BIG COTTONWOOD FORMATION 235 



Redstone and 200 feet lower than the granite outcrop on the Big Cotton- 

 wood, thus proving a considerable valley to lie buried here. 



From the direction of the cross-bedding in the sands, as already de- 

 scribed, the river is shown to have filled the valley just mentioned from 

 east to west, and the original valley may also run east to west. I have 

 searched for evidence of this valley and the Big Cottonwood formation 

 along the Minnesota river below Mankato, Minnesota, and along the Blue 

 Earth and Le Seur rivers, which together make a north-to-south section 

 crossing the line which the valley and formation must have followed if 

 they extended that far. The clays and sandstones which are to be seen 

 there, and which N. H. Winchell and Warren Upham descril^ed as Cre- 

 taceous, are in fact only the rotted or residuary portions of Paleozoic sedi- 

 mentary rocks in main. In places, 20 feet of clay occur with stratification 

 partially preserved and with quartz geodes, silicified oolite, fossils, and 

 other marks showing the original rock to have been the Oneota and Ska- 

 kopee dolomites ; or, again, the same clay extends into dejiressions, or verti- 

 cally into the dolomite as seams, or it fills "potholes," which same are 

 leached and enlarged joints. Occasionally the leaching has made resid- 

 uary clay in horizontal joints likewise. Original beds and strata of sand 

 remain in the red clay. Such materials correspond to the rotted granite 

 under the Big Cottonwood formation, and not to that formation. These 

 residuary materials occur between 800 and 875 feet above tide, the main 

 preserved part lying at 825 to 850 feet. On the Le Seur river (north 

 line of section 2, town of Eapidan, Blue Earth county) there are 12 feet 

 of clean gravel of finely polished siliceous pebbles, an inch or less' in diam- 

 eter, some of which are the remnants of Silurian and Devonian fossils. 

 This rests on coarse sand 20 feet thick over the Jordan sandstone. The 

 top of it is about 840 feet above tide. This gravel and sand correlates 

 evidently with the Big Cottonwood formation, but it lies in a shallow 

 valley, as the section along the river indicates. The cross-bedding in the 

 gravel is from east to west. At one place a small pocket or lens of fire- 

 clay was formerly discovered over the gravel. It has now been mined out. 



The evidence of the leached rock floor and the Cretaceous sand and 

 gravels along the Le Seur and Blue Earth rivers, and of the residuary 

 clays in the Oneota dolomite near Mankato, Kasota, and Ottawa, along 

 the Minnesota river, indicates that the general surface of the old land 

 along that section lies now between 850 and 900 feet above tide and was 

 originally high above the Big Cottonwood formation. From such a land 

 area the red and blue shales, white sands, polished siliceous pebbles, and 

 that which is now blue clay could have been derived. Between Kasota 

 and Ottawa, and especially north of Ottawa, the rock wall is missing along 

 the present Minnesota valley and possibly a pre-Cretaceous course, and 



