Rickwood's Most Obvious Sinkhole
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Located just off the Parking Lot at Rickwood Caverns State Park, this Earthcache should help you look closely at some of the Park's interesting karst terrain. The park is open 8:00 am to dusk and charges $2 for entrance. This is an Earthcache, no physical container to find. You must answer the questions as indicated in the cache description by emailing me via my profile. Please do not post answers in your log.
In order to log this cache, please:
1) estimate the visible depth of the sinkhole
2) Tell me what type of Sinkhole you believe it to be
3) Post of photo of you at the sinkhole with the "danger" sign visible in the shot. (Do NOT cross the fence).
The main source of all cave development is water. Some shelter caves may be developed by wind, but for all intents and purposes we will talk about caves that are developed in the limestone layers of the earth. Limestone is a rock that is highly soluble in water which is enriched with carbonic acid. The water comes from rainfall and when it enters the soil it mixes with plant material where it picks up carbon dioxide. The mixture of carbon dioxide and water forms a carbonic acid. The carbonic acid enriched water finds its way into the water table. This can be referred to as ground water. The ground water finds its way into tiny pores and cracks in the limestone layer and slowly begins to dissolve the limestone. As time goes on the pockets and cracks enlarge to form small chambers in the rock. There are occasions that the water table can create a hydrostatic pressure which will erode the limestone rock away quicker. As time goes on the erosion process creates larger crevices and passages for the water to flow in. More ground water is able to enter the passages and erode away the limestone even more quickly. The passage grows in size with each rainfall, but this is a slow process. No one is able to predict how long it takes a cave to form. Some caves are formed faster than others and this all depends on the geological layout of the area. Eventually in geologic time passages are formed which are large enough to walk in. All water that enters a cave must eventually leave the cave at some point due to the underlying geology of the earth and these areas are called springs or resurgences. The areas where the water enters the cave are called insurgences.
A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline or cenote, is a natural depression or hole in the surface topography caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth, and vary in form from soil-lined bowls to bedrock-edged chasms. They may be formed gradually or suddenly, and are found worldwide. These terms are often used interchangeably though many will distinguish between those features into which a surface stream flows and those which have no such input. Only the former would be described as sinks, swallow holes or swallets.
SINKHOLES and the AQUIFER
Sinkholes originate beneath the surface when groundwater moves through the limestone and erodes large voids, or cavities, in the bedrock. When water fills a cavity, it supports the walls and ceiling, but if the water-table drops, the limestone cavity is exposed to further erosional processes that eventually result in the collapse of the cavity, causing a surface indenture, or sinkhole. The sinkhole becomes a primary site of recharge, where surface water can enter the aquifer and replenish the groundwater supply.
SURFACE CHARACTERISTICS
On the surface, sinkholes may develop progressively as subtle, bowl-shaped depressions, or they may collapse suddenly into steeply sided, water-filled craters. The shape of the sinkhole, and the speed that it forms, depend on the size of the subsurface cavity and the thickness of the overburden (sediments or organic matter that rest on the limestone bedrock).
Three general types of sinkholes occur in Alabama: collapse, solution, and subsidence.
Collapse Sinkholes:
Collapse sinkholes happen suddenly where the overburden is thick with soils and heavy clay. Collapse sinkholes are deep, steeply-sided holes in the ground. They are frequently triggered by fluctuations in the water-table. As water levels fluctuate, the roof of the cavity is stressed and weakened. When the water-table drops too far, the cavity walls are unsupported and the ceiling becomes too weak to hold the heavy overburden. Eventually, the ceiling collapses and a sinkhole is formed. A conical debris mound left on the sinkhole floor is all that remains of the cavity ceiling. If the water-table rises, the collapse sinkhole can fill with water, and overflow like a spring. An off-set sinkhole will have an upstream and downstream conduit as water flows into the sink and siphons underground. If the water-table drops below the sinkhole, it will remain dry and accumulate sediments and vegetation.
Solution Sinkholes:
If the overburden is thin or absent, the surface of the limestone bedrock is broken down by erosion from wind and surface water. A bowl-shaped depression, or solution sinkhole, naturally forms slowly and continuously as chemical and physical processes erode the rock.
Subsidence Sinkholes:
Subsidence sinkholes form gradually where the overburden is thin. The dissolving limestone is replaced by sand granules that fall into the depression and fill the holes. They appear as a concave depression in the ground. Subsidence sinkholes are usually only a few feet in diameter and depth because the development of the cavities in the limestone is retarded since they are filled with clay and sand. As the sediments fill the depression, they restrict the flow of water through the bottom and the hole begins to retain water. As water accumulates, a lake is formed.
Countless families have peered over the fence and wondered...
Although this sinkhole is still active and takes large amounts of water at times, let me assure you that it is NOT an entrance to Rickwood Caverns.
In order to log this cache, please:
1) estimate the visible depth of the sinkhole
2) Tell me what type of Sinkhole you believe it to be
3) Post of photo of you at the sinkhole with the sign visible in the shot. (Do NOT cross the fence).
Additional Hints
(No hints available.)