38 GEOLOGY OF OHIO. 



be well designated. Newark, Lancaster and Chillicothe, for example, are 

 situated almost exactly upon the Glacial boundary. The Drift deposits 

 are separated by a vast period of time from the bedded rocks of the 

 geological scale of the state. Many millions of years would undoubtedly 

 be needed to fill the interval between the latest formation of the state, 

 viz., the Upper Barren Coal Measures, which mark the period when rock- 

 making in Ohio was brought to an end, and the deposits of the Glacial 

 Drift. These deposits consist of beds of sand, gravel and clay, variously 

 intermingled and distributed. Bowlders or large blocks of rock, make a 

 conspicuous contribution to the drift deposits. The clay, by reason of the 

 distribution of these bowlders through it in large amount, is named the 

 Bowlder clay. Another designation of it is Till, by which name a deposit 

 of similar age and origin is recognized in Great Britain. Although the 

 most recent of our geological formations, and separated from the present 

 but by a few thousands, or tens of thousands of years, the questions as to 

 its origin are still anomalous and perplexing to a remarkable degree. We 

 can give a much better account of the formations of Paleozoic time than 

 we can of this series that almost merges into the present. Suffice it to 

 say that all geologists now believe that the Drift series is the product of 

 two great lines of events which have worked separately in part, and in 

 part have worked in combination. 



The Bowlder clay, which is the "most characteristic of the Drift de- 

 posits, is now known to have been formed under land ice. It can only 

 be explained by the passage over the regions in which it is found of a 

 . sheet of land ice, advancing slowly from the northward. The beds of 

 gravel and sand, on the other hand, which| make the latest formation of 

 the drift, are the results of a re-arrangement of the materials of the 

 Bowlder clay in shallow basins of water. The bowlders which constitute 

 so marked a feature of the Bowlder clay can be, in multitudes of 

 instances, traced directly to the ledges of rock from which they were 

 derived. Many of them must have been transported from the Canadian 

 highlands, a journey of not less than five hundred miles. Upon the 

 University farm, within the limits of the city of Columbus, representa- 

 tives of various formations of Canada and northern Michigan have been 

 found, as, for example, portions of the conglomerate which has its out- 

 crops at the Bruce mines along the north shore of the River St. Mary's, 

 in Canada. Fragments of the Marquette iron ores and of Keewenaw 

 Copper are also found there; bowlders of the Pictured Rocks, or Pots- 

 dam sandstone of the south shore of Lake Superior also occur. All of 

 the material of the Drift is derived from regions to the northward. A 

 large portion of that which covers Ohio, at least in the central portion of 

 the state, is derived faom the formations that come to the surface in the 

 northern parts of the state. The limestone and the shale of these 

 regions has made very extensive contributions to these beds. The black 

 shale, especially, proved an easily wasted formation, as the ice sheet 



