LAND WATERS STREAMS 131 



a whole), as well as the valleys, represented on Plate VI, are in 

 their topographic youth. It is often convenient to recognize 

 various sub-stages, such as early youth, middle youth, and late 

 youth, within' the youthful stage of valleys or topographies. 



Youthful streams, as well as youthful topographies, have their 

 distinctive characteristics. They are usually swift; their cutting 

 is mainly at the bottom rather than at the sides, and their courses 

 are often marked by rapids and falls. 



As valleys approach base-level, they develop flats. As the 



Fig. 97. A valley much older than that shown in Fig. 96, Gray Copper 

 Gulch, southwestern Colorado. (U. S. Geol. Surv.) 



valleys and their flats widen, and as their tributaries increase in 

 numbers and size, a stage of erosion is presently reached where 

 but little of the original upland surface remains. The country 

 is largely reduced to slopes, and in this condition the drainage and 

 the topography which it has determined are said to be mature. 

 Mature topography is shown in contours in PL VII, where slopes, 

 rather than upland or valley flats, predominate. Mature topog- 

 raphy is also shown in Fig. 97, which illustrates the universal 

 tendency of rivers in regions of notable relief to develop new flats 

 well below the former surface of the region. 



The same processes which have made young valleys mature will 

 in time work further changes. When the gradients of the valleys 



