472 0. D. von Engeln — Studies on Ice Structure. 



facilitated by an accompanying increase in the volume of the 

 interstitial film in that area. 



Summarizing, the phenomena of a valley glacier may 

 be conceived as follows. In the reservoir area the increasing 

 depth of accumulating snow gradually augments the pressure 

 on its bottom layers. In the mass, meanwhile, the growth of 

 ice crystals proceeds by molecular transference of particles 

 and the absorption of small grains by adjacent larger nuclei. 

 At the same time the saline matter is being segregated in the 

 intergranular spaces. Compression itself involves a rise in 

 temperature and such rise is further increased by inflow of 

 surface heat and heat from the earth. When pressure and 

 heat attain sufficiently high points flow is initiated. During 

 the period of snow accumulation the snow and ice mass in the 

 reservoir is under practically uniform pressure. Once flow is 

 started non-uniform pressures are introduced with resulting 

 increases and decreases of the thickness of the interstitial films. 



The continued movement of the ice tongue so started results 

 both from the pressure due to its own thickness and thrust 

 from the continuation of the tongue up slope. If the snow 

 supply in the reservoir is cut off an equilibrium of pressures 

 will be established and the glacier will then melt away without 

 further flow. If the snow supply is insufficient to provide 

 great enough accumulations for continuous flow pressures, there 

 will be alternative waves of advance and stagnation. Appar- 

 ently the latter condition is only attained under exceptional 

 conditions because flow is more readily maintained than 

 initiated. 



The surface portions of the ice tongue are relatively rigid 

 and brittle. The viscous (?) underflow gives rise to tensional 

 and shearing stresses in the upper, rigid ice layers with 

 resultant crevassing. These crevasses are commonly developed 

 on a greater scale in the upper ice tongue, partly because slopes 

 are steeper, partly also, probably, because the ice of the upper 

 portions of the ice tongues is colder. In the lower expanded 

 portions of ice tongues pressure-temperature equilibrium has 

 been established throughout the thickness of the ice, hence 

 flow is possible over lower slopes and with thinner ice than at 

 and above the neve line. Near-surface melting in the lower 

 parts of the tongue is localized in the intergranular spaces, the 

 resulting water tends to drain away and thus leaves the ice 

 grains "loose in the socket." On the bottom of the glacier, 

 near the yery front, the establishment of pressure-temperature 

 equilibrium combined with the continued inflow of earth-heat 

 and friction may result in pressure melting and thus give rise 

 to submarginal glacial streams, whose volume is augmented 

 by the surface-melting water percolating through the frontal 



