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IOWA ACADEMY OF SCIENCES 
varied by placing the sand at the head of the hay instead of at its en- 
trance. In D the sand is spread out along the channel. Near E are the 
deposits and channels due to the currents as the water flows in and out. 
In the narrow passage way to P the erosion and deposition of the sand 
placed between the two partial obstructions illustrate river erosion and 
deposition; and, when E is partially closed, the current sweeping in 
near F washes the sand into a hook-shaped bar. 
If instead of very fine sand a coarser sand is used the further effects 
of sorting of material are illustrated. The effects may be further varied 
by removing A and then elevating and depressing that end of the tray. 
