280 R. S. TARR — RECENT ADVANCE OF GLACIERS IN ALASKA 



same in 1906 as in 1905, and in both cases it was far too low to account 

 for movement rapid enough to so break tlie ice (plates 8 and 19). It 

 is evident that the cause of the crevassing must be sought not in the in- 

 fluence of grade, but in an acceleration of motion due to a push from 

 upstream, acting on low grade glaciers whose forward motion up to 1905 

 was so slight as to admit of the concentration of surface morainic debris. 



HYPOTHESIS OF BREAKING BY EARTHQUAKE SffOGKS 



Two hypotheses involving earthquake effect were considered and quickly 

 dismissed. One of these was that some recent earthquake had so shaken 

 the region as to break the ice into the condition observed in 1906. This 

 hypothesis was readily disproved on inquiry, when it was found that there 

 has been no notable earthquake in the Yakutat Bay region since 1899. 



The second earthquake hypothesis was that the shock of 1899 was re- 

 sponsible for the conditions of 1906 by actually breaking the ice, though 

 the effects are only just now appearing at the surface. Numerous facts 

 disprove this improbable hypothesis, the most fatal being the convincing 

 evidence of forward movement actually in progress in the summer of 

 1906. As has been shown above, the charge is associated with and appar- 

 ently the result of a forward push, so that any hypothesis which does not 

 include this is necessarily eliminated. 



HYPOTHESIS OF SNOW SVPPTjY RESULTING FROM EARTHQUAKE SHAKING 



This hypothesis is, in a word, that during the earthquakes of 1899 the 

 mountains from which the snow supj^ly of these glaciers is derived were 

 so vigorously shaken that great avalanches of snow and rock were thrown 

 down to the neve, starting a vigoroiis wave of advance whose effects have 

 now reached the glaciers described above. Of all the hypotheses which 

 have suggested themselves, this alone seems capable of explaining the 

 phenomena. Opposing it no facts have been discovered, while there is 

 much in its favor. 



The earthquake of 1899 was of unusual vigor.* Throughout a period 

 of seventeen days the region was subjected to earthquake shocks, some of 

 which were of exceptional strength, notably those on September 10 and 

 September 15. The most violent shocks were so strong that they were 

 plainly recorded on the seismographs in Europe, Japan, and at cape of 

 Good Hope. During these earthquakes the coastline of Yakutat bay was 

 greatly deformed — ^^in one place, on the west side of Disenchantment haj, 

 the beach being hoisted forty-seven feet above sealevel. The people living 



* Tarr and Martin : Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64. 



