364 J. E. TODD — CONCRETIONS AND THEIR GEOLOGICAL EFFECTS 



been formed. In some cases they merge into one another and form con- 

 tinuous strata, but more frequentl}^ they are more or less detached. 

 Under the influence of erosion the latter become the caps of earth pillars 

 which may sometimes rise several feet in height. As the process of 

 erosion goes on rapidly these stand out at different levels on the steep 

 slopes, sometimes as detached pinnacles and at other times as buttresses 

 receding in steps one above another. 



Similar effects are found in the dark colored clays and loams of the 

 Laramie which comprises much of the Little Missouri *' bad lands." 



B UTTES 



Studding the plain west of the Missouri in the Dakotas are many 

 isolated buttes which stand out as conspicuous landmarks. They are 

 carved from the Laramie and Fox Hills formations. They remind one 

 of the pinnacles just described on a grander scale. They sometimes rise 

 to 100 or 200 feet in height and may have an area at the top of an acre 

 or more. With a little study one discovers that their tops are approxi- 

 matel}^ on a level, and that this level farther west becomes embodied in 

 a tableland of greater or less extent. The common explanation of these 

 features is that at some time a stratum of sandstone, which is frequentl}^ 

 found capping the buttes, formerly extended over the whole region, and 

 that the buttes are the simple results of circumdenudation. Further 

 study, however, reveals the fact that some of the buttes, especially those 

 of the Fox hills, are not capped b}^ continuous sandstone strata, but b}^ 

 layers of more or less separate concretions lying in a bed of sand or 

 loam. In other cases, where the formation is I^aramie, while the butte 

 may be capped by a heavy stratum of sandstone in some cases, in other 

 cases only concretions are found, sometimes of gigantic size. It seems, 

 therefore, probable that instead of the sand stratum having been con- 

 solidated over the whole region it has been consolidated only locally in 

 concretions which have merged into one another so as to form the 

 stratum capping the first mentioned form, and in other cases the}^ have 

 not become so merged, although they have been near enough together 

 to efficiently resist erosion: hence we may believe that the buttes owe 

 their existence to local consolidation of strata in the concretionary form- 

 This view is strengthened b}^ finding that in the regions farther west, 

 where erosion has not worked so actively, the tableland is capped b}' a 

 sand stratum, with the consolidation locally developed in the way de- 

 scribed. Figures 25 and 27, plate 49, illustrate this theor}^ 



KNOBS 



Another topographical effect, and rather more common than the last 

 mentioned, is the formation of knobs of more or less rounded form. In 



