THE GLACIAL HISTORY OF AN EXTINCT VOL- 

 CANO, CRATER LAKE NATIONAL PARK^ 



By Wallace W. Atwood, Jr., 

 Clark Vniversity 



[With 6 plates] 



THE GLACIO-VOLCANIO SEQUENCE 



Hidden away in the volcanic rocks of the Cascade Range of 

 southern Oregon is the record of Mount Mazama, an ancient vol- 

 canic cone that grew to great height and later disappeared entirely, 

 leaving a giant caldera in which the deep-blue waters of Crater 

 Lake have since accumulated (pi. 1). The story of this mysterious 

 mountain is recorded in the rocks of the region. Like leaves in a 

 book, the alternating layers of laval and glacial material in the rim 

 surrounding Crater Lake tell the story of the late monarch of the 

 Cascade Range. 



During the vulcanism of mid-Tertiary time small volcanic cones 

 develojDed in the Cascade region, one of which was destined to be- 

 come Mount Mazama (fig. 1). With continued igneous activity 

 the 3^outhful mountain attained sufficient altitude to cause heavy 

 precipitation on its slopes. Snows accumulated and remained 

 through succeeding seasons. Glaciers were born, and the ice fields 

 moved slowly down the slopes of the intermittently active volcano 

 (fig. 2). Evidence of these early glaciers is found in the form of 

 till deposits buried beneath several hundred feet of volcanic ma- 

 terial and younger glacial debris. 



Glaciers on the slopes of a volcano sooner or later are apt to fall 

 victim to renewed lava eruption. In the case of young Mount 

 Mazama, the glaciers were destroyed several times during the growth 

 of the mountain. The glacial landscape of figure 2 was changed to 

 the volcanic landscape of figure 3. Gradually this activity subsided 

 and the scene reverted to a glacial landscape (fig. 4) . This succession 

 of changes may be called the " glacio-volcanic sequence." While the 

 pen-and-ink sketches show only one sequence, the glacial deposits 



* Reprinted by permission, with slight alterations, from the Journal of Geology, vol. 43, 

 no. 2, February-March 1935. 



303 



