Marengo Cave EarthCache
Hidden : 6/10/2013
Difficulty:
1.5 out of 5
Terrain:
2 out of 5

Size: Size: other (other)

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Geocache Description:




How are caves formed?
 Most caves throughout the world are formed in sedimentary rocks. Sedimentary rocks are formed by accumulation of layer upon layer of sediments underwater. Marengo Cave is formed in the St Genevieve limestone which is made of compacted particles of mud, sand, gravel, and bits of broken shells. According to standard geological theory, this limestone was formed around 325 million years ago when a shallow inland sea covered the southeastern part of what is now the United States.  Later, the land rose above sea level and the more impermeable sandstone and shales overlying the limestone were eroded, exposing the limestone below. Marengo Cave began to form as water sank into the first exposed limestone rock layers north of Marengo. As this water moved along the bedding planes of the limestone toward the water table, the cave was formed by solution of the limestone

Surface geology and hydrology
Marengo Cave was formed as Blue River, the master stream in the area, and Whiskey Run Creek slowly eroded downward.  Water sinking into the ground in the Flood Creek Valley about 1.5 miles north of Marengo, where the St Genevieve Limestone was first exposed at the surface, flowed along the bedding planes of the limestone rock to emerge at the Marengo Cave Spring on Whiskey Run Creek at Marengo. The water sinks underground at the 645 foot elevation and flows generally downdip along the same rock strata where it reappears at the spring in the 580 foot elevation. The water follows the dip in the bedding plane that slopes to the southwest at approximately 25 meet per mile. This cave shows much evidence of various stages of solution, backfilling, and removal of cavern fill. These changes were the result of Flood Creek storm waters which enter the cave system though numerous sinkholes in the valley leading to Slush Tube Cave. The water in Slush Tube cave has been dye-traced to connect to the lower level of Marengo Cave.

How old is the cave?
According to standard geological theory, the St Genevieve Limestone in which Marengo Cave is developed was formed around 325 million years ago. At that time a shallow inland sea covered what is now Indiana. Geologists currently believe that the cave itself did not begin to form until the last million years when the impermeable cap rock overlying the limestone was eroded away. That means the cave is probably less than a million years old. The speleothems (cave formations) that are seen on cave tours have generally been formed since the water abandoned the upper level of the cave entirely. While there is much we do not know, most speleothems are probably from a few thousand to a few tens of thousands of years old.

Sinking stream
All the water in Flood Creek enters the underground cave system through numerous deep sinkholes in the valley floor. The cave serves as the drain of the area.

Whiskey Run Creek
Whiskey Run is one of the few surface tributaries of Blue River, which drains most of the southern Indiana cave area. Except during periods of heavy rainfall, most of this water in the creek is supplied by the Marengo Cave Spring and water from the Old Town Spring Cave located about 1000 feet upstream from the Marengo Cave Spring.

Main Passage – Dripstone Trail
This arched passage is an example of a phreatic passage, initially formed at or just below the water table. Originally a tube, the passage has been greatly enlarged by vadose (above water table) down cutting of a running underground stream. The large canyon in the floor of the tube formed as the area water table dropped. The orientation of the passage, generally perpendicular to the drip, is consistent with phreatic conditions present when the passage first started to form. The passage is nearly half filled with sediments deposited by the former cave stream. Later ponded flood waters left fine silt and clay over the gravel stream bed.

Lover’s Retreat
Lover’s Retreat is a good example of a keyhole passage. The keyhole shape is formed when a canyon forms in the floor of an older tube stream action. Apparently, the water table was dropping rapidly so the narrow canyon had little time in which to widen the passage

Stewart Hall
Stewart Hall is a large chamber formed by the collapse of the cave ceiling. These slabs and blocks of fallen rock are called breakdown. This collapse formed the largest room in Marengo Cave. The room is approximately 350 feet wide and over 250 feet long. The ceiling of the room is nearly 60 feet above the underground river which is slowly dissolving the pile of rock which has fallen from the ceiling. Numerous unique, white-tipped stalagmites have formed on the breakdown in the room.

Lower Stream Level
This large stream passage is an example of a large canyon formed by a flowing underground stream. This water flows above the water table, generally down the dip in the rock strata following the same rock layer along the entire level of the main passage. Notice the flat ceiling formed by a bedding plane along which the cave started to dissolve. Wide canyons form when the water table is dropping slowly, allowing solution at the same level over a long period of time. This passage may be the largest cave passage ever found in an Indiana cave.

Washington Avenue
This passage is a classic tube, formed along the bedding plane at or just below the water table in the saturated zone. These phreatic passages are tubular in shape and generally wider than they are high. Tubular passages have very little slope and are formed by slow moving waters

To log this Earthcache, send me the following answers:

1. Did the cave form from the top to bottom, or from the bottom to top?

2. What is the temperature of the air in the cave?

3. The cave has 3 entrances: Natural Entrance, Crystal Palace, and Dripstone Trail. Compare the elevations of the entrances. Also compare the types of entrances.

4. Based on what you have learned about how the other passages formed, which method do you think formed Music Hall on the Dripstone Trail tour.

If you take the Crystal Palace tour, describe the process of how the Rock of Ages formed.

5. Post a picture of yourself (face not required) or a personal item at the cave with your favorite formation or at the entrance.

Additional Hints (No hints available.)



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