74 IOWA ACADEMY OF SCIENCES. 



well below the general level of the region, the process of pen- 

 eplanation is yet far from being complete, and the confused 

 appearance of the land swells is due to the fact that the various 

 parts of the surface are not being degraded at the same rate. 



A few miles west of Julien, and sweeping around in a long, 

 prominent ridge a mile or two south of the railway line, are 

 the abrupt slopes and cliffs of the Niagara limestone which 

 overlies the Maquoketa shales. The Niagara, like the Galena, 

 is dolomitic. It resists weathering and forms mural cliffs only 

 slightly less pronounced than those of the older and more per- 

 fectly crystalline dolomite already noticed. The total thick- 

 ness of the Maquoketa formation is something more than two 

 hundred feet, but the area within which the shales are super- 

 ficial — the area between the outcrops of the upper beds of the 

 Galena and the base of the Niagara cliffs — is comparatively 

 narrow, rarely exceeding three miles in this part of Dubuque 

 county. All around the foot of the Niagara escarpment the sur- 

 face of the Maquoketa. area presents a series of long, rain- 

 sculptured, cultivated slopes, rather steep, but gradually blend- 

 ing into the more level area encountered after passing from the 

 rugged topography of the Galena limestone to the more soft- 

 ened forms of the Maquoketa, near Julien. 



A mile or two east of Peosta the deep trenches and steep 

 scarps characteristic of the margin of the Niagara limestone 

 are encountered. Erosion has produced picturesque effects, 

 not so pronounced maybe, but yet in a measure comparable 

 with those seen in the region occupied by the Galena limestone. 

 The base of ttie Niagara is somewhat softer than the Galena. 

 Not far above the base are rapidly weathering beds containing 

 a large amount of chert. As a result of differences in structure 

 there are fewer vertical precipices in the Niagara than in the 

 Galena area, and this is particularly true when the cliffs are 

 formed by erosion of trenches in the basal portions of the 

 Niagara formation. Talus material accumulates at the foot of 

 the scarps, and the upper part of the cliff faces recedes as an 

 effect of weathering. While the resulting slopes, as a rule, 

 are not vertical, they yet stand at high angles; and so the 

 transition from the area of Maquoketa shales to that of the 

 Niagara is marked by an abrupt and steep ascent of sixty to one 

 hundred feet. This sharply defined offset is one of the most 

 striking topographic features in this part of the driftless area. 



