GRIMSLEY. | Geography and Topography. dd 
seaward. Some of these little streams will gain an advantage 
over others, because of more favorable conditions, such as softer 
rock, greater slope, and the like. They will thus form deeper 
channels, and will take the water from the less favorably located 
channels. By increase in slope and in volume of water greater 
and greater erosion will result, and the channel will become 
deeper and deeper, causing the rivulet to grow into the creek 
flowing in its ravine, and this in turn will grow into the river 
flowing in a valley which it has worn. The smaller and less 
favorably located rivyulets become tributaries to the larger ones, 
and they increase in size as the main stream grows, so that 
there finally results a well developed river system, with main 
streams and tributaries branching in all directions, like a great 
tree. 
The smooth upland or plateau becomes furrowed with valleys, 
at first separated by broad divides. Where obstructions are 
met, there the water is held back, forming lakes which later will 
be filled up with sediment or destroyed by the stream breaking 
through the barrier. If the river crosses a hard stratum of 
rock in its course, it will cut slowly through it, and more 
rapidly in the softer strata below, and so a projecting shelf of 
rock will be left over which the water will fall in a cascade. 
This hard stratum finally will be eroded away and the water- 
fall disappear. 
This early period in the stream’s history, from analogy to the 
growth of an organism, has been termed the period of infancy. 
During this time the amount of erosion work accomplished by 
the stream is not as great as later in its history because the 
slopes are not as great, volume of water is smaller, and the 
tributaries fewer in number. At this time there is more or less 
change in course through the selection of more favorable paths. 
As the river cuts its channel deeper, the heads of the streams 
work backward in their different directions, narrowing and cut- 
ting across the divides. ‘The valleys are further widened by the 
action of rain, frost, wind, and all the forces of the atmosphere, 
until the plateau becomes dissected and extremely irregular. 
The river now does its maximum work of erosion and carries a 
