338 GLACIAL GEAVELS OF MAINE. 



REMARKS ON THE GLACIATIOST OF THE ROCKY MOUNTAINS. 



The glaciation of the Rocky Mountains throws some light on the 

 glaciation of Maine, and therefore a few of the principal valleys are here 

 briefly described. Questions pertaining to the problems of two or more 

 glaciations of the mountains and the water drift of Tertiary time are 

 omitted as not having a direct bearing on the questions arising in Maine. 



LA PLATA MOUNTAINS. 



These mountains are situated in southwestern Colorado (north latitude 

 37° 25'), and rise above 13,000 feet. They lie to the west of the San Juan 

 Mountains, and are the iirst high mountains encountered by the warm 

 southwest winds that bring moisture from the Pacific Ocean. To the west 

 and southwest lies the great plain of Arizona and southern Utah, out of 

 which rise here and there volcanic peaks and ranges of hills. The precipi- 

 tation in the form of snow is heavy in these mountains. Snowslides fre- 

 quentl}^ rush down the lateral ravines into the main valleys in such masses 

 that often they do not entirely melt during the summer, althougli situated 

 so far south. 



The mountains consist of a mass of upheaval due to igneous eruptions 

 through sedimentary beds which have been somewhat metamorphosed. 

 This makes it easy to recognize matter from the mountains as compared 

 with the unaltered sediments of the adjacent plains. The mass of upheaval 

 has been deeply dissected by a radiating system of sti-eams, so that the 

 mountains now consist of centrally connected ridges separated by profound 

 canyons ending above in rather narrow cirques, the ridges being only a few 

 feet wide on their tops and having their lateral slopes steep as talus will lie, 

 and often precipitous. Many of these slopes are so steep that lateral 

 moraines must have slid down the mountain sides as fast as the glaciers 

 melted, but here and there are broader parts of the ridges, or shelves on 

 their sides, or gutter slopes, where morainal matter could lodge. 



The evidence is conclusive that these valleys were once filled by 

 extensive glaciers. 



Glacial scratches. — Althougli tlic local rocks rcsist chemical decomposition 

 very well, they fracture readily. Hence, only here and there does the 

 exposed rock in place preserve the glacial scratches. Many places are 

 covered with a talus which shows slabs up to 4 feet in length that are well 



