2. Pre-Kansan or Albertan Stage. The Ozarkian stage, or stage of epeirogenic uplift, was followed in time by the first glacial stage which, for reasons to be noted later, is provisionally named pre-Kansan. Ice streams having their sources in remote northern highlands, at length invaded Iowa, but how much of the state during this first invasion was brought under the dominion of the Ice King has not been determined. The record of this first glacial stage has only been partially recovered. Evidence of a pre-Kansan ice sheet, however, is found in a bed of till of marked individual characteristics, varying from dark drab to bluish gray in color and charged with numberless pebbles derived from very obdurate crystalline rocks. This, the oldest known glacial drift in Iowa, is exposed at a number of points in the neighborhood of Thayer and Afton Junction in Union county. Mr. H. F. Bain has found it in Polk county. Till occupying the same relation to later Pleistocene deposits is found beneath a bed of peat at the base of the great railway cut near Oelwein in Fayette county. It is probable that it is somewhat generally distributed, but it is only recently that it has been recognized at all as a definitely differentiated glacial deposit. Its separation from the overlying Kansan drift may possibly be indicated by an old peaty soil and forest bed at a depth of 115 feet in the deep well at Washington, Iowa. In both Europe and America one stage of the glacial series stands out pre-eminently above all the rest as marking the time of maximum glaciation. Without doing violence to legitimate inference, it has been assumed that the stage of maximum glaciation on one continent coincided in point of time with the maximum glaciation on the other. This has led to the correlation of the Kansan stage of America with Geike's Saxonian stage of Europe. Until recently the Kansan stage, from the best evidence at hand, was believed to represent the first ice invasion-at least in the United States -while Geike has shown that the Saxonian was the second glacial stage of Europe. On the other side of the Atlantic, therefore, the time of greatest intensity, widest distribution and longest duration of glacial conditions was preceded by the formation of glaciers and distribution of drift on a less extensive scale. To this earlier stage of glaciation Geike has given the name Seanian; and it is quite possible that the preKansan drift of Union, Polk and Fayette counties in Iowa may be referable to the Scanian stage of Europe. Evidences of early glaciation, preceding the stage of maximum intensity. have been observed by Dr. Dawson in the province of Alberta in Western Canada. If our pre-Kansan drift can be correlated with that observed by Dawson, this first glacial stage in the Pleistocene history of Iowa will be called Albertan. At all events the discovery of a pre-Kansan drift sheet brings the glacial series of Europe, and for this interesting discovery science is indebted to Mr. H. F. Bain of the Iowa Survey. 3. Aftonian Stage. There are two distinct drift sheets in the hills and the sides of the valleys around Thayer and Afton Junction. Between the two sheets there are interglacial deposits of unusual interest. First, there is an old soil bed testifying to a long period of temperate conditions during which the surface was free from ice, and numberless generations of plants found the situation congenial. Then there are beds of stratified sand and gravel, 50 feet in thickness, that were laid down upon the old drift surface before the soil was formed. The Afton gravels were derived from the pre-Kansan drift, and show the effect of torrential action in connection with the melting of the pre-Kansan ice. These gravels have been extensively excavated for railway ballast, and very satisfactory sections are found at a number of localities within a radius of four or five miles. Some of the layers are so firmly cemented as to form a conglomerate hard and compact as the basal conglomerates of the Paleozoic or Algonkian. These gravels may be seen resting on the pre-Kansan drift in the valley of Grand river, a mile and a half below Afton Junetion; the same relation is seen at Thayer; while less than half a mile north of the Junction they are overlain by a heavy bed of Kansan till. South of the Junction, indeed, both drift sheets may be seen with the whole thickness of the intercal ated gravels between them, but here only a thin layer of Kansan drift has escaped erosion. A word of explanation may be necessary with reference to the position here assigned to the Aftonian beds. MeGee's studies of Pleistocene deposits in northeastern Iowa led to the recognition of two drift sheets that he called respectively the lower and the upper till. Prior to McGee's work the bclief in the unity of the Glacial period, a single ice invasion, and a single sheet of till, as far as Iowa is concerned, was very general. Science is always conservative and the announcement that our glacial period was dual and not single, and that the two ice invasions were separated from each other by a long interglacial period of comparatively warm climate, was regarded with more or less distrust. Gradually the evidence produced conviction in the minds of those observers who had personal knowledge of the facts. The recognition of the duality of the glacial period, however, was as far as conservatism could go at a single step; and therefore when the Aftonian beds were seen to lie between two sheets of drift, it did not seem possible that these two drift sheets could be other than the lower and upper till of McGee. Later McGee's lower till was called Kansan and the upper till Iowan, and the Aftonian beds came to be looked upon as representing interglacial deposits between these two stages. Recent detailed studies of the Kansan and Iowan tills have made it possible to recognize and differentiate them over extended areas, and to fix with a fair degree of accuracy the limits of their distribution. The Iowan drift does not extend southward beyond the latitude of Iowa City nor to Des Moines, but the Kansan till with easily recognized characteristics is continuous southward and southwestward far beyond the limits of Iowa. It is the Kansan till, as determined by Bain, and not the Iowan, that overlies the Aftonian gravels. The till beneath these gravels is therefore provisionally named pre-Kansan, and the Aftonian interglacial stage is transferred to a different position from that first assigned to it, a position. preceding, and not following the Kansan drift. The length of the Aftonian interglacial stage is not very clearly indicated. The gravel beds were piled up in connection with the melting of the pre-Kansan ice and do not necessarily indicate any great interval of time. The soil bed at Afton Junction, the peat and forest bed in the Washington well, the peat bed at Oelwein,-these, together with other beds on the same geological horizon elsewhere, are more significant. They tell of a protracted period of time during which forests were fully established, and the climate was at least as mild as that of Maine and New Hampshire. That the Aftonian interval was of considerable length is further indicated by the fact that the gravels near Afton were trenched and eroded on an extensive scale during the time of their first exposure, between the retreat of the pre-Kansan and the advance of the Kansan ice. It should be noted that since the Aftonian interglacial stage of Iowa preceded the Kansan instead of following it as was at first supposed, it must now be correlated with the Norfolkian stage of Europe, and not with Helvetian as has been done in some American publications. 4. The Kansan Stage. The second glacial stage in Iowa is represented by a very heavy body of drift that, excepting the Driftless Area in the northeast, occupies the entire state. During the Kansan stage the ice covering Iowa was but an insignificant fragment of the great mer de glace that spread over more than half the continent of North America. South of Iowa, the ice continued down to the Missouri river. During this stage it reached its extreme southwestern limit and spread a sheet of drift over all northeastern Kansas. The same drift sheet passes without break into the states north and east of Kansas and may be traced continuously to the Atlantic seaboard. The Kansan was the period of maximum glaciation for North America. It has been correlated with the Saxonian, the time of greatest severity of glacial conditions in Europe. As compared with other glacial stages there is evidence that the Kansan was long, the snow fall was excessive, and the ice sheet attained a great thickness as well as great horizontal dimensions. |