Places to see Indiana geology

Geologic Road Log

State Road 37 from Bloomington to Indianapolis

by Anthony Fleming and Todd A. Thompson

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State Road 37 from Bloomington to Indianapolis crosses several geologic terrains that formed under widely varying climate conditions and by glacial and coastal processes that are separated by 300 million years of time. This road log points out several features and deposits along the route that illustrate these conditions and processes. Buckle your seatbelt--we are on a ride through time.

Beanblossom Creek

The valley that Beanblossom Creek occupies is cut into limestone and siltstone that accumulated in Indiana about 340 million years ago (see diagram above and right). At this time, Indiana was about 10 to 15 degrees south of the equator in the equatorial tropics. The siltstones are part of a large delta that spread sediment westward into a shallow sea from rivers in the east and northeast part of the state. After the rivers stopped supplying sediment, the abandoned lobes of the delta were colonized by shelled and hard-bodied organisms that prefer to live in warm shallow seas. The remains of these organisms became the limestones. Road cuts along Highway 37 show these rocks south and north of Beanblossom Creek. Geologists rely on these road cuts and exposures along the banks of rivers and streams to view the rocks. Where there are no exposures, geologists bore holes and examine cores and rock chips that come from the hole.

Morgan-Monroe County Line, Bryants Creek

The county line marks the approximate extent of glaciers in south-central Indiana during the Ice Age. The slope to the west of the road is underlain by up to 50 feet of glacial deposits plastered in the lee of a bedrock hill. Bryants Creek formed along the margin of this glacier and contains abundant far-travelled rocks, such as granite, that were carried from Canada by the ice.

Martinsville

Martinsville is situated in a several-mile-wide plain of sand and gravel deposited by glacial meltwater flowing down the White River Valley. Somewhat elliptical bedrock hills of siltstone can be seen protruding through the outwash at several places as you traverse the plain. Many other similar hills are completely buried beneath the sand and gravel. The sand and gravel is a major source of ground water for many communities, such as Martinsville and Indianapolis. The large ridge that the road ascends just north of Martinsville is the remnant of a large outwash fan deposited by an ice sheet of Illinoian age, some 300,000 years ago.

Waverly

For several miles in either direction, the road traverses rolling to hummocky topography that flanks the flat valley of the White River just to the west. This topography reflects the most recent ice sheet to invade this area, during the Wisconsin age, some 21,000 years ago. The margin of the ice stabilized parallel to the White River, resulting in deposition of ridges and hummocks of sediment.

GEONOTE: Meltwater Torrents SR 37 between Martinsville and I-465 has many panoramic views of the White River Valley to the west. This valley was carved by massive torrents of glacial meltwater hundreds of times larger than the flow in the modern river. Some of the meltwater was derived from the fronts of ice sheets that stood in this area, but most probably was generated in catastrophic outbursts from the final melting of ice sheets north of Indianapolis. Small hills in and adjacent to the valley have an elliptical form that is indicative of streamlining by large volumes of fast-moving water. In some places, the meltwater followed and enlarged preexisting bedrock valleys, but in others, it overtopped low sags in the surrounding hills, thus cutting entirely new valley segments.

Marion-Johnson County Line

The large ridge that lies along the east side of the road just north of the county line is a remnant of a massive outwash fan deposited about 20,000 years ago. The fan is originally thought to have extended almost completely across the White River Valley to the west, but catastrophic outbursts of meltwater from the disintegrating glacier scoured the valley, leaving only the high-standing head of the fan remaining. It is interesting to note that the river here follows an unusually arcuate course that appears to be radially symmetrical about the fan, apparently marking the former edge of the fan. The fan is contiguous with a long, low ridge that lies to the east, locally known as the Greenwood Moraine. The fan and moraine mark the location where the ice margin stood for a long period of time.

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