S4 BUREAU OF AMERICAN ETHNOLOGY [BuU. 188 



Canyon at tlie east to Cow Springs Canyon at the west. Besides its 

 three major canyons, the plateau is incised with innumerable small 

 watercourses, all tending in a southerly or southwesterly direction, 

 toward Klethla Valley. Surface contour throughout most of the 

 Shonto area is fairly irregular, marked by numerous small detached 

 mesas, ridges, bluffs, and roclcy outcrops. Only in the extreme south, 

 on the Klethla Valley floodplain, is there an extensive level area. 



GEOLOGY 



The entire Shonto Plateau is capped by a stratum of Navaho Sand- 

 stone 200 to 300 feet in thickness. The region was at one time denuded 

 of all soil cover, presenting an unbroken surface of naked rock such as 

 may still be seen in areas to the east and west. Except in its three 

 major canyon drainages subsequent deposition has been chiefly in the 

 form of coarse wind-blown sand, which now covers the more level por- 

 tions of the plateau to a depth of 1 to 5 feet. However, many small 

 expanses of exposed bedrock remain throughout the Shonto area, and 

 nearly all slopes are devoid of soil. It is doubtful if the soil cover 

 anywhere exceeds 5 feet in depth. 



Because of the peculiar geology, the common type of gully erosion 

 found throughout the Navaho country does not occur in the Shonto 

 area, except in Kletlila Valley. Small watercourses on the Shonto 

 Plateau tend to have sloping, sandy walls and rock floors. Of the 

 major canyons, Shonto and Cow Springs both present steep, unbroken 

 sandstone walls 150 to 250 feet in height. Much deeper Tsegi is cut 

 down through the Navajo into underlying sandstone formations, so 

 that its steep walls descend in a series of terraces, having a total height 

 of nearly 800 feet. 



The floors of Tsegi and Cow Springs Canyons are formed by shallow 

 alluvial deposits, frequently overlain with aeolian sand and cut by deep 

 arroyos which reach bedrock.^ Shonto Canyon, by contrast, is unique 

 among the canyons in the Navaho country in that its alluvial floor is 

 aggrading instead of eroding. At some time in the past the mouth of 

 Shonto Canyon was blocked by an enormous rockslide, with the result 

 that most of the canyon became deeply filled with alluvium. The slide 

 has served to impound water as well as soil, and the floor of the canyon 

 is today so waterlogged that Shonto Trading Post is able to pmnp its 

 water from a depth of less than 10 feet. Extensive marshes occur 

 farther downstream. As a consequence, the small permanent stream 

 in upper Shonto Canyon flows upon the alluvial surface, free of any 

 natural channel. It is confined by artificial banks and by plantings of 

 Cottonwood. 



Aside from the canyons, alluvial deposits in the Shonto area occur 

 only in Kletlila Valley, the broad floodplain at the southern margin of 



" For a detaUed description of the geology of Tsegi, see Hack, 1945 



