EXTINCT VOLCANO — ATWOOD 307 



which carried glaciers during that period. On the basis of the 

 glacial record, the approximate height of Mount Mazama has been 

 established. 



Unlike Krakatoa and Katmai, Mazama did not blow itself to 

 pieces; but instead, it is believed, this mountain collapsed and was 

 engulfed. As early as 1901 Joseph S. Diller proposed this theory in 

 his presidential address delivered before the Geological Society of 

 Washington.^ A year later the results of Diller's field work ap- 

 peared as a professional paper of the United States Geological Sur- 

 vey.^ Although it is difficult to conceive of such a phenomenon, field 

 evidence to date affords no acceptable alternative. The mountain 

 certainly existed ; the mountain is now gone, and the 15 cubic miles 

 of material have not been found. The processes of engulfment were 

 probably slow. They may have been similiar to the caving-in which 

 takes place in Hawaii, where the huge calderas are from time to 

 time being enlarged by engulfment. 



Following the formation of the giant caldera near the close of the 

 Pleistocene, or shortly thereafter, there was a brief period of inac- 

 tivity interrupted by the building of Wizard Island cinder cone and 

 two other smaller cones reported to exist on the floor of the present 

 lake. Since the completion of the Wizard Island Cone, there has 

 apparently been no volcanic activity in the immediate region. 



With the cessation of vulcanism a lake formed in the bottom of 

 the caldera. The annual precipitation far exceeded the amount of 

 water lost each year by evaporation and seepage, and consequently 

 the lake level rose. Crater Lake is now nearly 2,000 feet deep, and 

 it maintains a relatively constant level throughout the year. A cer- 

 tain amount of water disappears through underground channels 

 and reappears in numerous springs on the lower slopes of the 

 mountain base. 



IMPORTANT LOCALITIES 



In order to unravel the long and complicated history of Mount 

 Mazama, many field observations were made on the precipitous cliffs 

 surrounding the lake. A few of the significant localities vsdll be 

 described and related to the glacio-volcanic sequence which preceded 

 the collapse and engulfment. The locations of all exposures men- 

 tioned are shown on the reference map, figure 7. 



Discovery Point (1). — On the rim at Discovery Point, partially 

 covered by overlying pumice material, are several beautiful polished 



' Diller, J. S., The wreck of Mount Mazama, abstract published in Science, n. s., vol. 15, 

 pp. 203-211, January-June 1902. 



» Diller, J. S., and Patton, H. B., The geology and petrography of Crater Lake National 

 Park. U. S. Geol. Surv. Prof. Paper 3. 



36923—36 21 



