Inland Seas

Inland Seas. . , (side 1) , The changing of global sea levels over millions of years was a major factor in determining the ancient geography of Minnehaha County. Through much of geologic time, salty warm-water seas covered the interior of the United States. , During the late Cretaceous Period, 65 to 80 million years ago, rising ocean levels several times produced flooding that created inland seaways. Extending from the Gulf of Mexico to the Arctic Circle, these seas divided North America into two parts. (Continued on other side). (side 2) (Continued from other side). The highest elevations of the Sioux Quartzite Ridge became exposed islands with irregular shorelines containing shallow, quiet-water bays. Sediments laid down in these bays contained fossils of wood, plant imprints, carbonized plant fragments, tiny mollusks, and fish parts. , During the Ice Age, these deposits were buried by glacial drift. Several exposures of the ancient bay deposits may be seen at points where the Big Sioux River, Split Rock Creek, and Slip-Up Creek have eroded through the glacial overburden. , Dedicated in 2003 by the Minnehaha County Historical Society and the City of Sioux Falls..