From 600-to-230 million years ago, Monroe was on the bottom of a large shallow sea that covered much of central North America. Today a peak back to that time can be seen in the limestone rocks that form layers along the sides of the North Dixie Highway Underpass. These limestone rocks are the remains of that ancient sea. From about 500-250-million years ago, Monroe's climate was probably much warmer than it is today because it was located father south. The seas eventually retreated and about a million years ago the first glaciers began advancing into the Midwest from Canada. Matter of fact, a little bit of Canada is under the feet of all Monroe residents. Glaciers advanced into the Monroe area from Canada and carried sand, silt, clay, and boulders. These materials were left here permanently when the last of the glaciers retreated north and left the upper Midwest about 10,000 years ago. Geologists say glaciers that covered Monroe were up to a mile-high. The glaciers' bulldozing power was so intense as they moved south that they gouged deep depressions and basins in the land. As the glaciers receded north from the Midwest, enormous amounts of meltwater from the glaciers filled the deep depressions --- eventually forming Lake Erie and the four-other Great Lakes. Monroe is Michigan's only Lake Erie commercial port. And just as the French Explorer LaSalle made his way west across Lake Erie, later so to did farmer and laborers who help build what was then 'the West.' From about 1826-1843 an existing but inadequate harbor was in use on LaPlaisance Bay (today Bolles Harbor) that disembarked people, goods, and materials from the East. In 1834 U.S. Army Corps of Engineers Captain Henry Smith submitted plans to the chief of engineers a plan for straightening the River Raisin so that the river would directly connect to Lake Erie. Captain Smith's plan called for digging a canal about 4,000' long by 100' wide. Work began in 1835 on what would be called the United States Canal. Congress appropriated money intermittently for the canal building project . The construction of all piers and revetments, and dredging was finished some time around 1888. Monroe's geographic location continued to play a role in its development when, in 1837, the Michigan Legislature appropriated money for the construction of three-railroads that would cross the width of the State east to west. To capitalize on the movement of people from the East Coast who were hoping to find their fortunes in the West and their need for supplies, the Michigan Southern Railroad was finished in 1841. The Michigan Southern Railroad was the southernmost of the three State-financed railroads and connected Monroe to New Buffalo on the eastern shore of Lake Michigan.