344 



GEOLOGY. 



siderable area in southwestern Wisconsin, and the adjacent parts of 

 Illinois, Iowa, and Minnesota, is without drift. The driftless area ! 

 of these states is neither notably higher nor lower than its surround- 



Fig. 478. — Figure to illustrate the disposition of the drift in such manner as to 

 increase the relief of the surface on which it lies. 



ings, and the agent which produced the drift must have been such 

 as could avoid this area. Glacial ice seems to be the only agent com- 

 petent to the result. (/) Stratified drift often extends beyond the 

 unstratified, in the direction in which the ice was moving, especially 



Fig. 479. — Diagram to illustrate (1) the disposition of drift, the drift being thick 

 in the valleys and thin or absent on the hills; (2) the effect of the drift on topog- 

 raphy, making it less uneven; and (3) the sharp contact between firm rock 

 below and the drift above. 



in valleys and on low land. This peculiarity of distribution is the 

 result of running water. 



The first five of these points, a-e, make strongly for the conclusion 

 that the drift is a product of glaciers, while the sixth (/), is consistent 

 with this conclusion. 



(5) Topography of the drift. 2 — Among the characteristic features 

 of the topography of the drift are: (a) Depressions without outlets, 

 and (?)) knobs, hills, and ridges, similar in size to the depressions, 



1 Winchell, Ann. Kept. Minn. Geol. Surv., 1876, pp. 35-38; Irving, Geol. of Wis., 

 Vol. II, pp. 632-633; Chamberlin, Ann. Rept. Wis. Geol. Surv., 1878, pp. 21-25, 

 and Chamberlin and Salisbury, Sixth Ann. Rept. U. S. Geol. Surv., 1885, pp. 199-322. 



2 This, as well as other characteristics of the drift, is discussed in 3d' Ann. Rept. 

 U. S. Geol. Surv. 



