172 /. HARLEN BRETZ 



IV. GENERAL CONSIDERATIONS 



Considering the altitudes of the terminal moraine only where 

 facing driftless country to the south, its crest is found to have no 

 great range in elevation above the sea. On the north slope of the 

 Bald Hills, near the headwaters of the Des Chutes River, the 

 moraine crest is probably nowhere more than 900 feet A.T., though 

 erratics occur 320 feet higher. Near Tenino, where the moraine 

 is most typically developed on the plain, the crest is probably less 

 than 600 feet in altitude. The existence of buried rock hills in 

 the moraine in this region has been noted. At Little Rock, the 

 moraine surface on the west side of Black River can hardly have 

 been lowered by erosion of escaping glacial water or subsequent 

 stream action, and is approximately 150 feet above the sea, the 

 lowest altitude in the moraine. From this altitude is a descending 

 slope southward, on which the ice ceased to advance. The oppos- 

 ing northern flanks of the Black Hills, deeply cut by valleys, did 

 not permit assumption of the moraine form. Drift, however, has 

 its upper limit in the re-entrant angle which they produced, at 

 an altitude of 1,460 feet. The flattened lobe northwest of these 

 hills has its moraine hills about Lake Nahwatzel at 450 feet A.T. 

 Puget Sound and Olympic drift damming Lake Cushman reaches 

 observed heights of 950 feet above the sea. 



The data available for an estimate of the thickness of the ice 

 and its frontal slope are meager. Three miles from Little Rock, 

 the glacier left its till at the eastern foot of the Black Hills at an 

 altitude of about 150 feet. From here it is 10 miles north to the 

 upper drift limit near Summit Lake, at 1,460 feet A.T. The slope 

 in this instance is approximately 130 feet per mile. Fifteen miles 

 east of Seattle rises the peak of Mt. Issaquah, about 3,000 feet A.T., 

 whose frost-riven summit bears no residual soil comparable to 

 that found on hills of much the same lava rock beyond the limit 

 of the drift. Scattered erratic pebbles were found on the summit, 

 their number increasing on the lower slopes. With the maximum 

 depth of the Sound near Seattle at 964 feet, we may conclude that 

 in the latitude of Seattle the glacier attained a thickness of 4,000 

 feet, allowing very little for central surface convexity, which would 

 increase the estimate an unknown amount. 



