RIVER EROSION 167 
amount of direct solution. Weathering prepares the 
bulk of the sediment, and the river removes it. The 
stream cuts its valley deeper, and weathering, because 
of its power to decay rocks, and because the stream 
is ready to remove the frag- 
ments thus given to it, con- 
tinually broadens the channel. 
Waterfalls. — As a stream 
cuts through the rocks, it often 
finds layers of different hard- 
ness. The softer ones it 
grooves more rapidly than 
the harder, and in this way 
may locally transform itself 
to a series of rapids, or even 
a direct waterfall. Most of 
the waterfalls of the world 
are due to variable rate of 
Fic. 83. 
work in lay ers of different Bird’s-eye view of Niagara River. 
E, Lake Erie; O, Lake Onta- 
hardness. Of these we may 3“) Ningava Falls; L, the 
take Niagara as a type (Fig. bluff, at Lewistown, caused by 
the hard limestone bed, and 
81). over which the water fell to 
Here the rock strata are form the first Niagara Falls. 
nearly horizontal, and at the crest of the Falls there 
is a hard limestone layer, beneath which are soft shales 
(Fig. 82). This limestone cap resists the action of 
weather and water, but the soft shale is easily worn 
