MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE. 385 
new geomorphic forms, of the bad-land type, are evolved with startling rapidity. This 
colrasive process is sometimes complicated by earth-slumping. 
The activity of earth-slumping as a degradational process is, in general, a function of 
the amount of rainfall in any given season. Thus in the winter of 1889-1890, in which 
the rainfall was exceptionally heavy, earth-slumps thruout the Coast Ranges were 
much more active than in seasons of normal rainfall, and many new ones were started. 
In all such earth-slumps the saturation with water of the soil and regolith, and in some 
cases of the underlying formations, is an essential condition. This water is the main 
agent in loosening or disintegrating the material preparatory to the slip. It is also a 
motive power on account of the large addition which it makes to the weight of the un- 
stable mass; and it is a transporting agent owing to the fluid or plastic nature which it 
imparts to it. 
The character of the movement in an earth-slump is noteworthy. The ground moved 
drops away from the slope in the form of a bite, leaving a lunate or horseshoe-shaped 
scarp overlooking the sunken area. As the mass moves down, it generally encounters 
the resistance of more stable portions of the slope below, and is thus crowded upon 
itself. ‘The plastic mass is in this way deformed, and the deformation amounts in many 
cases to an effective rotation of the moved portion upon a horizontal axis. The lower 
portion is thrust over the passive ground at its lower margin, and the slope of the sur- 
face of the moved part is greatly diminished and in many cases reversed. Between the 
reversed slope and the limiting scarp a depression is thus formed which may become 
a pool. The change in the slope thus occasioned gives rise to the landslide terrace.! 
This kind of movement may be slowly continuous for considerable periods, or it may be 
fitful, depending upon the supply of water. In a slumping tract the movement may 
be repeated at various levels, giving the slope an irregularly stept or terraced profile; 
and if the movement has been recent, numerous cracks and fissures traverse these terraces, 
particularly where they break away from the upper limiting scarp. 
The instability of the mass is an essential feature of the earth-slump. When not 
actually moving, its movement is imminent at all times, but with varying degrees of 
imminence, depending upon local conditions. This instability and imminence of move- 
ment is true of many slopes where no actual earth-slump has appeared, but where move- 
ment may be inaugurated at any time by an exceptionally heavy winter or by some 
other precipitating cause. Severe earthquakes constitute one of these precipitatory 
causes. ‘Thruout the Coast Ranges of California the small residual stability of many 
earth-slumps was overcome by the vibration of the ground at the time of the earthquake 
of April 18, and they were caused to slump forward. In many other instances new 
earth-slumps were started, owing to the same general cause. Besides the earth-slump 
movements which were the immediate effect of the earthquake shock, there were doubt- 
less others which were indirectly referable to the same cause. As will be shown in an- 
other part of this report, one effect of the earthquake was the derangement of the normal 
movement and amount of flow of underground waters, the general result being a tem- 
porary increase of flow. Inasmuch as many earth-slumps depend for their water upon 
springs, there can be little doubt that the increased flow had its effect upon these, and 
promoted their activity several days or possibly weeks after the shock itself. 
Another way in which the shock conduced to the activity of earth-slumps at a later 
date than the shock itself was by opening cracks and thus rendering the deeper portions 
of the unstable mass more accessible to the rains of the following winter. The move- 
ment of earth-slumps at the time of the earthquake was abnormally large and sudden, 
thus leading to the development of numerous open cracks, not only in the landslide 
proper, but also in the surrounding slopes above the limiting scarp. The effect of this 
1See U.S. G. S. Monograph, I, Lake Bonneville, by G. K. Gilbert, p. 83. 

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