42 IOWA ACADEMY OF SCIENCES. 



about five miles a well more thau 300 feet deep was bored and no bed, 

 definitely recognized as Cretaceous, was shown in the record; though it is 

 possible that some of the upper clays there met were of this age. No 

 samples of the borings were seen. 



It must be remembered that the surface of the Upper Carboniferous at 

 the incursion of the Cretaceous sea in Iowa was not regular; perhaps even 

 more irregular and broken than the surface of the strata is to-day. Deep 

 channels, gorges, depressions, and rises marked the entire surface. The 

 Cretaceous as a shoi'e deposit maj'^ have wholly filled these Carboniferous 

 channels and hollows, spreading itself in great depth near the floor, or 

 partly leaving protuberances and ridges of higher elevations uncovered. 

 However this may have been, the friable Cretaceous was, after the time of 

 its laying down, greatly modified both by the preglacial weathering agen- 

 cies and the glacial grinding and corroding. Daring those stages new 

 channels were cut, others more deeply corroded, many extending through 

 the entire thickness of the formation; large areas were disturbed, only to be 

 obliterated by the i-epeated advancement and retreat of the glacier, and the 

 high and low points were alike mantled with drift debris. The southern 

 and southeastern limits, would, since the glaciers traveled in a southeasterly 

 direction, naturally be more altered than would other portions of this shore 

 deposit, the original shoi-e line would be wholly displaced and a new line, 

 probably a number of miles northward, left to mirk the present irregular 

 boundary. Thus it may be seen that the Cretaceous is not one persistent 

 bed everywhere of the same thickness with its boundary an unbroken line, 

 and its character unvarying. 



Now extending from some of the outliers noticed the topography presents 

 itself, just as it appears at the outlier, sometimes for several miles in length. 

 To cite a case, consider the outlier which is exposed at Coburg and in that 

 vicinity. Here for several miles to the southwest, between the WestTarkio 

 and the East Nishnabotna rivers the upland topography such as at Coburg, 

 continues without any abrupt change. Again, while no Cretaceous has been 

 noted as occurring between the East Tarkio and the West Nodaway rivers, 

 the topography in Page county between these streams resembles, in many 

 respects, that between those streams to the west, along which Cretaceous 

 beds have been found in Montgomery county. 



It would appear, therefore, from surface features of this county, that the 

 upland between these four streams are made up largely of Cretaceous depos- 

 its covered only by a mantle of drift. If this is so, it is probable that in the 

 two counties lying in the most northwestern portion of Missouri, along 

 the northern border, and through the entire length of Page county, Iowa, 

 will be found jnst such beds of Cretaceous age as occur farther northward in 

 the latter State. Additional examination of the region at hand may bring 

 out definite results and prove that Cretaceous beds do now in reality exist 

 in the doubtful localities just mentioned. 



In doing this work it must be borne in mind from what has been said, 

 that because the topography appears so in any place it does not necessarily 

 follow that under such topography rests the Cretaceous; the marginal shore 

 deposits may have been so modified and the debris from the Ice age so 

 unevenly laid down that the existence or non-existence of the Cretaceous can 

 no longer be recognized by mere topographic features of the land surface. 



