138 GEOLOGY OF EASTERN WISCONSIN. 



chiefly from the yielding Potsdam sandstone, the direction of drift 

 movement being here to the westward. It is probable that a river 

 channel existed there jireviously, which enabled the ice to act with 

 greater efficiency. 



Green Lake, Puckawa l/ike and Bush Lake all lie in one valley 

 along which the glacier plowed its way. Eush Lake was eroded from 

 the soft St. Peters sandstone, having the harder Lower Magnesian 

 limestone for its bed, while the Trenton limestone borders it on the 

 east. The rise that separates this from Green Lake is scarcely per- 

 ceptible. The east end of the latter lake occupies the same geological 

 horizon, but the dip of the strata is such at this point that, in its 

 length, the lake cuts across the Lower Magnesian limestone, the Mad- 

 ison sandstone, the Mendota limestone, and into a still lower division 

 of the Potsdam sandstone. At the west end it is confined by a range 

 of drift hills crossing the valley. To these this beautiful lake may be 

 said to owe its existence. If they were removed, the lake would dis- 

 charge itself into Lake Puckawa, to the west, its elevation being the 

 greater. The contour of these hills shows their morainic character 

 and indicates that they were heaped up there by the tongue of ice that 

 filled and in part eroded the valley. 



Lake Puckawa was eroded in the same way, but from still lower 

 strata. On its south side there rises a cluster of hills of Archsean 

 rocks, which, by their hardness and powers of resistance, may have 

 forced the ice mass to more deeply erode the softer sandstone repos- 

 ing on their flanks. 



Lake Shawano lies along the northern base of an east and west line 

 of bluffs capped by Lower Magnesian limestone resting upon Potsdam 

 sandstone, from which the basin of the lake has been excavated. Its 

 longer axis harmonizes with the direction of glacial movement, which 

 in this region was from the east to the west. "We have, then, an 

 easily eroded stratum, adjacent to a more resisting one, with a com- 

 jjetent eroding agency acting in a favorable direction, resulting in the 

 formation of Lake Shawano. 



Lake Koshkonong likewise lies in the direction of glacial progress 

 and is due to the ease with which the St. Peters sandstone was exca- 

 vated. An ancient stream had probably cut down to and perhaps 

 through it — for such is the fact in reference to the preglacial Eock 

 river channel farther south — and the ice plow, taking advantage of 

 this, furrowed and filled, leaving a wide, shallow basin. 



The foregoing lakes, it will be noticed, have a definite relationship 

 to the geological formations adjacent and subjacent to them, and are 

 all to be accounted for on essentially the same principle, viz. : The un- 



