ROCK ISLAND COUNTY. 219 



the Mississippi river flowed a mightier stream, both in its present chan- 

 nel and in the Maredosia slough, it was a broad headland sand-bar. 

 The bluff bounded highlands above described then rose as an islaiKl 

 from the broad lake-like river. The drifting sands lodged against its upper 

 end. and the sand-plain under consideration was gradually formed, just 

 as sand-bars of the present day are formed against the upper end of 

 river islands. 



This sand deposit took place during the long ages, while the Missis- 

 sippi valley was occupied by a mightier stream than the present river, 

 and a part of its waters found a channel through the Maredosia bot- 

 tom, and the present valley of Rock river below its outlet. I have dis- 

 cussed at length, in the geology of Whiteside county, the proposition 

 that the Mississippi once flowed through this latter bottom, and into 

 Rock river at Erie, and need not now make farther remarks upon that 

 subject. 



The narrow bottom from Hampton to Camden is an ancient shore or 

 beach. It is dry, and in some places rocky. But the most curious phe- 

 nomenon along the Mississippi bluffs is the old shore-line marked along 

 their sides. At Cordova, the principal part of the town is built upon 

 this ancient beach or terrace. It is here some fifty feet above the pres- 

 ent low water mark of the Mississippi river. It is distinctly marked all 

 along the blurt's to Camden. but runs lower as it passes from Cordova 

 to the latter place. 



The bluffs and hills of Rock Island county are composed in part of the 

 whitish buff' clays, sands, and marly deposits known as the loess. Re- 

 ceding back from the bluff lines the loess thins out, and is succeeded by 

 tine laminated drift clays, such as cover most of our upland barrens and 

 high prairies. Large boulders are of rare occurrence j so are genuine 

 drift gravel beds. Beds of recent river gravel mark the present shore 

 lines of the streams, but these are of very recent formation. 



J Me a s u res. 



In that portion of the county lying west of Rock river, the Coal Meas- 

 ures are found as outliers, overlaying and resting uncouformably upon. 

 the Devonian and upper Silurian limestone, as far north as the vicinity 

 of Port Byron, where they finally terminate. The most northerly point 

 where a workable bed of coal has been found on this side of the river is 

 at Rapids City, where the seam, probably the same as that two miles 

 east of Hampton, and at Carbon Cliff', is said to be from four to five feet 

 thick, and overlies the Niagara limestone, with only a few feet in thick- 

 ness of shales and fire-clav between. 



