THE 0VERLA:N"D route COUNCIL BLUFFS TO OGDEN. 



23 



West 



is only a few feet below the track level. The flood plain is here 10 

 to 12 miles wdde and is confined between bluffs 100 feet or more 

 height. It thus hes about 100 feet below the level of the Great 

 Plains, which extend far to the north and to the south. The small 



m 



Clarks. and Thummcl 



passed before the next city is reached. 



passes 



the Niobrara limestone, of Cretaceous age, to the formations of 



Tertiary age.^ (See table on p. 15.) If the younger 

 Central City. Cretaceous formations, the Pierre shale, Fox Hills 



Elevation l/)99 feet. 

 Population 2,42S. 

 Omaha 132 miles. 



Laramie formation 



ited here, they were eroded away before the Tertiary 

 beds were laid down. The contact therefore denotes 



period of time 

 formations were being eroded. 



a very 



long 



during which 



the older sedimentary 



not bo developed until some time after 



the pre-Kansan ice and all its climatic 

 effectahad disappeared fromBOuthweatern 



Iowa/' 



The character of the shells of the fresh- 

 water and land moUuskfl found in the 

 Af tenia n bedri shown that the climate was 

 similar to that of the present time. 



After tliis mild 



gtage the Keewatin 



8 feet, and caused many of the granitic 



pebbles to decay. 



After the melting of the Kansan glacier 

 the continental ice sheets did not again 

 reach as far as the line of the Union 

 Pacific Raihoad. At the last or Wiscon- 

 sin stage one lobe of the Keewatin glacier 

 invaded north-central Iowa, extending to 

 Des Moines, nearly as far south as the 

 latitude of Omaha, and another lobe 



glacier again spread southward and in- latitude of Omaha, and another lobe 

 vadedthereeion. The ice reached at this covered the northern and eastern parts 



Btage its greatest extension in northern of the Dakotas southward to a P«"^t =.bout 

 ^r:"...: ...a .....T,...t«vT, T^.n^as whence 90 miles north of Omaha, but Nebraska 



Kansas 



tliLs is kuomi as the Kansan stage of gla- 

 ciation. As shown on the accompany- 

 ing map (sheet 2) the western limit 

 of the glacial drift croests Platte River 

 near Columbus, Nebr. The Kansan 

 glacial drift that was uncovered in the 

 cuts made in South Omaha for the Lane 

 cut-off is blui.-h-gray clay containing red- 

 dish and purplish bowlders of qnartzite, 

 popularly known as "Sioux Falls gran- 

 ite," brought by the glacier from the 

 ledges exp«*ed near Sioux Falls, S. Dak. 

 This drift is not now well exposed m these 

 cuts, but it may be seen at a place U 

 miles we.t of PapilUon Creek, where it 

 forms the lower 10 feet of the section ex- 

 posed. Long exposure after the meltmg 

 of the Kansan ice has changed the original 

 blue-grav color of the upper part of this 

 drift to rasty red, dissolved out the solu- 

 ble calcareous ingredients for a depth of 



was not again invaded. 



An interesting deposit overljing the gla- 

 cial drift is exposed about 7^ miles north 

 of Omaha and at several places farther 

 west. It consists of volcanic ash which 

 must have accumulated after the melting 



Kansan 



with 



}ns, possibly those ot tne v^iuaier- 

 volcanoes of northeastern New 



nary 



Mexico. 



1 In marked contrast ^\ith the Cre- 

 taceous forma tious, which 

 down in shallow marine 



were 



laid 

 ■water and 

 which are regular in thickness and 

 character over vast areas, the Tertiap^ 

 deposits of this region are irregular in 

 thickness and character, are nonmarine, 

 and were deposited along streams or in 

 shallow lakes. During the Cretaceous 

 T^Priod Nebraska and certain other parts 



