Nebraskaland

48 Nebraskaland • March 2021 other rock types deposited by glaciers. During the Pleistocene Period — the Ice Age — roughly 2.6 million to 10,000 years ago, four ice sheets descended southward upon the Central Lowlands: from oldest to youngest these were the Nebraskan, Kansan, Illinoian and Wisconsinan. Only the Nebraskan and Kansan reached Nebraska, covering the eastern fourth of our state with a mile-high sheet of slowly creeping ice for thousands of years. The eff ects the glaciers had on the land were immense. They sheared off hilltops, fi lled valleys and pushed rivers southward. A few miles west of Rock Glen, Diff endal knows of a sand and gravel outcrop on a hillside. It was the remnants of the Platte River, which during the Nebraskan Glaciation was forced south 100 miles of its present location. As the glaciers bulldozed their way across our state, they carried rocks, gravel, sand and clay from as far north as Canada. Later, as they melted, this "glacial till" was left on the land surface. Throughout most of eastern Nebraska, the till was later covered by a thick layer of wind-blown silt, known as loess, and remains so today. The silt was fi rst deposited by Rocky Mountain and continental glacial meltwater in large river fl oodplains, mainly those of the Platte and Missouri, and then blown onto the surrounding uplands as a fi ne dust, accumulating by fractions of an inch per year. Far from rivers draining the glaciers, the Rock Glen region received little loess deposition, leaving the till uncovered and exposed to the elements. Smaller boulders eventually broke down and water washed away the clays and sands, leaving only the larger boulders. Erosion also carved ravines and canyons into the sandstone hills. Hard, erosion-resistant Sioux quartzite composes most of the WMA's remaining glacial boulders. Diff endal said the pinkish to purple, billion-year-old rocks originated from the Sioux Falls, South Dakota, area. There, Sioux quartzite outcrops still bear scars where the ice gouged out the boulders. At Rock Glen, these now lichen-covered boulders stand testament to the power of ice. Prairie and Woods A mere 18,000 years ago, the Wisconsin glacier ground to a halt in far southeastern South Dakota, its meltwater cutting a section of the Missouri River Valley. With frigid winds blowing south off the ice, Nebraska's climate was cold. The land was cloaked by northern spruce forest and grassland and roamed by mammoths, giant ground sloths and other Ice Age creatures. During the next few millennia, as the climate warmed and the glaciers retreated, prairie and deciduous forest plants slowly migrated northward from glacial refuges to continue their pre-Ice Age battle for control of the Great Plains. Simultaneously, humans began migrating to North America from Asia, traversing the Bering Land Bridge from Siberia to Alaska, becoming the fi rst Americans. As they moved Pinkish-red Dakota shale is exposed in a ravine at Rock Glen. Glacial rocks lie on the shale's surface. A glacier-deposited Sioux Quartzite boulder graces the prairie at Rock Glen. PHOTO BY GERRY STEINAUER PHOTO BY GERRY STEINAUER PHOTO BY GERRY STEINAUER

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