EXTINCT VOLCANO — ATWOOD 



311 



south, is a glacially polished bedrock surface overlain by boulder 

 moraine. The position of these various evidences of glaciation be- 

 neath the dacite flow Avhich produced Llao Rock is conclusive proof 

 of glaciation on Mount Mazama prior to the eruption which pro- 

 duced the Llao Rock flow. From a vantage point on the lake or on 

 the opposite rim, an excellent view of Llao Rock is obtained which 

 shows very clearly the topography which existed before the Llao 

 Rock flow descended the slopes of Mazama (pi. 5, fig. 2). On the 

 basis of the glacial evidence reported above, the pre-Llao topog- 



CARBONIZED LOG 



PUMICE AND 

 FRAGMENTAL 

 MATERIAL 



GLACIAL 

 DEBRIS 



FiGDEB 9. — A carbonized log buried beneath pumice and fragmental material. Locality 



raphy presented a glacial landscape and the U-shaped form of the 

 central portion of the Llao flow is due to glacial scour in the valley, 

 which was later occupied by the lava stream. Because of inaccessi- 

 bility it was impossible to determine whether glacial material exists 

 beneath the central portion of the flow. 



Steel Bay (7). — After considerable difficulty a section was investi- 

 gated at the western edge of Steel Bay. The very steep walls pre- 

 vented complete examination, but the record uncovered was never- 

 theless of value. There are two distinctly striated surfaces — one 

 buried beneath approximately 100 feet of pumice and the second 

 several hundred feet down the cliff. 



