148 UNIVERSITY OF COLORADO STUDIES 



The whole topography of the region indicates that the basin was 

 formerly a mountain valley and its laterals, formed in granite by 

 stream erosion, and hence thoroughly drained, in this respect resem- 

 bUng the original valleys of glacial lake basins. This is obvious to' 

 the physiographer from an examination of the accompanying map. 

 The ramification of the lake into lateral valleys and consequent tortuous 

 outhne gives a great length of shore line in proportion to the area. 

 It consisted of two somewhat distinct bodies of water connected by 

 a narrow strait and placed at such an angle with each other as to 

 form a rude l; or, to state it another way, the lake was nearly divided 

 at the angle by two promontories which jutted into it from opposite 

 shores. 



It seems certain that a lake could originate and continue its exist- 

 ence in such a valley for a sufficient length of time to permit the depo- 

 sition of these beds only by the formation of a very resistant barrier 

 in such a position as to interrupt the drainage. Such a barrier of 

 rock could have been formed in several ways: (i) by volcanic accu- 

 mulations choking the valley, (2) by a sharp fold thrown across the 

 channel, (3) by the general tilting of the region resulting either from 

 an uprising to the southeast or a depression to the northwest, or both- 

 From the evidence so far at hand, it seems more hkely that the last 

 is what occurred. As soon as such a change in level had progressed 

 far enough to raise a downstream portion of the valley to a position 

 higher than any portion further upstream, the water would begin 

 to accumulate, the vertical longitudinal section of such a valley being 

 concave. The accumulation would continue until either the tilting 

 ceased or the lake overflowed its rim at some point other than its origi- 

 nal outlet; but that does not necessarily mean that the lake would 

 attain great depth. The character of some of the fossils, the Sequoia 

 stumps standing erect in the deposits, and reported sun-cracks in 

 the shales indicate that the filhng of the lake by detritus progressed 

 approximately as rapidly as the supposed tilting, so that the lake was 

 maintained in a rather shallow condition. 



In the accompanying diagram (Fig. 2) an effort is made to illustrate 

 graphically this supposed change of level and its effect, the vertical 



