738 Vals Wil JEAN INGE S ATI 
tances from their point of origin and at all levels in the drift, it 
seems sufficiently clear that we cannot measure the duration of 
any single ice invasion by the period required for the transpor- 
tation of a single erratic from its northern origin to its outer 
verge, even allowing for all the retardation in the ground 
moraine. Every one of the Archean fragments so commonly 
seen along the southern borders or the drift must have required 
some four thousand years, even if we allow it to have advanced 
two feet a day, to reach its present. position and probably a 
much longer period, for there is no good reason to suppose that 
the mass of the ice-sheet itself advanced at any such rate. We 
can also allow a somewhat more rapid transportation of débris 
near the margins by floods, subglacial drainage, etc., and yet 
find our time limit tending to be too small. The Alpine and 
Scandinavian glaciers, with their steep gradients affording full 
play to the action of gravity, move on an average only a few 
inches a day. How the continental glacier derived its move- 
ment, except at its elevated origin and near its periphery, is one 
of the questions that no one has yet satisfactorily answered, and 
the inevitable conclusion from the known facts is that while 
motion undoubtedly occurred it must have been extremely slow. 
The formation of the ground moraine must have required a 
very prolonged period of time, involving as it did the grinding 
up and working over of the rocks and other material that 
together make up the till. It does not matter whether it is held 
that it was mainly deposited under the ice-sheet by stagnation 
of contained débris, as suggested by O. P. Hay,’ or as a contin- 
uous terminal moraine as the glacier retreated, as was held by 
Newberry.* In either case a long time must have been required. 
In what has been said I have tried to show that a great ice- 
sheet thousands of feet in thickness, extending over a third 
of a continent, expanding from its center in the direction of 
least resistance towards its periphery, having over the greater 
portion of its area a very slight slope, and probably none at all 
SILO, Cit 
2 Geol. Survey of Ohio, Vol. Il, 29; III, 34. 
