no 



PHYSICAL GEOLOGY 



mature stage of its erosion cycle, 

 block is sea level. 



The bottom of the 



subjected to stream erosion. It was formerly the site of a lake 



(p. 656) whose bed was covered evenly with sediment. After the 



lake was drained the 

 W&m bed was exposed to 

 erosion, and a drain- 

 age system was de- 

 veloped whose stream 

 courses were deter- 

 mined by the in- 

 equalities of the 

 bottom. Later in its 

 history new tribu- 

 taries will erode side 



valleys, the main valleys will be widened, and a mature topography 



will result. 



Maturity. — A mature valley is deep, but has flaring sides and 



gently rounded upper slopes. A region in full maturity (Figs. 95, 96) 



is in decided contrast to a youthful region. Instead of few tribu- 

 taries and consequently wide 



divides, the land is thoroughly 



dissected by valleys, the 



divides are narrow, the valley 



sides are less steep than in 



youth, and the streams are 



accomplishing their greatest 



work both in erosion and 



transportation. In such a 



region the rainfall runs al- 

 most immediately into the 



streams; lakes have practi- 

 cally disappeared, having 



been drained by the cutting 



down of their outlets or filled 



by stream sediment and 



organic matter. In this stage 



the relief is greatest, and 



arable land is at a minimum; 



roads are difficult and must 



i" II -11 MILES 



follow either the valleys or t? «* A /r l l 



^ ric. 96. — Map showing the stream courses 



the narrow divides, and the in a mature region. 



