364 



GRASSES OF IOWA. 



territory, and more than 600 feet below the highest points in the region. 

 The sharp-walled valleys, narrow, tortuous ravines, the network of 

 streams, and the occasional uninvaded flats are all characteristic of stream 

 action on beds of variable hardness. 



The strata of the region are of Paleozoic age, and include hard lime- 

 stones and dolomites, and soft sandstones and shales. These variations in 

 hardness have resulted in the production of well marked structural plains, 

 the surface rising to the west in a series of steps or benches. In Dubuque 

 county there is an abrupt rise from the Mississippi to the top of the 

 Galena cliffs, about 230 feet. At this level there is an open, rolling plain 

 rising very gently 150 to 175 feet, to the base of the Niagara. This plain 

 marks the horizon of the Maquoketa shales. Above it rise the Niagara 

 cliffs 100 to 200 feet high, and out on it stand the Mounds, isolated 



Fig. 254. View near Graf showing the effect of the Maquoketa shales on the topography. 

 The rounded swells and long, cultivated slopes are underlain by shales ; the steep, wooded 

 hill in the distance is composed of the overlying Niagara limestone. 



patches of Niagara, which are so characteristic a physiographic feature of 

 the region. A view showing the effect of the Maquoketa shales on, the 

 topography, near Graf, appears in figure 254. 



In Allamakee exactly similar features may be observed, the struct- 

 ural plane here being developed on the easily eroded St. Peter sands- 

 tone, and the Oneota and Galena-Trenton limestones forming the 

 lower and upper cliffs respectively. 



While the valleys are, as compared with the interstream areas, narrow, 

 they have nevertheless many of the marks of considerable age. As con- 



