622 GEOLOGY OF CENTRAL WISCONSIN.' 



east of the drift limit and south of the line of the Wisconsin Central 

 railroad, in Portage and Waupaca counties, and also to an indefinite 

 distance further north. It is most abundant in the Kettle Range it- 

 self, but is not entirely restricted to it. Even northward, into the 

 region of the Archaean rocks, the gravel is partly of limestone peb- 

 bles, which have been brought from the limestone formations to the 

 eastward. In the region north of the driftless area and west of the 

 Kettle Range including the valley of the Wisconsin as far north 

 as the northern line of Marathon county, and the country lying be- 

 tween the Wisconsin and Black rivers, in northern Wood, and in 

 Marathon and Clark counties whilst erratics are often seen, some- 

 times in clusters of very large bowlders, the coarse limestone gravel 

 appears wholly wanting. The fine gravel consists, more largely than 

 the coarse, of pebbles of quartz and various crystalline rocks. It is 

 to be seen, finely stratified, in the drift of stream valleys, and in some 

 places far away from the streams, as, for instance, on the divide be- 

 tween Black and Yellow rivers, where it occurs interstratified with 

 sand and clay to a thickness of over 100 feet. 



Sand appears to make up by far the largest part of the drift de- 

 posits. It is commonly light-colored and purely silicious, but is often 

 mingled with more or less clayey material, both when in the plainly 

 stratified and the more or less unstratified conditions. Occasionally 

 it is stained brown with hydrous iron-oxide, and when stratified alter- 

 nates in different colored bands. The explanation of the large pre- 

 ponderance of sand over clay in the Central Wisconsin drift will ap- 

 pear hereafter. 



Clay occurs, as already said, to a considerable extent mingled with 

 the sand, over which it sometimes preponderates greatly, forming a 

 firm, tenacious clay, which is stuck full of scratched and polished peb- 

 bles and bowlders, and appears to be identical with the '' till " of the 

 Scotch geologists. Such a clay, however, is not often to be seen. 

 Something like it appears in the heaps that lie on the high prairies 

 of northern Dane and southern Columbia, but the only places where 

 an apparently true till has been noticed are in the vicinity of Devil's 

 Lake, for a better understanding of whose position reference should 

 be made to Plate XIX, and the descriptions accompanying it. The 

 lake lies in a perpendicularly walled gorge, 500 to 600 feet deep, 

 which passes entirely through the main quartzite range of the Bara- 

 boo. This gorge is about three-quarters of a mile in width, and be- 

 tween three and a half and four miles in length. At the northern 

 end its course is nearly due north and south for over a mile, when it 

 turns nearly at right angles, and runs for the rest of its length but 



