The Dr. Rane L. Curl Sinkhole Trail

The Sinkhole Trail is a short loop trail starting near the east end of the parking area along Norton Camp Road.

Trails policy

The objective of the Michigan Karst Conservancy is to protect karst areas in Michigan and to educate the public about the value of karst lands and the safe and proper use of these lands. One aspect of this objective is to provide educational trails on its karst preserves.

These trails are narrow footpaths that often go through wet or muddy areas; across or around rocks and boulders;and often have steep, narrow, hilly, or sideways tilted surfaces. Some cross bare rock or cross a stream on a narrow rustic bridge.

These trails are not suitable for personal assistance mobility devices such as walkers, manual or motorized wheelchairs, power scooters, golf carts, off-road or all-terrain vehicles. Use of such equipment on MKC trails is not reasonable and is likely to do damage through soil erosion, trail widening, plant and root damage, and is thus prohibited.

Visitation to MKC preserves is at the visitor's own risk. MKC normally will not have staff or volunteers or emergency phones or equipment present at a preserve. The preserves are a significant distance from emergency medical facilities. Cell phone coverage is limited or non-existent, especially in the Upper Peninsula.

Treat the features of the Preserve in such a way that their interest and appearance are not altered.

A permit is required to collect animal, plant or mineral specimens.

The MKC recommends that you wear long pants, a long-sleeve shirt and sturdy shoes; carry insect repellent (in season), water and a compass.

Hiking the Trail

Proceed south from the Trails sign, until you see the post marked "1." Turn west (right) and follow the trail and marked posts from there. The trail will return to Norton Camp Road about 200 yards west of the parking area. If you lose the trail, you can return to Norton Camp Road by heading due north through the forest.

The trail is divided into seven segments.

1. Trailhead

From here the trail goes approximately west along the northern margin of an area in which many sinkhole features have developed in the Fiborn Limestone bedrock.

The swamp to the south probably once encroached on the higher ground in this area, assisted by ponding due to beaver dams. The water dissolved limestone in joints in the bedrock and worked its way underground to the south branch of the Hendrie River, about a mile to the northeast.

2. Shallow sinkholes or solution pans

This area, with many shallow sinkholes, may represent early karst development in the bedrock. Other sinkholes must have pirated the main water flow before these became very deep.

The upland forest throughout the Preserve, which you will pass along this trail, is a second-growth beech-maple hardwood forest, consisting of sugar maple (acer saccharum) scattered beech (fagus grandifolia), hemlock (tsuga canadensis), balsam fir (abies balsamea), and white cedar (thuja occidentalis) in the lower, wetter areas. Red-berried elder (sambucus pubens) is common, while the ground cover consists of woodland fern (dryopteris austriaca), northern wood sorrel (oxalis montana), and ground pine (lycopodium obscurum). Red raspberry (rubus strigogus) is common in open areas.

3. Flat Creek Sinkhole

Flat Creek flows north from a swamp adjacent to Fiborn Pond and goes underground here.

Flat Creek was probably natural, but appears to have been dug deeper while Fiborn Quarry was operating to help drain the swamp water around the quarry. These days, waters level in the swamp are lower, and Flat Creek doesn't flow rapidly.

Please do not walk on the steep sides of the sink, because the soil is very sandy and loose, and will quickly erode.

The sinkholes developed during the past 8,000-9,000 years, after the glacial Lake Algonquin receded. The relatively great speed of the development of the sinkholes may have been due partly to the acidity of the swamp water that ponded upon the limestone, since limestone dissolves more readily in acid solutions.

The limestone bedrock here has been dissolved to some depth so it is buried in moderately deep sand and is not exposed.

4. Cone Sink

Here is an example of collapse induced by a cave passage below, by which sand is being transferred into the subterranean karst system and being carried away by a cave stream.

It is probably the Flat Creek stream, underground, that caused this sinkhole, but with the reduced water flow it cannot be currently active. Erosion of the sandy walls of this sink by people would quickly fill the bottom with sand and reduce the steepness of the sides, destroying the original appearance of the sinkhole. Please stay back from the edge.

5. Brushy Sink

Water ponded upon limestone will form a distributory network of streams until the openings in the limestone become large enough that one main stream can carry the available water. Until that happens, many sinks will develop.

This is another sink like Flat Creek Sinkhole, but less well developed.

6. Bog Creek and Reluctant Sink

Surface drainage in this area became better integrated at some time in the past, and a single surface stream from the swamp cut a small canyon into limestone bedrock, heading into this blind valley sinkhole. The stream has also been much more active in the recent past.

The sinkhole was named "Reluctant" because early explorers hoped it would provide access to another cave, but it is too narrow and filled with sediments.

Please do not climb down into the sink, in order to keep it from being further eroded and filled with sand.

Bog Creek upstream curves around to the west as a small canyon until it blends into a shallow, boggy channel.

7. Sinkhole Trail End

This former two-track road is one of the many logging roads cut into the forest when it was timbered selectively in 1987 and earlier.

To the south, it crosses Bog Creek and the beginning of the narrow canyon leading to Reluctant Sink.

Proceed north (right) on the forest road to return to the parking area.