IOWA ACADEMY OF SCIENCES. 75 



On the line followed by the Illinois Central railroad the edge 

 of the Niagara does not extend far beyond the margin of the Kan- 

 san drift; but south of the valley of Catfish creek, and conspicuous 

 even from the car window, the Niagara gives character to a high 

 ridge, noted above, which extends two or three miles into the 

 driftless area; and at Sherrill's Mound, in the northern part of 

 the county, the Niagara escarpment is nearly ten miles east of 

 the edge of the drift. The drift-covered area is entered by the 

 railway line less than a mile east of Peosta. A cut, thirty to 

 forty feet in depth, is made in the thickened margin of the 

 Kansan till one-half mile east of the village named; and at the 

 station the traveler has at last an uninterrupted view to the 

 western horizon. He has reached the upland plain, a plain which 

 varies but little in altitude all the rest of the way across the state. 

 He has passed from the driftless area where the topographic 

 forms are the resultant of erosion acting on indurated rocks 

 of var j7ing degrees of hardness, to an area of comparatively incon- 

 spicuous topographic forms developed by erosion of a body of 

 drift which was originally left by. the retreating ice fields, with 

 a surface approximating a plane. It was not a true plane, how- 

 ever, but a drift plain diversified by numerous undulations 

 which were due to two facts — the ice deposited more material 

 in some places than in others, and the mantle of glacial detritus 

 was not in all cases sufficient completely to disguise the high 

 ridges and the deep valleys of the preglacial topography. The 

 thickening of the Kansan 'drift along its ultimate border has 

 given rise to a well marked ridge which curves so as to trend 

 northeast and southeast from Peosta. The drift plain is 

 inclined very gently to the west, and all the drainage streams, 

 for some miles back from the drift border, take a westerly or 

 southwesterly course. A new type of topography now engages 

 the attention of the observer. The landscape stretches away in 

 an unbroken plain to the far horizon, but the surface of the 

 plain, as in all the Kansan drift areas of Iowa, is carved into 

 a dendritic system of miniature hills and valleys. Compared 

 with the driftless area which was traversed only a mile or two 

 east of Peosta, the visible -effects of erosion are very small. 

 Except along the permanent streams the rain sculpture has 

 affected only the drift; it has not exposed any of the indurated 

 rocks. Making allowance for the original undulations of the 

 plain, it may be said that the symmetrically rounded swells 

 of the surface, in striking contrast with the tumultuous and 



