866 GRASSES OF IOWA. 



Deep borings and mine shafts show, however, that the rock surface is 

 nearly as deeply trenched by streams as is that of the driftless area. Deep 

 down 400 feet below the high, upland prairies, the drill plunges into the 

 sands and gravels 01 old river beds. The streams which, before the ice 

 came into the country, flowed in valleys as deep, narrow and tortuous as 

 those of the Oneota, Little Maquoketa or Tete des Morts, are now 

 blotted out. The whole country has been smoothed over, the valleys 

 filled and the uplands cut down. The general effect has been a levelling 

 up. A new upland surface, approximately 100 feet above the old rock 

 hilltops, has been produced. Below this newer upland the present streams 

 have cut to an average depth of about 200 feet. Occasionally these new 

 streams are the lineal descendants of their preglacial prototypes, and run 

 in. rock cut valleys whose sides are veneered with drift. The valley sides 

 have, however, gentler slopes and show soft, rounded contours. The up- 

 land belts are narrow and, especially in the southwestern part of the 



Fig. 255. Loe.-s topography near Iowa City. 



state, become narrow divides between streams. The bottom lands are 

 broad and the streams are bordered by alluvial plains. The secondary 

 ams and their numerous branches are regularly and generously devel- 

 oped. Everywhere there is evidence of long-continued action. The 

 surface shows the typical erosion topography such as is developed on soft, 

 homogeneous materials. An example of this is shown in figure 255. Along 

 the Missouri river borders the erosion has been very sharp, and the coun- 

 try is minutely dissected. Here, too, the wind has intervened and has 



