of repose, whether in air or water, would vary within the limits 
of from 35 c to 48°, or, considering the proportion of perfectly angular debris, it would 
in this case more probably be about 40°. But the matrix of the detrital matter consists 
of sand and clay, of which the angle of repose, when dry, is from 21° to 37°, and 
when saturated with water as low as from 14° to 22°. The angle of repose of the com¬ 
bined soil and stones would therefore depend on the relative proportion of the several 
materials, and on the amount of saturation. 
Therefore the angle of repose of the mass of detritus set in motion would, as the 
water drained from it, tend to become greater; and as an equilibrium was established, 
commencing on the upper part of the slope first left uncovered, that part would be 
gradually stayed, while the portion, on the contrary, which continued to slide would, 
on the lake coming to rest, bulge forward at the water-level, where it had its maximum 
saturation, at a minimum angle of repose. 
Owing to the variable nature and proportions of its elements, the problem is a very 
complicated one. The angles of the detrital slopes between the “roads” are, in fact, 
dependent both on the gradients of the underlying rock surfaces, and on the above- 
mentioned varying conditions of the detritus ; while the angle of inclination of the 
“ roads ” is dependent on the combined effects of an arrested slide, and a simultaneous 
change of the state of equilibrium in the falling detrital mass, above and below the 
water-line. 
The effects of these slides is represented generally in diagram fig. 11. 
Fig. 11. 
a, b. Original detrital slope. 
a', V. Slope after slide of detritus d to d'd" and fall of lake to Z. 
a, b". Further modification of slope after another fall of the lake and slide of d". 
r. Shelf or “ road.” ?. Lake level. 
I do not suppose, however, that the present inclination of the “ roads ” is exactly that 
of the slipped detritus, but that the subsequent fall of debris from the slope above, 
