Where’s The Water Go? EarthCache

Visit a sinkhole, known as Tussey Sink, and estimate the amount of limestone that has dissolved in order to create this geological formation.

Sinkholes
Sinkholes form in areas where soluble carbonate rocks, limestone being one such rock, are dissolved by weakly acidic water. The dissolution of the limestone leads to a sinking of the local ground structure, creating a hole that allows the water to pass deep into the rock structure below.

Geological Origins: Formation Of Limestone
The limestone that led to the formation of this sinkhole began in the Ordovician Period over 400 million years ago. At that time, the world’s fauna consisted of sea creatures, with plants only beginning to make inroads to the land. This area, as was much of present-day North America and Europe, was covered with shallow continental seas. Those shallow oceans were teeming with life, including trilobites, brachiopods, bivalves, and graptolites, as well as other early marine animals. Many of those life forms grew shells composed of calcium carbonate (CaCO₃) for both structure and defense.

When these creatures died, their numerous hard shells remained on floor of those seas. Over time those shells compacted and eventually formed a solid mass of rock, composed mostly of calcium carbonate, which we know today as limestone. Remains of those shells can still be seen in many limestone samples, an example of which is shown below.

Fossils in limestone

Later in the Ordovician Period, the shells which would be become limestone were covered over by fine mud and clays resulting from erosion of land based features. Those deposits later hardened into a solidified rock, known as shale. Similarly, on top of those mud layers, larger sand-like erosion particles settled over those mud layers, eventually forming rocks commonly known as sandstone, the resultant structure shown below.

Schematic diagram of the rock layers

Formation of the Appalachian Mountains
The Appalachian Mountains began to form in the Ordovician period when the Iapetus oceanic plate collided and sank below the North American plate. This created a subduction zone, and the North American plate folded in response causing the rise Appalachian Mountains. The action of those plates (as well other interactions over millions of years) would result in the bending and folding of those initially flat rock beds, as shown schematically below.

Folding of the rock beds

Millions of years of plate tectonics would change the landscape, with the Appalachian Mountains growing to heights similar to the present day Himalayas, and in time wearing away to an almost flat plain, then only to be thrust back up yet again, and worn down by erosion and weathering. A schematic of the present day configuration of the site of the earthcache, is shown below.

Erosion of the folded rocks

CO₂ + H₂0 = H₂CO₃ (Carbonic acid)

This mildly acidic water forms a stream on the mountainside to the south, and flows down the mountain ending in the sinkhole. Where the stream begins, the underlying rock is composed of shale, which is impermeable to water. The water continues to flow down the stream and eventually encounters the limestone. In contrast to shale, the limestone generally contains numerous fractures.

Schematic of the Current Site Showing the Mountainside and Stream

Shale

Limestone, showing fractures

The fractures in the limestone allow the water to penetrate and the mildly acid water dissolves the calcium carbonate in the limestone. This action is shown in the figure below. As the acidic water flows over the limestone (left), it permeates through the cracks and fissures and percolates downward into the formation. As the water moves it dissolves small amounts of the limestone. In time (center), the fractures in the limestone grow larger, allowing for more water to penetrate while reducing the height of the limestone and causing the ground to sink. This action continues, (right) until a sinkhole is formed and all of the water is capable of percolating into the formation.

Dissolution of the Limestone and Formation of the Sinkhole

For this Earthcache you will estimate the amount of limestone that has been dissolved to form this sinkhole.

The coordinates for this earthcache will take you to a bridge that crosses the stream at the top of the sinkhole. (You should find ample parking along Taylor Hill Rd) Take a look off the bridge and notice if there is any water running in the stream, note that all of the water will go into the sinkhole.

The bridge on Tussey Sink Rd

The amount of limestone dissolved will be approximated using two rectangles. In order to measure the amount of limestone dissolved, you will need three measurements, the width of the sinkhole, and two lengths.

Map of the Site

For the width you will need to measure/estimate the distance from the walnut tree at the corner of Taylor Hill Rd and a point on Tussey Sink Rd across from the third tree from the bridge, which should have an orange flag on one of the branches (see map and photo).

Third Tree

The estimate for the length of the sinkhole requires two measurements, as the sinkhole has a bit of curve to it. The first length is the distance from the walnut tree to a point across from the Stop Ahead road sign along Taylor Hill Rd. As you walk this length you should be able to get a good view of the sinkhole. The second length is the distance from the point across from the Stop Ahead road sign to the end of the sinkhole, near the house number sign (2074). Here at the end of the sinkhole you can see that there is no stream that continues on anywhere and confirm that the water does indeed go underground.

End of Sinkhole (2074)

Estimated Volume of Limestone Dissolved (cubic feet)=
Width X
Length (Length 1 + Length 2) X
Depth (10 ft)

Finally take a photo of yourself at either the bridge at the top of the sinkhole –or– at cross road marker with the Tussey Sink sign in the background.

Email me your estimated width, lengths (1 and 2) and your calculation of volume. Post the photo with your log.

The author reserves the right to delete your post if you fail to complete the requirements of this earthcache.

As this earthcache is located along public road right-of-ways, it is possible for this earthcache to be completed from a car. I have listed the cache as handicap accessible from this perspective.

References