STREAM VALLEYS AND THEIR MEANING 477 



undercut bluffs and slip-off slopes with resulting forms like those 

 illustrated in contours in Fig. 4/ 



Combinations of processes. — As already stated, all three of the 

 above processes are normally in operation at the same time along 

 a stream valley. The analysis of the form produced when any one 

 is dominant is a necessary preliminary to an understanding of the 

 more complex forms resulting when two or more of these processes 

 act in combination. Where down-cutting is vigorous the effects 

 of the other two are, as a rule, masked to a greater or less degree. 

 A gorge-like valley is the result. When, in the history of a stream, 

 down-cutting ceases or becomes very slow, lateral cutting con- 

 tinues to widen the valley^ while sweep tends to clear it out if it is 

 not already relatively straight. Once a valley assumes the open 

 form, whether as a result of initial conditions or as a consequence 

 of the clearing effect of sweep, the constant down- valley procession 

 of meanders tends, throughout the remainder of the cycle, to main- 

 tain this form by shifting continually the locus of attack of the 

 stream, and to produce a valley with fairly uniform width and 

 with relatively straight walls. 



Between the stage marked by predominant down-cutting and 

 that marked by predominant sweep there must be an intermediate 

 stage when sweep, down-cutting, and lateral cutting bear such a 

 relation to one another that whatever width of meander is gained 

 by the stream in its lateral cutting is retained because of the con- 

 temporaneous down-cutting, while the spurs between the meanders, 

 with their characteristic slip-off slopes, are not cleared out by 

 sweep. In valleys developed under such conditions, the slip-off 

 slopes would be conspicuous and the undercut bluffs well developed. 

 The form would be that of the in-grown meander valley (type 3 

 above). Sweep, in this case, would be subordinate because, as will 

 be explained more fully in a subsequent paragraph, in the interval 

 between the sweep of successive meanders past a given point, 



' W. M. Davis, "Incised Meandering Valleys," Bull. Geog. Soc. Philadelphia, 

 IV (1906), 182-92. 



^ A. Penck, Morphologie der Erdoberflache, II. Buch, I. Abschnitt, S. 350: "Lang- 

 same Vertiefung ist das Erforderniss zum gleichseirigen Verschieben der Flussbetten. 

 Sobald erstere rasch geschieht und auch letztere erfolgt, so wird eine ungeheuere Trum- 

 mermasse dem Flusse zugefiihrt werden, die er nicht zu bewaltigen vermag." 



