250 THE ICE AGE IN NORTH AMERICA, 



performed is as vague as possible, and demands the attention 

 of hydraulic engineers.* 



Elsewhere f I have briefly considered the way in which 

 this upward movement of bowlders imbedded in the glacial 

 current might be produced. The subject is of so much in- 

 terest and importance that it will be well to recur to it here. 



As a result of the differential motion in a glacier in which 

 each higher stratum of ice is moving slightly faster than that 

 which is immediately below it, it follows that the inclosed 

 bowlders are subjected to a differential strain, in which the 

 upper portions are impelled forward with greater rapidity 

 thau the lower portions. The result of this differential strain 

 upon the upper and lower portions of the bowlder, combined 

 with the friction and viscosity of the ice, must produce a 

 movement slightly upward as well as forward. The bowlder, 

 being inelastic, or at any rate less elastic than the ice in 

 which it is imbedded, must move as one mass, while each 

 particle of ice moves in independence of all the others. 

 Now, the portion of ice lying immediately in front of a 

 bowlder, being protected from the differential pressure of 

 the ice behind by the interference of the inelastic foreign 

 object, offers more resistance in that direction than is pre- 

 sented along a line leading diagonally upward across the lines 

 of greater movement above. In other words, the frictional 

 pressure of the moving ice upon the upper portion of a bowl- 

 der is greater than that on its lower portions, and the greatest 

 of all resistance is immediately in front. The line of least 

 resistance is consequently always in a direction slightly up- 

 ward. If, for example, the movement of the ice at the upper 

 surface of a bowlder be represented by 100, and that at the 

 lower portion of the bowlder by 99, then one one-hundredth 

 of the force might be supposed to be expended in producing 

 a diagonal upward movement. Thus we can conceive that 

 fragments of rock were picked up from beneath the glacier, 



* See " Second Geological Survey of Pennsylvania, Z," p. xxiii et seq. 

 + " Glacial Boundary in Ohio, Indiana, and Kentucky," p. 31. 



