PLAINS TOPOGRAPHY. 9 



toward and down the river. The present stream has carved its channel 

 within this terrace, its flood-plains being about 30 or 40 feel below the 

 ancient terrace. The ancient terrace is with difficulty distinguishable 

 along the west hank of the Platte. Similar but less prominent ancient 

 river terraces are found along Clear and Bear creeks, lint are wanting 

 along the streams entering the Platte from the east, where the topograph- 

 ical tonus are those of simple modern erosion. 



The lake terraces are more prominent in the northwestern portion of 

 the area mapped, owing probably to its having keen less deeply cut away 

 by modem erosion. They are particularly well marked in the area between 

 Ralston and South Boulder creeks, where there is a blending of lake and 



river terraces. Here five distinct terraces are traceable, the lake terraces 

 extending from 100 yards to .'! miles eastward from the foothills, while 

 those more distinctly of stream origin are but from •_'()<) to Too feel in 

 width. 



In the southern half of the area, the conditions for their preserva- 

 tion being less favorable, the remains of the lake terraces are less contin- 

 uous, being found here and there on the flanks of the hogback and on the 

 granite slopes within the hogback valleys. Above the 6,000-foo1 contour 

 there is a frequent recurrence of terrace remains along the foothills and on 

 the flanks of Green Mountain, but they are not found against the Tertiary 

 bluff along and beyond the southern border of the area mapped. 



The influence of faulting upon die topography of the plains is less 

 evident than might have been expected. No doubt much of its effect has 

 been obscured by more recent formations, deposited since the faulting took 

 place. Nevertheless, it is readily observable that in the northern part of 

 the area the trend of the leading topographical features has a northeasterly 



direction, which is also that of the greater faults that have been detected in 

 the coal areas round Marshall and Erie. Many of the faults that have 

 been traced in this region, however, have had no perceptible influence 

 upon the present surface configuration, and were detected only as a result 

 of artificial excavations, such as ditches or mine workings. 



In the region east of the Platte and north of Sand ('reek within tin- 

 area mapped, and on the same side of the Platte beyond this area, sand 



