ORIGIN 305 



terials, sedimentary structures, distribution of deposits, and nature of 

 the underlying surface. 



COMPOSITION AND ASSORTMENT OF THE MATERIALS 



The materials of the Windrow formation fall into two general groups : 

 (1) the pebbles and associated sands and clays, and (2) the iron ozides. 

 The chert pebbles are shown by their fossils to have been derived from 

 Paleozoic limestones, none of which was younger than the Devonian, 

 Most of them came from the Niagara dolomite. They are generally 

 highly polished, and well rounded specimens are extremely rare. At 

 Devils Lake a few chert fossils were collected which show practically no 

 wear. These facts prove that the transportation of the cherts has greatly 

 varied, and, as so few of them are well rounded, it follows that they have 

 not been subjected to much washing and are probably of stream rather 

 than of beach origin. The quartz pebbles do not appear to have been 

 derived from any of the Paleozoic formations now exposed in the upper 

 Mississippi Valley, and it is quite probable that their mother rock is in 

 the pre-Cambrian. All are well polished and rounded, but not one of 

 lenticular shape has been found. It is concluded that they have been 

 brought a great distance by streams and were not washed |long a beach. 

 If the surface on which the Windrow formation rests be projected north- 

 ward, it will be seen that the nearest possible source for the quartz pebbles 

 is the pre-Cambrian of northern Wisconsin, Michigan, or Canada. It is 

 possible that the pebbles did not make their journey in one period; they 

 may have been formed into a conglomerate which was later destroyed to 

 make the Windrow formation. If so, no direct evidence of the existence 

 of such a conglomerate has been discovered. 



The assortment of the gravels and associated sands is very poor. Chert 

 and quartz pebbles are intimately intermixed, and in a small hand speci- 

 men variations in size from a tenth of a millimeter to two or three centi- 

 meters is common. The sandstones show 'similar imperfect assortment. 

 These characters, while they do not preclude marine deposition, strongly 

 suggest river origin. 



The iron oxides are present as concretionary masses which range up to 

 several tons in weight and as a cement for the pebbles and the sands. 

 They are characterized by a variable state of hydration and by the pres- 

 ence of a variable percentage of manganese, phosphorus, and clay. In- 

 cluded in the iron ores are boulders of the underlying rock, unworn 

 silicified fossils, and lenses of conglomerate. The intimate association of 

 the different kinds strongly suggests, if it does not prove, ■ that their 

 deposition took place simultaneously. The iron oxides are of the bog ore 



