PHYSIOGRAPHY OF THE REGION 75 



same height and looking as if some mighty carpenter had planed away 

 the crests of the mountain folds, leaving here and there a knotty place, 

 which had been too tough for his plane to cut. These knots, or " mo- 

 nadnocks," as they are called in New England, from the typical form of 

 mount Monadnock rising above the New England peneplain in southern 

 New Hampshire, wherever studied by the writer in the Urals, whether 

 from personal observation or from the testimony of the Russian geolo- 

 gists, are always of more resistant rock than the surrounding masses. 

 The writer introduces the term " monadnock" at this point because it was 

 largely from the form of these " knots " that he concluded that the Urals 

 had been subaerially baseleveled. 



Other very characteristic monadnocks are Alexandrovskaia, IremeL 

 and several less striking summits on the Urenga ridge. 



THE EASTERN SLOPE 



The plain of the summits slopes down much more rapidly on the east 

 than on the west from the Ural Taou or main chain of mountains. 

 This is shown in figure 3, which gives a generalized section across the 

 Urals, showing its typical forms. As shown in the section, there is a 

 rather steep fall-off from the summit level to the planed surface of crys- 

 tallines over which the Tertiary deposits of the Siberian plain lie on its 

 eastern margin. This fall- off is not everywhere found, but in some 



FiouKE 3. — Diagrammatic Section of the Urals. 



places farther north the summit-level plane seems to dip down beneath 

 the Tertiary deposits. Any theory for the planation of the Urals must 

 take into account the presence of this fall-off in some places and its 

 absence in others, and also include the planed crystallines with their old 

 stream courses, now frequently occupied in part by lakes, as well as the 

 very important deposit of Tertiary sediments 'farther east. Enough 

 work has not been done to solve this problem, so that the discussion here 

 presented is simply in the line of a tentative hypothesis. 



The ver}'' natural suggestion has been made that the steep fall- off, as 

 seen, for example, at Umen mountain, represents the attack of the Ter- 

 tiary sea upon the Ural mountains, but there are some topographic forms 

 which it is almost impossible to explain on this hypothesis. How were 

 the valleys back of Umen mountain formed, and what became of the waste 

 if the sea came up to this point? The absence of a sea-cliff in certain 

 exposed places must be explained. Another difficulty is the surface of 

 the granite, now covered with decomposed rock to greater or less depths 

 and dotted with lakes. The lack of marine deposits in front of this fall' 



