Mining became significant after the 1850s, when iron mines opened in the Lake Superior region of the upper Midwest. These helped spur rapid development of iron and steel mills located on the shores of the Great Lakes.

• 19世纪50年代，上中西部的苏必利尔湖区开始采掘铁矿，从此以后采矿业兴起，促使五大湖沿岸的钢铁厂迅速发展。