FOURTH ANNUAL YEAR BOOK — PART II. 123 



glacial epoch, when the ponderous ice mills were grinding the primeval 

 rocks, ' 'slowly but exceeding fine, " to provide a covering of drift as the 

 bed of the richest deposits of soi! found 0:1 earth. 



PHYSIOGRAPHY OF IOWA, 



By Prof. Samuel Calvin, State Geologist. 



General Statement . —\\. would neera that a very short chapter ought to be 

 sufficient to include all that can be said concerning the physical features of 

 Iowa; for the state is simply an extensive plain— over large areas a very 

 monotonous plain — lying between the great rivers and rising but little above 

 them at any point. The relief is small. The zero point on the river gauge 

 at Keokuk has an elevation above tide of 477 feet; the elevation of Sibley, 

 the highest important railway station in Iowa, is 1,572 feet. It is possible 

 that Ocheyedan mound or some of the morainic prominences in Osceola 

 county rises 100 feet higher than Sibley, but even then there is less than 

 1,200 feet of diflference between the lowest and the highest points in the state. 

 One hundred feet is gained at once by ascending the bluflfs at Keokuk and 

 passing on to the upland a short distance northwest of the city, and so there 

 is left but about 1,100 feet as the sum of all the variations in level occurring 

 over the general surface of the whole great state of Iowa. There are stretches, 

 many miles in extent, so monotonously level that differences in altitude are 

 scarcely perceptible. 



TOPOGRAPHY. 



Larger Features. — Looking at the state as a whole there are a few con- 

 spicuous topographic features worthy of special note. On the eastern border 

 the Mississippi flows in a gorge which, at New Albin and Lansing, measured 

 from the summit of the bluffs facing the valley, is 400 feet in depth. From 

 the level of the divides a short distance back in the interior, the depth 

 exceeds 600 feet, an amount equal to more than half the sum of all the 

 variations in altitude encountered in the entire state. The depth of the Mis- 

 sissippi gorge diminishes toward the south. Instead of measuring from 600 

 to 700 feet between the flood plain and the higher levels as in the northeastern 

 corner of the state, there is a difference in altitude between Dubuque and 

 the upland at Peosta of only 430 feet; between Davenport and Walcott the 

 difference is 190 leet; between Keokuk and New Boston, 140 feet. Further- 

 more the valley is a curious patchwork of newer and older parts. At New 

 Albin, Clinton and Burlington the valley is old, wide and deeply filled with 

 mud. It is comparatively young at Dubuque, and younger still at Le Claire. 

 Twice at least in the course of recent geological history the great stream has 

 been forced to abandon parts of its old valley and cut several miles of chan- 

 nel relatively new. The narrow, rock-bottom gorge above and below Le 

 Claire is yet unfinished; adjustment of stream to valley is not yet complete. 



The valley of the Missouri river is very different from that of the Missis- 

 sippi. It is bordered by a series of bluffs unique in appearance and more 

 unique in structure, for they have been built up largely of fine dust trans- 

 ported by the winds. The constantly shifting meanders of the stream and 

 the great width of the level alluvial flood plain are among the striking char- 

 acteristics of this peculiar valley. 



