TOPOGRAPHY 35 



forests, are usually formed of fragmented rock, mingled with 

 the products of its decomposition and small additions of humus. 

 The different phases of vegetation on soils of this character are 

 usually expressions of reactions to moisture content in relation 

 to exposure. 



While the uplands of eastern Montana are largely of fine 

 soil or gravels the bottom lands are often of fine heavy soil 

 commonly known as "gumbo" formed by sedimentation from 

 decomposed shales (40). These in the aggregate involve a con- 

 siderable area. They are usually more or less heavily impreg- 

 nated with soluble salts and are the so-called alkali lands (52). 

 The salts most commonly present are sodium-chloride (very 

 slight) sodium sulphate, magnesium chloride, and calcium sul- 

 phate. Sodium carbonate is practically absent. Much of this 

 sort of soil is found in the lower Yellowstone Valley, where 

 strong erosive action is still going on, especially in the "Bad 

 Lands," where the bluffs are dissected into a rugged wilderness 

 of hills and gullies. 



Wind action also is still evident. Fantastic forms of 

 sandstone stand isolated here and there, caverns are carved in 

 the faces of cliffs, boulders are gradually reduced and obliterated. 

 Along the crumbling cliffs and on the margins of broken terraces 

 is a fringe of forest growth, seant, scattering and depauperate. 

 The benches above and below support only herbaceous or shrubby 

 vegetation. The stream bottoms invariably support cottonwoods 

 and other hard-wood species. 



The eastern slope of the Continental Divide in Montana is 

 drained by the Missouri River, the western slope by the Clark 

 Fork of the Columbia. Along the peaks of the main and outlying 

 ranges are the innumerable sources of these streams ; some take 

 their rise in high sub-alpine bogs and meadows, others flow 

 from cirque basins where quiet lakes lie under forested slopes. 

 Of these there are many of varying size and depth. Again 

 there are countless small streams that flow from melting snows 

 all summer, flowing freely by day arid ceasing by night. The 

 slopes of all the ranges are thus watered abundantly; every 

 canyon has its brook or torrent which soon unites with others 

 forming swift, clear streams uniting into the main tributaries 

 of their respective systems (17, 49). 



