Cement Ponds
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"Cement Ponds"  were present at every mill.  The most complete
 I found is at the Woodchuck Mill.  They were used to separate
the mineral from the water that had been used in the mill. 
They had an overhead bridge-like
construction that held a large electric motor, moving a "sweep",
much like found in today's waste water treatment plants.
The sediment was pumped back into the mill, and re-ran to
pick up anything that had gotten through the first time.