The posted coordinates for this earthcache will lead you to a sinkhole on the Yellow Trail within Saarinen Preserve. Sinkholes can form in several different ways and can either be gradual or catestropic collapses. This earthcache is designed to teach you about the basics of sinkhole formation and to look at and think about the sinkhole at this location.
SOLUTION SINKHOLES
Solution sinkholes form where soluble bedrock (i.e., limestone, dolomite, marble, and rock salt) is exposed at the land surface and thus subjected to weathering by dissolution. Surface water collects in natural depressions and slowly dissolves a sinkhole.
SUBSIDENCE SINKHOLES
Subsidence sinkholes are similar to solution sinkholes, except that the soluble bedrock is covered by a thin layer of soil and/or sediment. Surface water infiltration dissolves cavities where the bedrock is most significantly fractured, and the sediment fills in the void left by the disolved limestone. The sediment often will restrict the flow of water through the bedrock in these and it allows the sinkhole to become water filled.
COLLAPSE SINKHOLES
Collapse sinkholes form when surface materials suddenly sink into a subsurface cavity or cave. These tend to be the most catestrophic types of sinkholes. The cavities form slowly over time, as groundwater moves along fractures in soluble bedrock and enlarges them through dissolution, and the actual collapse can occur either when the weight of the overlying ground becomes more than the "roof" can support or if an underground cave is normally water filled and groundwater levels drop, the resulting air pocket may allow for collapse.
To claim credit for this earthcache, send the answers to the following questions to the cache owner through their profile above. Please do not post answers in your log, even if encrypted. If answers are posted in your log or you do not send answers, your log will be deleted.
1) What are the elevation readings at the top and bottom of the sinkhole? How deep is it?
2) Estimate the distance from the west rim to the east rim of the sinkhole at the widest point.
3) Based on the types of sinkholes listed in the description and from looking at the sinkhole, how do you think this sinkhole formed?
4) As you walk along the trails to and from this earthcache, what clues do you see that the soil layer may be thin in this area?
5) Optional: Please feel free to post pictures in the area.
References:
http://water.usgs.gov/edu/sinkholes.html
http://www4.uwsp.edu/geo/faculty/ozsvath/lectures/Sinkholes.htm