EXTINCT VOLCANO ATWOOD 



317 



PROJECTED STRIAE 



In plotting the striae readings found on the rim, certain very in- 

 teresting facts came to light. All striae that appeared to be associ- 

 ated with the last advance of ice were recorded on the map. This 

 selection included the striae that were buried by pumice and frag- 

 mental material, but not those beneath lava flows. When the read- 

 ings were complete, the lines indicating direction were projected 

 toward the lake, with the result shown in figure 12. It is not safe 

 to place too much significance on this type of mapping; neverthe- 



MILES 



Figure 12. — The direction of ice movement during the final stages of glaclation on Mount 

 Mazama. The projected striae suggest that there were two centers of ice dispersal — 

 one somewhere above the middle of Crater Lake, and the other above Wizard Island. 



less, certain results appear convincing. Most of the lines projected 

 from readings on the north, south, and east rims roughly converge 

 above the center of the lake. This fact, coupled with the volcanic 

 evidence in the rocks surrounding the lake, suggests rather definitely 

 that the summit of the original Mount Mazama was somewhere 

 above the center of the present Crater Lake. 



A second center of intersection of projected striae appears over 

 Wizard Island. This is certainly no mere coincidence but instead 

 seems to prove the existence of a secondary cone which, like Shastina 

 on the side of Mount Shasta, grew on the slope of Mount Mazama, 



