Approximately how many years ago did the Great Lakes begin to form in the upper east US region?

The 'Great Lakes' began to form at the end of the last glacial period approximately 14,000 years ago, as retreating ice sheets exposed the basins they had carved into the land which then filled with meltwater.

Also known as the 'Laurentian Great Lakes' (named after the massive sheet of ice that covered the land) and the 'Great Lakes of North America', these are a series of interconnected freshwater lakes primarily in the north central upper region of North America, on the Canada-United States border. Together, these lakes connect to the Atlantic Ocean through the Saint Lawrence River also sometimes identified on maps as the Saint Lawrence Seaway.

Often, the US state of Michigan is described as a left-hand mitten with the lakes surrounded it. The five lakes consist of Lake Michigan, located on the west side of the state of Michigan. On the top, farthest north or above Lake Michigan is Lake Superior. Lake Huron is located on the right side of the mitten extending down past the thumb. To the lower right, on the bottom of the mitten is Lake Erie. Extending upward from Lake Erie on the far right is Lake Ontario.

Hydrologically, there are only four lakes, because Lakes Michigan and Huron join at the Straits of Mackinac located at the top of the mitten.

Together, the lakes have been a major source for transportation, migration, trade, and fishing serving as a habitat to many aquatic species in a region with considerable biodiversity.