MIGRATION HISTORY OF GREEN MOUNTAIN BEACH PARABOLIC DUNE, SOUTHEASTERN SHORE OF LAKE MICHIGAN

Large parabolic dunes line the southeastern shore of Lake Michigan, and unraveling their history is critical in understanding the development of the lake shore. Radiocarbon ages from paleosols exposed in the interior of Green Mountain Beach Dune, 9 km southwest of Holland, Michigan, indicate that it has migrated 320 meters inland since 1930 - 1740 cal. YBP. The axis of the parabola rotated 15^o from an orientation of 86^o to 71^o over this time period. This migration was interrupted by 3 periods of stability marked by paleosols. The geometry of paleosol exposures along the lakefront can be explained by a model involving episodic inland migration of parabolic dunes for roughly 4000 years and a net local shoreline recession. The growth of the depositional lobe has been monitored since September, 2001 using erosion pins set on the slip face supplemented by sand traps at the crest. During fall and early winter sand was deposited almost exclusively on the upper and mid portions of the slip face leading to an over-steepening (37^o) of the top compared to the bottom slopes (31^o). Ice between grains allowed these steep slopes to be preserved until spring. Most movement of sand to the bottom of the slope was by mass wasting with over 75% of the movement occurring between February 1 and April 20. Sand was deposited in an arc around the dune axis with the maximum thicknesses of the deposited layer occurring at the axis (1 m by July 2) and falling away to less than 10 cm at those points along the limbs where the walls of the interior trough are greater than 10 m high. The two storms with strongest winds (max. wind speeds of 43 and gusts of up to 55 mph measured at Tulip City Airport, 7 km east of the dune) were associated with 49% of the deposition. The dune migrated inland 1.7 m along its axis from September to July with essentially all migration occurring before April 20. This compares with 1.45 m/yr from 1938 to 1999 (estimated from aerial photographs) and 0.52-0.86 m/yr for the last 500 - 300 years (estimated from the position of the youngest paleosol). Structures exposed by erosion in the trough of the dune are identical to those observed forming on the modern slip face suggesting that there has been little change in the pattern of sand deposition for the last 2000 years.