Written by OLC Board Member Tom Zeller
The area around Atherton Island Natural Area has an interesting geologic history. Atherton Island is an upland immediately east of Clinton, Indiana. It’s only about 300 feet above the surrounding area, but the edges rise quite noticeably from the floodplains on either side, of the Wabash River to the west and the much smaller Big Raccoon Creek to the east. As the name suggests, it was once a literal island in the Wabash River, but of course is no longer. The “island” is about fifteen miles long, north to south, and is about five miles wide. The feature is quite visible using Google Maps’ satellite view, as it is outlined by the roads on both sides that hug the flat terrain directly adjacent to the upland.
The Indiana Geological and Water Survey (IGWS) has determined that the Atherton Island Natural Area lies at the very southern limit of the most recent glacial episode, called the Wisconsin glaciation, which lasted from 50,000 to 10,000 years ago. The Atherton area was covered by glacial ice, but, being at the edge of the glacier, by a much thinner covering than even a few miles to the north.
As the climate warmed and the glacial ice retreated from the area, large southward flows of meltwater surrounded Atherton Island, while Big Raccoon Creek fed into the eastern of the two large flows. The eastern flow became blocked either by ice or by deposits of sand and gravel carried by the meltwater, causing a lake to form along the entire eastern edge of the island.
Eventually the lake water rose high enough to spill back into the Wabash River on the north end of the island and the flow of water was reversed, as the lake emptied from south to north. Now, the water of Big Raccoon Creek still flows north through the waterway initially formed by the southward-flowing post-glacial melt.
The deposits from the southward glacial flow and later lake environment partially filled the valley between Atherton Island the highland to the east. After the stream flow reversal, Big Raccoon Creek carried much of these deposits away. However, the highest parts of the previous valley fill were not removed by the much smaller flow of Big Raccoon Creek and still hug each side of its valley in features known as glacial terraces.
A good place to see a terrace is the road north of Mecca, which is perched on the terrace material that the Creek did not scour away. The west side of the road faces the fairly vertical face of the Atherton upland. To the east, the floodplain of the current Raccoon Creek is about 150 foot lower than the road.
When the glacier first retreated, the land was barren and wind had a free hand in moving fine-grained silt, creating deposits tens of feet thick. This material is called loess. Pronunciation of the term varies around the world, but in these parts, it’s pronounced “luss.” Loess has the odd property of sticking together in a manner that allows steep slopes to form, even though one could easily scoop out the material with a spoon. This makes it a popular location for birds nests, especially for swallows.
The post-glacial wind also created a series of sand dunes which are still present as the sandy hills just east of Sandcut and west of Greencastle Road.
From a creek bed in the Atherton Island Natural Area, one can see the gray deposits left directly by the glacier. While most stream and lake deposits are composed of material well sorted by size, depending on the energy in the environment at the time, these glacial deposits are a jumbled mix of silt, sand, and large pebbles. Above this thirty-foot thick layer is a ten foot brown layer of the wind-deposited loess.
Core drilling by the IGWS found glacial deposits made by the much earlier Illinoian glaciation (300,000 to 140,000 years ago). The older Illinoian deposits weathered into soil which was later covered by the Wisconsin glacier.
For more information about the ice ages and Indiana, see https://igws.indiana.edu/Surficial/IceAge.