CENTURY OF IOWA GEOLOGY 435 



by a master-stream much in the same position as it is today. By 

 the elevation of the Rocky mountains the rivers in the west must 

 have been directed eastward down the long gentle slope until they 

 finally reached the old Mississippi. Their present ending with the 

 Alissouri river is a later, or Glacial, consequence. At any rate 

 several of the primitive streams must have continued entirely 

 across the states of Iowa and Missouri. •^■* 



In marked contrast with the present smooth surface of our 

 state the relief which prevailed at the end of Tertiary times and 

 'niimediately before the first great ice invasion presents extreme 

 differences of altitude of between 300 and 500 feet. Over the old 

 elevations the drift is often scarcely more than a score of feet in 

 thickness. In some of the old depressions and valleys the Glacial 

 deposits are as much as 500 feet thick. The disposition of the low 

 places is such that they lie in long belts or gorges having relatively 

 steep sides. Some of "these primitive troughs are manifestly the 

 paths of extinct rivers. The one so well know at Des Moines is 

 now followed far beyond that neighborhood. Its narrow belt is 

 traceable northwestwardly to Sioux City where it unites with the 

 gorge of the Missouri river. 



EPI-CONTINENTAL ORIGIN OF GYPSUM DEPOSITS 



Iowa's massive beds of gypsum being among the most famous in 

 the country the mode of genesis attracts wide attention. So closely 

 is their deposition always associated with the drying up of em- 

 pounded waters of the ocean that any possibility of their having 

 been formed under conditions other than those involved in the 

 evaporation of sea-water is expressly precluded. 



Quite recently to be sure, we now find that gypsum beds ac- 

 cumulate on a large scale far away from the influence of the sea. 

 They are being extensively developed today on the highest and 

 driest part of our 'continent. Dunes of gypsum sands that are 

 vastly more pretentious than any of our Iowa beds accumulate 

 under the activity of the winds. The fact that in Tertiary times 

 western deserts probably extended eastward over the state sug- 

 gests that our gypsum deposits, too, may have been segregated 

 under conditions of excessive aridity. 



The recent determination of the geologic age of the Iowa gypsum 

 deposits, that the date of their origin is not Carbonic, nor Permian, 

 nor Triassic, nor even Cretacic, as has been repeatedly advocated 



*Proc. Iowa Acad. Sci., Vol. XXV, pp. 551-561, 1919. 



