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A Scottish engineer named John McAdam, in the early part of the nineteenth century, would perfect the technology of road building using layers of stone and gravel and give his name (slightly misspelled as macadam) to the process, which was widely used in both the United States and Britain. Toward the end of the nineteenth century, when engineers began mixing the top layer of gravel with tar to provide a relatively waterproof surface, the roads were said to be tarmacadamized, or tarmacked.
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John Steele Gordon (An Empire of Wealth: The Epic History of American Economic Power)