PUBLIC PARKS OF IOWA 165 



On the wide outer curve of the valley the walls are steep and high. 

 In places they are timbered from base to summit, but elsewhere they 

 are too rocky to support a forest cover. In some places the rock ledges 

 form straight horizontal lines across the face of the bluff, while in other 

 places the entire slope is strewn with rock fragments, the wastage from 

 the solid strata beneath. 



The Backbone reveals a most interesting fragment of the geological 

 history of the region, a fragment which is writ large and clear for him 

 who would interpret its meaning. The rock strata belong to the Mis- 

 souri stage of the geological column, sometimes called the Upper Coal 

 Measures. The limestones over which the river is flowing and also 

 those which make up the upper part of the ridge tell of quiet clear 

 oceanic conditions, when myriads of humble types of life peopled the 

 waters and the ocean floor. These rocks are built very largely of the 

 shells and other hard remains of these animals of a far distant day. 

 Between the two ledges of limestone is a layer of dark gray to black 

 shale. This tellis of a relatively brief period when the waters were more 

 turbid and mud and silt were being carried in from nearby shores and 

 rivers. It is through these shales that the tunnel was dug to supply 

 water to the Tunnel Mill. Following the retreat of the ocean from the 

 region there is a long gap in the history, a gap which extends to the 

 Pleistocene period, the time of the advance of the great glaciers. Either 

 in the period just preceding the Pleistocene or else in the interval be- 

 tween the first two ice advances, the Nebraskan and the Kansan, Middle 

 river cut its valley in the plain. The valley was filled by detritus from 

 the glacieir and the river has been steadily engaged in clearing this out. 

 The great size now attained shows how successful it has been. When the 

 stream was flowing up on the level of the plain, nearly 200 feet above 

 its present position, it began meandering and as it sank into the loose 

 surface material and then into the rock its meanders became fixed and 

 continued to increase by the cutting of the stream into the outer walls. 

 In this way were formed the steep outer walls of the valley facing the 

 Backbone, while on the inner curves were being cut the long gentle slopes 

 at the end of the Backbone and the face of the terrace within the wide 

 bend to the east of the Backbone. 



The whole assemblage of phenomena gathered here, the long high 

 narrow ridge, the steep walls of the valley, the rock strata with their 

 contained fossils, the peaceful river with its flat flood plain, the variety 

 and abundance of the plant and bird life, with their rare and unusual 

 varieties, all of these make a grouping which renders the region of the 

 Backbone of great value scientifically and sceniically as well as from the 

 standpoint of pure pleasure. The region is certainly well worthy of 

 being set aside for a state park, and such action should be taken soon. 

 Enough should be included in the reservation to insure the perpetuation 

 of the beauty and safety of both sides of the river valley as well as of the 

 Backbone itself. 



