GEOMORPHOLOGY OF COASTAL DUNES NEAR SOUTHERN SHORES OF LAKE MICHIGAN
From west to east the dune belt extends for 45 km length, and it changes in width from 2.4 km to0.5 km. There are no dunes located west of Gary, Indiana. Most common dunes are parabolic dunes which appear, in order of their abundance, in the following forms: hairpin, lobate, compound, digitate, and windrift. The apices of parabolic dunes point in easterly direction, from 85-105 azimuth, indicating prevailing westerly winds in their formation. The hairpin parabolic dunes have arms that reach up to 550 m, while arms in lobate parabolic dunes have 250-300 m length. The average height of parabolic dunes is 22 meters, with tallest as much as 58 meters found in the western part of Indiana Dunes State Park. The OSL ages of parabolic dunes suggest their development ~ 3,500 to 4,000 years ago, with some episodes of dune reactivation ~3,000 years ago. Nearest the shore are linear dunes which represent arms of parabolic dunes modified by wave erosion and mass wasting. These dunes have a very steep lakeward and gentle landward slopes. A few linear dunes further inland are arms of breached windrift dunes, with steeper landward than lakeward slopes. The linear dunes in this system extend, on average 600-700 m in east-west direction. The longest linear dune is 1,337 m long, and shortest only 200 m. On average their relative height is 20 meters. The OSL ages of linear dunes are ~3,500 years ago, similar to ages of parabolic dunes.
In many places along Lake Michigan southern shoreline, blowouts have developed in some of the linear dunes and in the northern arms of the older parabolic dunes. The apices of the blowouts point in south-easterly direction (120-135 degrees azimuth), indicating their formation by north- westerly winds. The axis of the blowouts average 145 meters in length, but the largest reach 650 meters in length. The OSL ages of sand collected within blowouts range from 800 to 50 years ago.