388 A\ F. Griggs — Divided Lakes in Western Minnesota. 



Art. XXXIII. — Divided Lakes in Western Minnesota ; by 

 Robert F. Griggs. 



No feature of the physiography of western Minnesota is 

 more notable to the casual observer than the large number of 

 twin lakes. Throughout Becker and Ottertail counties and 

 presumably over a much larger area a very large proportion of 

 the lakes are partially or completely divided by narrow necks 

 stretched across from shore to shore. This feature is very 

 striking to the traveler in the region, because these narrow 

 ridges cutting the lakes in two are almost always traversed by 

 the main roads, which thus connect the country lying on oppo- 

 site sides of the lake, the importance of the road depending on 

 the size of the lake and the consequent amount of country 

 benefited by the short cut. These dividing ridges are ice- 

 pushed ramparts, features which, on lake margins, are well 

 known and have been fully described by others. But no one, 

 so far as the writer has found, has shown the possibilities of ice- 

 shove in building new shore lines and cutting up the lakes in 

 which it works. 



The mechanism of ice action and its effects in building ram- 

 parts of sand and bowlders along the shore have been most fully 

 described by Bulkley ('00) and Gilbert ('90, '08). Briefly it 

 is this : If the ice covering a lake is subjected to further 

 declines in temperature after freezing, it contracts and is broken 

 up by cracks running in every direction ; water from below now 

 seeps into these cracks and freezes. Thus the shrinkage in 

 volume is taken up by new ice, so that when the temperature 

 rises and the ice expands the whole sheet most enlarge. This 

 enlargement causes the ice, carrying with it whatever bowlders 

 or other material it may grip, to shove up onto the shore if it 

 be low, or if it be a resistant cliff, to pile up against it, or if the 

 lake be long and narrow, to buckle up in the open water where 

 the major cracks due to lateral friction and other causes create 

 zones of weakness. The shove may be very slight in the begin- 

 ning but as it is repeated with every change of the weather 

 until the ice is broken up, its cumulative effects are so great as 

 to push, up large ramparts of bowlders or to cause considerable 

 destruction along the shore. 



A typical example of an arm of a lake cut off from the main 

 body by an ice-pushed rampart occurs near the northeastern 

 angle of Lake Pelican (fig. 1), where a small pond is separated 

 from the lake b} T such a barrier. This rampart, though very 

 narrow, stands quite high above the water and supports a row 

 of trees. Its back side slopes somewhat gently into the swampy 



