(;|':m;i:ai. (ii;()i.(KiV oi' riii'; iM':(ii(>\ l!)ll 



fcionu'tiiucs even reacliini!,- a vortical position. Tiio Kcwt'onawaii rocks 

 coiitiiuic to outc'ro[) at rare intervals ahuii:; the lake shore for a (lislaiuu! 

 of foiii- miles. At Clinton point the Canil)riaii sandstone takes their 

 l)laee and continues to he the only rock formation ex})osed on the lake 

 shore until .Minnesota is reached, 'i'he sandstone, locally termed the 

 Lake ►Superior sandstone, is of such importance that it will receive sep- 

 arate treatment, and need not he further considered at this point. 



Cieneral!}' the consolidated sediments are masked h}' glacial nuiterial, 

 either till or washed deposits of silt and gravel. The clay predominates 

 in all the clifis, although washed deposits replace it in a few instances. 

 This is especially true of the sliores at the foot of Chequamegon l)ay. 

 Tht> thickness of the glacial debris is great and elTectually hides the un- 

 derlying formations in most localities, except where wave action has 

 removed the glacial deposits. A well-boring at Ashland shows that the 

 glacial drift at that point is about 300 feet thick. 



LAKE SUPERIOR SANDSTONE 



Equ'ivdlency and general character. — The Lake Superior sandstone has 

 been derived from the Penokee and Douglas ranges, whose ridges still 

 rise consi)icuously above the sjeneral level of the region. The sandstone, 

 which is probably the equivalent of the Potsdam, is a highly ferruginous, 

 thick-bedded rock. The iron present does not act as a cement ; it simply 

 coats the grains of silica, which chiefly compose the formation. The 

 cement is silicious and is usually })resent in such qu^mtities as to render 

 the rock ver}'^ resistant to weathering. Fine clay frequently forms the 

 matrix of the sandstone, and in such cases the sandstone is friable and 

 weathers readily. The clay often becomes so abundant that the rock 

 takes on shaly characteristics and becomes laminated. This is true of 

 the exposures at the north end of Sand island. In addition to its dis- 

 semination throughout the sandstone, the clay often occurs in lenticular 

 masses, var3dng in size from a fraction of an inch to a diameter of sev- 

 eral feet. Apparently they are not nodular in character, but are masses 

 of clay which accumulated as the sand was deposited. Weathering 

 readily removes these accumulations, giving the rock a characteristically 

 pitted appearance. 



Cross-hedding . — Cross-bedding is a common feature of the sandstone ; 

 it occurs in all of the layers exposed to examination. Its most char- 

 acteristic habit is the large scale on which the flow and plunge structure 

 is developed. This becomes especially noticeable along the shore of the 

 lake, where the waves have removed the soil and left a large area of rock 

 surface exposed. In such localities the structure reminds one of the 

 arrangement of blocks in an ice jam. 



