Detroit was not a seaport until the completion of the St. Lawrence Seaway in the 1950s. Niagra Falls and Rapids in the St. Lawrence prevented Ocean Ships from reaching Detroit in early times. Freight had to be "portaged" around these obstacles and loaded into craft upstream. They would not have carried rocks. Pavement cobblestones were probably a 'gift' of the Glaciers found in farm fields and Great Lakes.
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