294 thwaites and twenhofel the windrow formation 



Introdootion 



At widely separated points on the higher uplands of the Driftless and 

 adjacent areas of Wisconsin, Iowa, and Minnesota, there occur conglom- 

 erates and gravels with associated limonite, clay, and sandstone. 



The interest of the writers was drawn to these gravels while they were 

 engaged in the study of the geology of the Tomah and Sparta quadran- 

 gles, in western Wisconsin. As the prohlem of their origin could not be 

 solved within the limits of that area, the study has been extended over a 

 wider field and the available literature has been examined for descrip- 

 tions of similar formations in adjacent States. These studies have led 

 to the assembling of a considerable amount of information and to con- 

 clusions relating to the origin and age of the deposits. 



As these deposits merit a name, it is proposed to call them the Windrow 

 formation, the name being selected because of an excellent exposure on 

 the top of Windrow Bluff, near Tomah, Wisconsin. 



Description of the Windrow Formation 



The Windrow formation consists of quartz and chert pebbles in a 

 matrix of quartz sand and l^rown iron oxide, iron oxide cemented sand- 

 stone, concretionary limonite, and at some localities blue and white sticky 

 clay. In many places only pebljles are present, as the matrix has weath- 

 ered away. 



The pebbles are mainly quartz and chert. Those of quartz are univer- 

 sally well polished and rounded to spherical and elliptical shapes. The 

 chert pebbles are also Avell polished, but are mainly of subangular shapes. 

 Most of the pebbles are small ; examples of greater than an inch in diam- 

 eter are rare, although a few chert boulclers up to a foot in diameter have 

 been observed. These are not rounded or polished. The relative abun- 

 dance of chert and quartz varies widely. At an occurrence southeast of 

 Sparta, Wisconsin, a count gave 50 per cent chert, 45 per cent yellow and 

 milky quartz, and 5 per cent pink quartz. At a closely adjacent locality 

 it was found that 75 per cent consists of yellow and milky quartz, 24 per 

 cent of black, gray, and brown chert, and 1 per cent of pink quartz, with 

 an occasional pebble of dolomite. The quartz pebbles vary considerably 

 in shades of color and are utterly unlike any material found in the Paleo- 

 zoic rocks of the region of their occurrence. They closely resemble the 

 vein quartzes of the pre-Cambrian rocks to the north and northeast and 

 it is probable that such is their source. 



Fossils are rather common in the chert pebbles. Not uncommonly they 

 are well preserved and in a few instances a fossil makes an entire pebble. 



