428 WALTER H. SCHOEWE 



EXTINCT LAKE CALVIN 



Lake Calvin was formed at the time of the third or Illinoian 

 ice invasion. The great ice sheet coming from the state of Illinois 

 found its way westward, filled the vaUey of the Mississippi River, 

 and advanced into the southeastern border counties of Iowa. In 

 its advance, the mighty river had to abandon its course and was 

 forced to find the new one described above. As the present course 

 of Iowa-Cedar River was occupied by the great ice mass forming 

 a dam across its valley at Columbus Junction, the combined waters 

 of Mississippi, Maquoketa, Wapsipinicon, Cedar, and Iowa rivers 

 and those flowing from the edge of the ice were ponded back, 

 giving rise to the large lake labeled "Lake Calvin" on Figure i. 

 During the long existence of the lake, the surplus water found its 

 way to the unfilled valley of the Mississippi below Fort Madison 

 by way of the abandoned channel south of Columbus Junction. 



The shape of the basin formerly occupied by Lake Calvin, 

 considered in broad, general outline, is that of a huge letter V, 

 made irregular by numerous ramifications, especially in the northern 

 part of the lake site. The arms of the V extend in directions parallel 

 to Iowa and Cedar rivers, and meet near the junction of the two 

 streams at Columbus Junction, Louisa County. 



Without including the numerous river-like irregularities north 

 of Iowa City, West Liberty, and Moscow as part of the lake basin 

 proper, the lengths of the Iowa and Cedar river arms of the V are 

 28 and 24 miles, respectively, with corresponding average widths 

 of 4.4 and 5.8 miles. The widest portion of the old lake is 2^ miles 

 south of Lone Tree, where the bluff lines are separated by a low, 

 flat stretch of country nearly 15 miles wide. At the time of their 

 greatest expansion, the waters of the lake covered an area of approxi- 

 mately 325 square miles. 



The site of former Lake Calvin, which is now an extensive low- 

 land, has been described on pages 423 and 425. At least three sets 

 of terraces occur in the lake basin, two of which represent the ancient 

 floor of the lake. Laminated silts 35 feet thick characterize the 

 lake terraces, whereas sand and gravel with typical fluvial cross- 

 bedding and pocket-and-lens type of structure mark the third 

 terrace. 



