THE WORK OF RUNNING WATER 65 



slow and winding, and are said to meander Figs. 2 and 3, (PL XVII, 

 p. 64). A meandering stream often flows against the base of 

 the slope of its valley, and wearing it back (PI. VIII, p. 13), makes 

 the valley flat wider. Most valley flats were developed chiefly by 

 the side-cutting of the streams. 



The lowest level to which a stream can lower its flat is base-level. 

 Any valley flat is a sort of base-level, though the first one developed 

 by a stream is not necessarily the lowest level to which it may bring 

 its valley bottom. It is the lowest level to which it can bring its valley 

 under the conditions which exist when the -flat is made. It is, therefore, 

 a temporary base-level. Later, under changed conditions, the stream 

 may sink its channel below its first flat. 



The lengthening of valleys. Valleys are lengthened in various 

 ways. One way is illustrated by the gullies developed on hillsides 



Fig. 53. A gully developed by a single shower. 



during heavy rains. The gully made during one rainstorm (Fig. 

 53), is often lengthened at its upper end during the next, by the water 

 which flows in at its head. This process of lengthening may some- 

 times be seen even during the progress of a single storm. The heads 

 of valleys often have the characteristics of ravines or gullies, and 



