CONVEGHINV: OF, HILEETORS 34 
On 
another, and (2) because one particle cannot slide on another without 
developing friction. Spherical frictionless particles would flow on 
the faintest of slopes, and subangular frictionless particles would 
flow on a moderate slope. Whatever diminishes friction promotes 
flow. Whatever disturbs the arrangement of particles, permitting any 
motion among them, also promotes flow, because gravity is a factor 
in the rearrangement and its tendency is down the slope. Violent 
agitation by an earthquake suspends for the time the structural 
arrangement, surcharge by water greatly reduces friction, and each 
of these may cause flow, the flow phenomena being of the land-slide 
type. 
In creep the chief disturbing agencies are expansion and contrac- 
tion, and these are caused by freezing and thawing, heating and cool- 
ing, wetting and drying. If expansion were equal in all directions, 
and extended indefinitely downward, the arrangement of the particles 
—or the structure of the formation—would not be affected; but 
dilatation is resisted in all directions except outward, and expansion 
in a single direction modifies the structure. The structure is again 
modified during the ensuing contraction, and during both changes 
gravity enters as a constant factor tending downhill. 
Prominent among other disturbing agencies are plant roots, 
which alike in growth and decay occasion soil movements; and roots 
also act on soils when trees are swayed by the wind. Animals pro- 
mote creep in a more direct way, for as they walk either up or down 
a slope their feet push harder downhill than uphill. 
Consider now the effect of creep on the law of slope. As we are 
Speaking of mature topography only, we may assume the rate of 
degradation to be the same on 
all parts of the slope, so that the 
two lines in the diagrammatic 
Section, Pigs a, represent ’ the 
surface of the ground at two Fic. 1.—Diagrammatic section of a hill- 
: top, indicating the zone of creep, and the 
epochs. In the interval be- position of the surface at two epochs. 
tween the epochs, there has 
been no transportation at the summit, D; but a volume of material 
equivalent to the prism enclosed by the lines between D and A has 
been carried past A; and a volume equivalent to the prism between 
