The Valparaiso Moraine is a recessional moraine (a land form left by receding glaciers) that forms an immense U around the southern Lake Michigan basin in North America. It is a band of hilly terrain composed of glacial till and sand. The Valparaiso Moraine defines part of the continental divide known as the Saint Lawrence River Divide, bounding the Great Lakes Basin. It begins near the border of Wisconsin and Illinois and extends south through Lake, McHenry, Cook, DuPage and Will counties in Illinois, and then turns southeast, going through northwestern Indiana. From this point, the moraine curves northeast through Lake, Porter, and LaPorte counties of Indiana into Michigan. It continues into Michigan as far as Montcalm County.The moraine was formed during the Crown Point Phase of the Wisconsin glaciation. At this time the glacier covering the area had grown thin, so it was restrained by dolomite rock layers in the Lake Michigan basin. Where the glacier stopped, glacial till and sand was deposited, creating the hills of the moraine. After the Valparaiso Moraine was formed, the glacier retreated and formed the Tinley Moraine.
Many towns in northwest Indiana and northeast Illinois are named after the Valparaiso Moraine or the Tinley Moraine. The moraine itself was named after the city of Valparaiso, Indiana where the moraine is at its highest and the city lies on the moraine.
As part of the continental divide, many creeks or rivulets have their origin source in the Valparaiso Moraine. Water on one side of the moraine flows into Lake Michigan, through the Great Lakes, and eventually into the Atlantic Ocean via the Saint Lawrence River, while water run-off on the other side of the moraine flows into tributaries of the Mississippi River, which eventually flows into the Gulf of Mexico. The moraine divide was breached at the Chicago Portage gap by a canal beginning in the mid-19th century.
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