N 0 II T II W E S T 



S II 0 R E 



OF LAKE SUPERIOR. 



339 



height of six hundred feet. These bays owe their existence, shape, and extent, to 

 the numerous trap dikes, with various bearings, which traverse all this region. 



The streams which empty their waters into the Lake along the northwest shore, 

 are all small, and the valleys through which they flow, narrow. From six to ten 

 miles before they reach the Lake, most of them cut their way through the softer 

 trap and metamorphosed rocks, and often flow through narrow gorges, with mural 

 walls of considerable depth. The wear of the rocks which form the beds and walls 

 of the water-courses, has been great throughout the whole District, but especially so 

 in that part lying south of the Fond du Lac and Mountain Lake Range ; where, 

 after following the strike of the valleys for various distances, they cut through the 

 rock ridges, and finally mingle their waters with those of the Lake, after a descent 

 of from seven hundred to a thousand feet in the course of a few miles. The pre- 

 vious higher level of the beds of many of these streams, is well marked by the pot- 

 holes which have been left on the ledges, which once formed the river-beds, but are 

 now many feet above the present level of the waters, and are bounded by perpendi- 

 cular walls of rock, which once formed the sides of the channel. 



The ravines and gorges through which most of the streams flow, after entering 

 the hilly range near the Lake, are only a few feet in width at the top, and often 

 appear to widen as they descend. Generally, however, they are almost, or quite 

 perpendicular, especially where they cut through ridges of hard rock. In many 

 cases, the gorges have evidently been cut out by means of boulders, which have 

 been arrested in their descent at various points, and under the action of the stream, 

 have excavated the lines of potholes just alluded to. These potholes gradually in- 

 creased in size until they coalesced, their walls were broken down, and the channel 

 thus deepened. Some of the gorges have, doubtless, been formed by fractures and 

 dislocations of the rocks, during some part of the eruptive period, others by the dis- 

 integration of veins and dikes, but many of them owe their existence to potholes in 

 the way mentioned. 



Some of the streams enter the Lake through narrow gates, with perpendicular 

 walls, while beyond the gate, landward, the river flows gently through a wide, flat 

 bed, for half a mile or more, until the first high ridge is reached, where there is, in- 

 variably, a fall, or a series of cascades. The mouths of many of the streams are 

 continually changing, partly in consequence of the detritus brought down from the 

 high ridges during freshets, but principally from the action of the Lake during high 

 winds, or when lashed by storms, the waves heaping up pebbles and boulders to the 

 height of several feet, and forming, frequently, complete barriers across them, 

 through which, however, their waters percolate into the Lake. These barriers re- 

 main until, after heavy rains in the highlands, they are swept away by the descend- 

 ing torrents. On a few of the rivers, at the distance of four or five miles from the 

 Lake, the valleys become wider, and along some of the streams tracts of what might 

 be called " bottom land" occur, rising with very gentle slopes to the base of the 

 hills which environ them. 



As before stated, these streams are often only a few feet in width, and ordinarily 

 discharge but little water. The largest river explored did not exceed forty yards 

 at the widest part seen, and this, two miles lower down, ran through a gorge only 



