334 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



termination of the Yellowstone glacier. Although several terminal 

 moraines may be distinguished in places, there is too general an over- 

 lapping to permit of definite mapping of the successive positions of the 



ice front. In this part of the valley the sharply limited extent of the 

 glaciation will be at once noted. To the east an extensive develop- 

 ment of the ovcnvash plain is seen, forming a tlat dotted with farms 

 between Mill creek and the river, with the hummocky surface of a strip 

 of valley moraine at the base of the mountains. 



East of the valley, the steep slopes of the Snowy Mountains show 

 tine examples of mountain moraines upon their Hanks, while their 

 deeply incised canyons and polished slopes of gneiss show the abrading 

 power of the tributary glaciers. West of the valley the rocks arc 

 wholly volcanic and readily distinguishable from the glacial drift. 

 Near Fridleys (Emigrant station) the ice covered only the valley bot- 

 tom, but as we ascend the valley toward the south the drift rises to 

 higher altitudes, though nowhere does it creep far up the mountains. 

 It is evident that the Yellowstone glacier once tilling the valley was 

 not reinforced by streams from the west. 



Near Fridley's (Emigrant) the columnar cliffs of a recent basalt tlow 

 are seen. The lava caps Pliocene lake beds, and its upper surface is 

 polished, planed, and striated by the ice sheet. Aross the river to the 

 eastward a good example of a subglacial stream channel can be 

 seen indenting the undulating niorainal slopes. Approaching Daileys 

 another exposure of basalt, capping lake beds, is seen to the east; boul- 

 ders of basalt from the two localities are very abundant in the drift 

 down the valley. To the west a remarkable series of terrace lines is 

 seen, some thirteen being plainly distinguishable. They are found only 

 within the glaciated area, and are due to diverted drainage, marginal to 

 the ice. The valley presents very different features on its two sides. 

 To the east the serrated summits of the Snowy mountains and the high 

 point of Emigrant Peak rise abruptly from the valley. These moun- 

 tains held tributary glaciers at one time confluent with the great gla- 

 cier tilling the valley, and at its decline pushing westward across the 

 valley and depositing erratics of gneiss and limestone upon the slopes 

 of volcanic agglomerate. The mountain Blopes are strewn with scat- 

 tered erratics, and striking examples of moraines of angular debris 

 may be seen upon their Hanks. 



To the west of the valley, however, tin; limit of the drift is very 

 sharply defined. The dark slopes of volcanic agglomerates rising grad- 

 ually to the crest of the Gallatin range are free from drift and show no 

 evidences of glacial sculpture. It is certain that the ice filling the 

 valley extended but a short distance up the slopes. Within the drift- 

 covered area the slopes are very generally terraced, and the evidence 



