PRE-GLACIAL RIVER VALLEYS OF MINNEAPOLIS 457 



The character of the material fihing the ancient gorge in the 

 north part of the city between the mouth of Bassett's Creek and the 

 north city hmits is strikingly different from that which constitutes 

 the valley filling at all other points in the course of the main gorge 

 and its tributaries. Along this particular portion of the river 

 course the old valley is filled with fine laminated clays and silts 

 instead of the heterogeneous drift which characterizes the filling at 

 other places. The upper portions of these clay beds have been used 

 for many years in the manufacture of brick and tile. These fine 

 silts seem to extend from the bed of the present river (which at this 

 place flows directly over the pre-glacial gorge) down to the bottom 

 of the buried valley, a distance of about 160 feet. These silts also 

 are found back from the main channel, where they cover many acres 

 along the flats on both sides of the river. Their thickness is much 

 less, however, away from the river. 



This sudden change in the nature of the material filling the old 

 valley, the suggestive laminated structure of the upper portion of 

 the deposit, the variation in thickness of the material, and its occur- 

 rence back from the river channel, along the flood plain of the river, 

 are all evidences which point to an original deposition in a lake. 

 The change from gravel and drift to clay and silt occurs immediately 

 above the point where the present river abandons the pre-glacial 

 course. It seems probable that at a late stage in the recession of 

 the ice a moraine or dam was formed across the old gorge near the 

 mouth of Bassett's Creek. This resulted in the formation of a tem- 

 porary lake which filled the river valley, overflowed the banks, and 

 spread over the low land which flanked the valley on both sides. In 

 this lake the fine clays and silts could be deposited. Eventually an 

 outlet was formed for the impounded waters at the point where the 

 present river diverges from the filled gorge. After the outlet was 

 once estabhshed the erosion of the new post-glacial gorge was 

 started. But deposition of fine sediments could continue for a long 

 time after the new gorge was started, because the lake would still 

 remain and the current would continue to be slackened to the point 

 where clays and slits could accumulate. The boundaries of this 

 lake have not been traced north of the city limits, but it is probable 

 that it extended a considerable distance beyond them. 



