IOWA ACADEMY OF SCIENCES. 73 



gorges cut in the Mississippi bluffs, and residences perched 

 airily on their summits, seem, at first sight, to be altogether 

 inaccessible. Deep, lateral valleys and ravines afford the only 

 practicable routes for highways of every sort, leading to the 

 higher levels west of the city; and all of these, for some dis- 

 tance, follow tortuous courses, and ascend at very steep grades. 

 The railway uses the -valley of Catfish creek as the sinuous 

 pathway whereby it is enabled to make the four hundred feet 

 of ascent necessary to reach the upland plain at Peosta. From 

 Dubuque almost to Julien, the fantastically weathered crags 

 and castles of Galeaa limestone, and the steep-sided hills, lit- 

 tered with loose fragments of the dolomite partially embedded 

 in the thin, residual soil, are prominent features of the land- 

 scape, and at the saoie time are easily understood characteris- 

 tics of the driftless area topography. The "Three Towers," 

 massive columns of dolomite, forty to fifty feet in height, 

 rising from the comparatively level bottom of the stream valley, 

 are seen a short distance west of Rockdale; and in the same 

 neighborhood aremaay picturesque escarpments and buttressed 

 walls and bistioned fortresses, carved by natural processes 

 from the living rock, and lending charm and variety to this 

 unique, drif cless landscape so strangely set in the midst of the 

 great prairie plains of the middle west. 



Before reaching Julien the surface inequalities become 

 toned down to a marked extent. The relief is very much less. 

 The outlines of the hills are more flowing, the curves more 

 gentle. The weather-beaten crags disappear. The surface 

 is yet rolling and irregular, as compared with the ordinary 

 prairie; but the land is susceptible of cultivation to a greater or 

 less extent, and most of it has been brought under the plow. 

 The ascending grade of the small valley followed by the rail- 

 way brings the surface here above the upper limit of the Galena 

 limestone, up to the softer Maquoketa shales, and it is this 

 shale formation that expresses itself in the cliffless, cragless, 

 softened landscape. The billowy sarface bears little resem- 

 blance, however, to even an eroded drift plain. There is a 

 certain tumultuoasness and confusion manifest in the swells 

 which mark the sarfac3. There is no common level to which 

 thair summits rise, but a kaob here and a ridge there may stand 

 conspicuously above all the rest, and it is altogether impossible 

 to pick out any set or series a ay where, which can be referred 

 to a common plane. The valleys of the larger streams are cut 



