Cedar Sink

The rocks of Cedar Sink and the surrounding area are from the Mississippian (also known as Lower Carboniferous or Early Carboniferous) subperiod that occurred 358.9 to 323.2 million years ago.[2]

The Big Clifty Sandstone caprock has been breached exposing the limestone Girkin Formation resulting in a relief between approximately 200 feet (61 m) below the main plateau surface. The bottom of the sink lies another 100 feet (30.5 m) below and exposes the top of the Ste. Genevieve Limestone.[3]

The weakening of the ceiling from the breaching of the caprock and the formation of Smith, Cedar Spring, and Woolsey Valleys caused the ancestral master trunk to collapse. Continuous flow in the master drainage line further undermined the sink and triggered further collapse.[1]

Collapse sinkholes, like Cedar Sink, form after an initial subsurface collapse occurs. This collapse commonly happens at the intersection of two passages underground where the roof span is the widest and therefore weakest.[4] The process continues as the underground stream repeatedly undercuts the cave walls, inducing collapse of the weight-bearing cantilever beams resulting in a wider passage. Erosion continues and eventually the void reaches the surface.

Water in an ephemeral stream can be seen from the overlook and occasionally at six other places within this sink, based on the season and recent precipitation events.[4] Exposure of the subsurface stream makes Cedar Sink a karst window [5] or Karst fenster.

Cedar Sink is derived chiefly from two similar-sized sub-basins: Mill Hole and Procter Cave. Prior to the major storm of May 1984, all the water from Cedar Sink flowed only to Turnhole Spring.[4]

Cedar Sink area is karst topography. Karst topography is a landscape formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes, dolines, and caves.

The development of karst occurs when acidic water chemically and physically breaks down the rock surface. The acidic water is created when atmospheric carbon dioxide and falling precipitation create a weak carbonic acid (H2CO3). This process occurs near the bedrock cracks or bedding planes. Over time, the fractures will widen and eventually create a drainage system network in the bedrock. The karst landscape will continue to develop over time as the water wears away at the rocks. Karst aquifers display drainage characteristics that in many ways appear similar to surface networks.[6] Scientists have studied parameters of surface-water hydrology and geomorphology for years, but it was not until recently that the underground water system was understood. Extensive drainage-network analysis using GIS and other tools has developed a more well-rounded view of the karst topography and the subsurface hydrologic influences occurring in Mammoth Caves National Park. Over the past four decades, an extensive investigation of the Mammoth Cave Watershed useful for providing the opportunity for broader quantitative research in the organization of a large, highly developed, karst-drainage network.[6]

The trail to Cedar Sink follows the Smith Valley floor for about ¾ mile. The trail is 1.6 miles round trip and circles the southern edge of the sink and eventually leads to a platform overlooking the sink. The trail is clearly delineated with many stairs leading to the bottom of the sink and there are several vista platforms along the ridge.[1] The trail offers many glimpses at the once underground river system. The views from the plateau change depending on the weather; there are many ephemeral springs at the south end of the sink. The overflow route is dry at times and flowing strongly at other during high precipitation events. The main channel flow is often not at the surface throughout many parts of this region.

Geocaching is also a common treasure-hunt like recreation activity in Cedar Sink. Several caches are hidden in Cedar Sink and Mammoth Caves area.