Key to Red River Valley story reading puzzle:

7)  mixture of silts, sands, and plant debris
6)  well sorted clay-rich mud
5)  mixture of silts, sands, and woody tree fossils
4)  well sorted clay-rich mud
3)  poorly sorted mixture of mud, sand, and gravel
2)  well sorted clay-rich mud
1)  poorly sorted mixture of mud, sand, and gravel

1)  First the glaciers were here (glaciers leave behind a poorly sorted mix of mud, sand, and gravel as we learned in the last section on rocks).

2)  Clay is very fine grained sediment, indicating a very still water environment, like a deep lake.  Thus, the next thing was glacial lake Agassiz.

3)  The return of the poorly sorted glacial till reveals the readvance of the glacier.

4)  The lake returns as the glacier retreats again.

5)  silts, sands, and woody fossils indicate that neither glacier nor lake came next, but rather rivers and forest.

6)  The lake returns.

7)  The lake finally drains away for the last time (or at least the most recent time!), and we get the modern rivers and prairie of today.

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