138 NATURAL HISTORY OF THE FARM 
from the solid rock, gravity disposes of them: removes them 
almost as fast as formed from the vertical face of the cliff: 
lets them lie on the level summit: sweeps them down the 
slope: spreads them out over the flood plain, making level 
fields; or carries them far away with the rushing flood to 
dump them into the bottom of the sea, where, removed from 
light and air, they are lost to our use. 
Thus the rugged and geologically ancient outlines of 
topography are softened by erosion and the more level 
places are overspread by a mantle of productive soil. 
Erosion rounds off the sharp edges of the headlands; 
silting fills the low places; delta building covers the shores 
about the mouths of streams; everywhere as time runs on, 
sinuous lines replace the sharp angles, and verdure replaces 
the gray pristine desolation. 
Let us go to some good point of outlook, some hill-top or 
housetop or tower, and view the topography of our own 
neighborhood, to see how the land lies. We will let our eyes 
wander slowly from the near-by fields upward to the summit 
of the distant hills, and downward to the level of the valley; 
we will follow the stream that meanders across the valley 
floor, back to its more turbulent tributaries, and on to the 
little brooks that run among the hills. Upland and lowland 
levels, and intervening slopes:—these are the natural divi- 
sions of the land; and their boundaries are all laid down by 
gravity. Water runs down hill, and loosened soil materials 
move ever withit. They may glide unnoticed as tiny films 
of sediment trickling between the clods of the fields; or they 
may move in great masses of earth and stone as a landslide, 
scarring the face of the steep slope; but ever, with the aid of 
water, they move to lower levels, and slowly the form of the 
hill is changed. Flood-plains broaden: valleys are filled; 
the slope grows gentler; and the upland plains are narrowed 
by invading rills. 
