44 PUBLIC PARKS OF IOWA 



of the life forms of the time, forms which dying left their shells on the 

 bottom and in the muds beneath. These shells and their casts today 

 bear clear witness to the abundance of the life of those days, as the ac- 

 companying figure shows. When we realize that these types persisted 

 so long that their fossil remains are found through sixty feet of lime- 

 stone we can understand what hosts there must have been. 



Once and again in all probability the seas covered our area, but the 

 materials which were spread over their floors have long since been swept 

 away, at least from the particular region with which we are now con- 

 cerned. Rocks of the Devonian period are present in southwestern Dela- 

 ware and the southwest half of Buchanan counties, and unquestionably 

 they once extended much farther to the northeast. Whether the seas of 

 Mississippian or later ages advanced thus far to the northeast we do not 

 know. However, for age upon age, northeastern Iowa, as a part of a 

 great central land-mass, lay exposed to all the destructive forces of na- 

 ture until its 'once level plains were dissected into deep valleys and high 

 hills. Then the great Nebraskan continental glacier advanced from 

 the north, crept over these hills and valleys and buried them beneath its 

 load of debris. In time the long winter merged into spring, the ice cap 

 disappeared and vegetation covered the bare gray plains. After a long in- 

 terval a second glacier, the Kansan, gathered its forces in the far north 

 and again covered valley and hill and plain with its icy mantle, and, melt- 

 ing away in turn, left its load of glacial drift spread over all the invaded 

 lands. Upon these filled up and leveled off plains young streams at once 

 set to work and in time cut a new series of valleys, very largely if not 

 entirely independent of former systems of drainage. 



The third glacier of the series, the Illinoian, did not feach the Back- 

 bone region, but the fourth one, the lowan, coming from the northwest, 

 extended beyond the Backbone and across most of Delaware county. A 

 remarkable feature of this glacier was that it seemed to have avoided cer- 

 tain areas in its path, as is evident by the absence from them of any drift 

 of lowan age. The Backbone is in one of these areas and so it is that while 

 all about are smooth gentle slopes and shallow swales of the lowan area, 

 within the Backbone region itself are the deep-cut valleys and vertical 

 rock-walled cliffs of the Kansan drift area and of the Driftless area to the 

 northeast. The reason for these anomalous conditions probably lies in 

 the fact that the Backbone region rises above the surrounding country, 

 that while its hills and ridges reach heights of 1,150 to 1,200 feet above 

 sea level, in the country round about similar altitudes are not reached 

 for several miles distant to the north and northwest. Thus the lowan 

 glacier, which carried only a light load of debris, and which probably was 

 itself but a thin sheet of ice, was unable to surmount these outstanding 

 prominences and never covered them with its veneer of mingled clay and 

 gravel. However, there was laid down over these rugged hills a layer 

 of fine wind-blown dust known as loess which covers but does not con- 

 ceal the topography of an older time. 



The question naturally arises as to the reason for this great loop in 

 the river which causes the Backbone. The most probable answer seems 

 to be that when drainage began on the level Kansan drift plain its course 



