THIS LAND 



St. Croix River flows south into the distance, separating the western, Minnesota side 

 of Interstate State Park (cliff in foreground) from Wisconsin, on the far side. 



Alon 



Pothole Trails 



A river that runs between Minnesota 

 and Wisconsin has left a legacy of its wild youth. 



By Robert H. Mohlenbrock 



P 



_ 



feet 



otholes are saucer-shaped or 

 cylindrical depressions 

 scoured into hard rock sur- 

 faces by whirlpools of water carry- 

 ing fine gravel. Such abrasive 

 whirlpools can form in swift streams 

 or in waters stirred by the wind; in 

 either case the hollow you see today 

 may be the result of thousands of 

 years of weathering. On the east 

 side of the St. Croix River, near St. 

 Croix Falls, Wisconsin, there are 

 dozens of potholes of various size 

 and shape, including what is billed 

 as "the world's most perfect pot- 

 hole": a circular hole about four 

 and .1 halt feet wide and eighteen 



feet deep, with spiral striations on the 

 inside surface of its walls. On the 

 western, Minnesota side of the river 

 are numerous other potholes, many of 

 irregular shape and some as deep as 

 sixty-seven feet. All of the depressions 

 are known as glacial potholes because 

 they were carved by rushing glacial 

 meltwater. The potholes, along with 

 scenic gorges, fanciful rock forma- 

 tions, moist and dry woods, and other 

 natural attractions, are protected 

 within the boundaries of Interstate 

 State Park, which spans the St. Croix 

 River and parts of both states. 



Before the potholes could be 

 sculpted, the rock had to be in place. 

 The oldest rocks exposed here are 

 volcanic, part of an accumulation of 

 lava that flowed from deep within the 

 earth into what is now the basin of 

 Lake Superior. The lava flows, which 

 began about 1 . 1 billion years ago, are 

 14,000 feet thick at Interstate State 

 Park. (Closer to Lake Superior, 1 ,000 

 miles away they are 60,000 feet 

 thick.) Following the formation of 

 the lava rock, great seas flooded the 

 region several times, leaving deposits 

 of gravel and sand. The deposits con- 

 solidated into sandstone, shale, and 

 conglomerate rock. The seas subsided 

 about 70 million years ago, and rivers 

 began to cut into the layers of rock. 



About 1.8 million years ago the 

 climate cooled, snow fell faster than it 

 could melt, and continental glaciers 

 began to form. Four rimes glaciers 

 ground southward across the site of 

 what is now Interstate State Park, be- 

 fore they melted and retreated, leaving 

 gravel, stones, and other debris be- 

 hind. Then, about 10,000 years ago, 

 as the last ice age was ending, glacial 

 Lake Duluth began to fill. The lake — 

 the precursor of Lake Superior — 

 spawned the St. Croix River, whose 

 raging waters began to break through 

 cracks in the lava rock. The river and 

 its branches eventually carved canyons 

 that can be seen in Interstate State 

 Park. Waterborne sand and gravel, 

 swirling in whirlpools, gradually 

 carved out the glacial potholes. As 

 the rivers dug deeper, narrower 



54 NATURAL HISTORY June 2006 



