EXTINCT VOLCANO — ATWOOD 309 



wrapped up in the growth of Mount Mazama. The lower glacial 

 horizon is related to a stage of glaciation which was abruptly ter- 

 minated when the mountain erupted the lavas which now cap the till 

 deposits. The upper glacial horizon dates to the last glacial stage 

 which occun-ed after the surface lavas had cooled and before the 

 mountain collapsed. 



Identification of the striae and glacial till deposits which occur 

 over and over again within the Crater Lake region has been made 

 only after considering all alternative possibilities. Structure within 

 the volcanic rocks has been carefully eliminated as a factor in pro- 

 ducing the polished and grooved lava surfaces. It should be stated, 

 however, that certain exposures exhibit scratches which are definitely 

 due to causes other than glaciation. All such exposures have been 

 eliminated from consideration in the present study. The recogni- 

 tion of till deposits has been made on the basis of physical and 

 lithologic composition and on the existence of subangular and 

 striated stones. While the writer is aware of the difficulties asso- 

 ciated with the identification of glacial scratches, he feels certain 

 that the variety and kind of evidence available at Crater Lake points 

 conclusively to a glacial origin for the features here described. 



Glacier Point (2). — Close to the trail which follows the rim east 

 of Discovery Point is an excellent exposure illustrating three stages 

 of glaciation. Three lava flows of different ages exhibit striae (fig. 8) • 

 The lower surface is capped with till containing subangular and 

 striated stones, while the other levels are relatively free of glacial 

 debris. 



South of the Watchman (3). — ^About half way between Discovery 

 Point and the Watchman, resting upon a striated lava surface, is a 

 thin layer of glacial till and in turn some 50 feet of stratified 

 pumice and fragmental material. In the pumice is a carbonized log 

 standing in upright position. The stump and roots appear to have 

 decomposed, allowing a portion of the log to settle. D. S. Libbey, 

 the park naturalist, in collaboration with Albert E. Long, exca- 

 vated the log during the summer of 1933. It is believed that the 

 tree was growing in a thin layer of glacial till when volcanic ash 

 and pumice buried it. The roots and base of the tree were buried 

 first by cool pumice, but subsequently hot volcanic ash and pumice 

 settled around the tree so fast that air was excluded, combustion 

 was prevented, and carbonization resulted^ (fig. 9). 



North of the Devils Backbone (4). — Beside the rim road a short 

 distance north of the Devils Backbone is a beautifully polished and 

 striated surface of lava (pi. 4, fig. 1). Resting upon this is a thin 

 layer of till and a deposit of pumice, locally several feet thick and 



^ Libbey, D. S., Carbonized tree found within the rim. Nature Notes, Crater Lalje 

 National Park, vol. 6, no. 3, August 1933. 



