FLOODS CONFINED TO THE PLAIN. 103 



or two in thickness for several miles from the mountains and rest on an 

 eroded surface of non-decomposed mountain rocks. 



Later the 1894 expedition passed up the broad valley extending north- 

 westward from Hermosillo between two outlying ranges for about 100 

 miles. The greater part of this valley is a single torrential plain tilting 

 up laterally into the bounding ranges and rising gradually northwest- 

 ward from an altitude of perhaps 100 feet to over 2,000, where it seems 

 to drown the mountains, save a few peaks rising sharply from its gentle 

 surface; the regularity of the valley and its apparent lining being inter- 

 rupted in four or five places by sharp-cut drainage-ways which have 

 retrogressed through the westerly bounding range. About midlength of 

 the valley (south of the tanque known as Agua Nueva) the route crossed 

 obliquely the trail of a sheetflood, marked by flotsam and gullies, several 

 miles broad. In this case the nearest upslope mountains were 20 or 30 

 miles away and not more than 2,500 feet higher than the flood-marks, 

 while the surface inclined for 50 miles directly to the lower reach of Rio 

 Sonora, which is never wet save by local showers or the storm freshets 

 descending from the Sierra Madre during midwinter or midsummer 

 rains. Accordingly it seemed clear that the sheetflood had been confined 

 to the plain — that the waters of a single storm had accumulated, rushed 

 down the 1:25 slope for ten or more miles, and then disappeared through 

 absorption and evaporation. 



During the exceptionally humid autumn of 1895, the second expedi- 

 tion experienced a single rain, or rather a succession of showers, not suf- 

 ficient to produce either streams or sheetfloods, on the gentle slopes of 

 Altar valley about the settlement and entrenched buttes of San Rafael de 

 Alamito, where camp was pitched for several days. Yet this plain was 

 marked for an area of at least 100 square miles by bunches of flotsam 

 and driftwood lodged against the sparse shrubbery. The area was alto- 

 gether out of reach of possible floods in Altar wash, to which, indeed, 

 the marks did not extend, and the slope and direction of flow were 

 nearly at right angles to its line. Except the geographically insignifi- 

 cant buttes, there were no mountains within a dozen miles in which the 

 torrent might have gathered; and when the party ascended the 30-mile 

 slope toward the Altar-Sonora divide the torrent-marks were found to 

 diminish and gradually disappear in such manner as to demonstrate that 

 the waters had gathered mainly, if not wholly, on the plain itself, and 

 then rushed down toward, but apparently not quite to, the sand wash 

 of the Altar. Where the flotsam was accumulated most abundantly the 

 slope was probably 50 feet to the mile or less ; and the fan-ended gullies 

 were few and small, becoming more conspicuous further up the incline 

 where the flotsam was less abundant. 



XV— Bull. Geol. Soc. Am., Vol. 8, 1896 



