GEOLOGICAL HISTORY OF THE YELLOWSTONE PARK. 141 



found. lu the Teton Range several well-dettiied cliaracteristic glaciers 

 still (>xist u]><)ntlio abrupt slopes of Mount Haj'den and Mount JMoran. 

 They are the remnants of a much larger system of glaciers. Tlie Park 

 region i)reseuts so broad a. mass of elevated country that the entire 

 plateau was, in glacial times, covered with a heavy capping of ice. 

 Evidences of glacial action are everywhere to be seen. 



Over the Absaroka Range glaciers were forced down into the Lamar 

 and Yeirowstone valleys, thence westward over the top of Mount Evarts 

 to the Mammoth Hot Springs Basin. On the opposite side of the Park 

 the ice from the summit of the Gallatin Range moved eastward across 

 Swan Valley and passing over the top of Terrace Mountain joined the 

 ice fiehl coming from the east. The united ice sheet plowed its way 

 northu'ard down tiie valley of the Gardiner to tlie Lower Yellowstone, 

 where the broad valley may be seen strewn with the material trans- 

 ported from l)oth the east and west rims of the Park. 



Since the dying out of the rliyolite eruptions erosion has greatly 

 moditied the entire surface features of the Park. Some idea of the ex- 

 tent of this action may be realized when it is recalled that the deep 

 cafions of the Yellowstone, Gibbon and Madison rivers — canons in the 

 strictest use of the word — have all been carved out since that time. 

 To-day these gorges measure several miles in length and from 1,000 to 

 1,500 feet in depth. 



To the geologist one of the most impressive objects on the Park 

 l)lateau is a transported bowlder of granite which rests directly upon 

 the rliyolite near the brink of the (rrand Canon, about 3 miles beh)w the 

 Falls of the Yellowstone. It stands alone in the forest, miles from the 

 nearest glacial bowlder. Glacial detritus carrying granitic material 

 may be traced upon both sides of the canon wall, but not a fragment 

 of rock more than a few inches in diameter, older than the recent lavas, 

 lijjs been recognized within a radius of many miles. This massive 

 block, although irregular in shape and somewhat pointed toward the 

 top, measures 24 feet in length by 20 feet in breadth and stands 18 feet 

 above the base. The nearest point from which it could have been trans- 

 ]»orted is distant 30 or 40 miles. (Joining upon it in the solitude of the 

 forest with all its strange, surroundings it tells a most impressive story. 

 lu no ])lace are the evidences of frost and lire brought so forcibly to- 

 gether as in the Yellowstone National Park. 



Since the close of the ice ]>eriod no geological events of any moment 

 have brought about any changes in the physical history of the region 

 other than those produced by the direct action of steam and thermal 

 waters. A few insigniticant eruptions have probably occurred, but 

 they failed to modify the broad outlines of topographical structure and 

 present but little of general interest beyond the evidence of the con- 

 tinuance <d" volcanii; action into <[uaternary times. Volcanic activity 

 in the Park maybe considered as long since extinct. At all events in- 

 dications of fresh lava-tiows within historical times are wholly want- 



