SHEETFLOODING PAILS IN THE LOWLANDS. 109 



ing either in configuration or in covering to indicate that streams have 

 flowed over them, and extended consideration has yielded no other sug- 

 gestion as to the eroding agent than that found also in analogy with the 

 observed sheetflood. Moreover, these planed surfaces are not rare or 

 exceptional ; they occur under a definite law of distribution (with re- 

 spect to sierras on the one hand and alluvium-lined valleys on the other) 

 in all parts of the Sonoran region ; their area may be estimated as two- 

 fifths of the entire tract, or over 100,000 square miles. The efficiency of 

 the sheetflood as a corrading agent is connected with its efficiency as an 

 agent of transportation, for the rapid corrasion constantly furnishes ma- 

 terial to be carried down the slopes, and this material in turn is available 

 for cumulatively increasing the effective work of the agent. 



The efficiency of the sheetflood in deposition would appear to be sub- 

 ordinate. In the miniature examples observed, a relatively considerable 

 sheet of sediment was indeed laid down, yet it was no more than might 

 be taken up and carried further down the slope by the next torrent, so 

 that deposition in this case would seem to be little more than the mark 

 of decadence ; and in the various torrential plains on which the traces 

 of past sheetfloods w T ere found the conspicuous marks were those of deg- 

 radation rather than aggradation. So far as the characteristics of the 

 sheetflood are susceptible of anatysis, too, it would seem that this form 

 of flowage cannot maintain its distinctive attributes unless the detritus 

 mantle is of limited thickness, so that deposition may be considered as 

 essential and characteristic onl}^ to the extent of supplying abrasive ma- 

 terials for the next debacle within the sheetflood zone proper, while the 

 external or peripheral deposition would appear rather to represent ordi- 

 nary delta-building or stream-work. It is significant in this connection 

 that many of the valley -plains (e. g., San Luis valley, east of Baboquivira 

 range) are much more strongly diversified by waterways and their at- 

 tendant bluffs in their medial portions than half way up the gentle slopes 

 toward the bounding mountains. 



When the functions of sheetfloods are combined, it is found that their 

 tendency is to recede or retrogress from the valley interiors, as these are 

 progressively clogged with sediments, toward and into the bounding 

 ranges ; and this retrogressive cutting is one of the most significant 

 features of the erosive process, partly in that it tends to produce anoma- 

 lous profiles, passing abruptly from steep mountain side to nearly flat 

 plain, in lieu of the gently sloping concave profile characteristic of 

 stream-work. When the torrent filling the mountain barranca em- 

 bouches on the plain, the first effect of the slackening in flow is the dis- 

 charge of the detritus in a fan, which of itself tends to spread the water ; 

 and this fan protects the subjacent rock from corrasion. The slackened 



