A grist mill is a mill where grains are ground into flour and middlings. The term "grist" refers to grain that has been separated from its chaff in preparation for grinding. The term can refer to either the grinding mechanism or the building that holds it. The earliest grist mills in America were built in the early 1600s, but the grinding of grain goes back centuries before that. The milling of flour grew to be Pennsylvanias most important export industry, and Philadelphia continued to be the center of the milling trade in America.

Key facts about grist mills include:

• Grinding stones: The heart of a grist mill was its grinding stones. Grinding stones were used in pairs. The bottom stone, or bed stone, was fixed into position, while the upper stone, or runner stone, moved. The stones were connected to the power source (water or wind) by a wooden "counterwheel," or contrate wheel, wedged on the horizontal drive shaft, which ran the trundle head, a small wooden lantern gear.

• Water power: Both the Romans and later the French Benedictine monks in the 11th Century used water to power the milling of grain. The American image of the turning water wheel, the flour-covered miller, and the chattering of people picking up their cornmeal and flour is accurate.

• Automation: Each step of the milling process was united into one automated system that provided mill owners with a way to lower their labor costs while maintaining high production volumes. The reconstructed gristmill at Mount Vernon is the only mill in America that has a working Oliver Evans automated system.

• Profitability: Despite all the challenges in building, maintaining, and operating the mill, the enterprise proved to be a highly profitable one. In 1797, the gristmill produced 117 pounds of profit.

Today, many of the countrys grist mills are no longer operable or only their ruins remain.