What resources does a coastal city need?
Tulum is a walled city that was once inhabited by the Maya people. Tulum is on the Yucatan Peninsula of Mexico. The beautiful blue Caribbean supplied the Maya with abundant fish. What else would the Maya people have needed? Fresh water is found in cenotes, sinkholes that are common in the Yucatan limestone.
Some water soaks into the ground. It travels down through tiny holes in soil. It seeps through cracks in rock. The water moves slowly, pulled deeper and deeper by gravity. Water in an underground rock or sediment layer is groundwater. Underground water can also erode and deposit material.
Rainwater absorbs carbon dioxide (CO2) as it falls. The CO2 combines with water to form carbonic acid. The slightly acidic water is especially good at dissolving the rock limestone. Groundwater creates landforms by dissolving away rock.
Florida is unique for groundwater erosion. The state is extremely flat and is made mostly of limestone. Due to the wet climate, groundwater surfaces in many locations. In the Everglades, rivers create a wide floodplain and groundwater comes to the surface (See Figure below).
A cypress forest in Everglades National Park needs water to thrive. 
Caves are one of the types of landforms created by groundwater erosion. Working slowly over many years, groundwater travels along small cracks. The water dissolves and carries away the solid rock. This gradually enlarges the cracks. Eventually, a cave, like the one seen in the Figure below may form.
Water flows through a limestone cave. 
You can explore a fantastic cave, Kartchner Caverns, in Arizona, by watching this video: http://video.nationalgeographic.com/video/player/science/earth-sci/exploring-kartchner-sci.html.
As erosion by groundwater continues, the ceiling of a cave may collapse. The rock and soil above it sink into the ground. This forms a sinkhole on the surface. Some sinkholes are large enough to swallow up a home or several homes in a neighborhood.
THE LOCAL TIE IN: Did you know that throughout the karst valley of Chester and Montgomery Counties, sinkholes occur. Evidence of past sinkholes within the park can be found on the Grand Parade grounds. The most notable sinkhole occurance within the park occurred during the Great Ice Age, approximately 750,000 years ago.
OFFICIAL VALLEY FORGE DOCUMENT:
Caves and Karst
Bone Cave is a vertical 12-m-deep (40-ft-deep) solution cavity (paleo-sinkhole) in the Cambrian Ledger Formation that was briefly opened to the surface, creating a natural trap for the accumulation of remains (Santucci et al. 2001; Daeschler et al. 2005). More than 1,200 fossils from the cave are held at the Academy of Natural Sciences in Philadelphia. The Bone Cave site is one of the most significant Middle Pleistocene (approximately 600,000 to 670,000 years ago) vertebrate fossil finds in North America. The fossiliferous deposit was not exhausted before flooding groundwater inundated the cave in 1896. The quarry was later filled with 9–12 m (30–40 ft) of industrial waste from a nearby factory (Daescher et al. 2005).
Edward Drinker Cope was among the leading paleontologists describing fossils from the cave in the late 1800s. Numerous fossils were recovered, representing 14 plant species, nine coleopterid insects, five reptiles (four turtles, one snake), one bird, and 33 mammal species (Daeschler et al. 1993; Santucci et al. 2001; Baughman et al. 2006). Significant fossils include the only known representatives of a skunk (Osmotherium spelaeum) and a muskrat species (Ondatra hiatidens). The diluvian fisher (Martes diluviana) is also only known from the site. Other notable fossils include ground sloths, mammoths, the earliest record of American black bear
(Ursus americanus), the type locality of the gracile sabertooth (Smilodon gracilis) (fig. 6), and the only population sample of large North American Pleistocene tapirs (fig. 7) (Baughman et al. 2006; Santucci et al. 2001).
A description of all the fossil remains and history of excavation of Bone Cave is beyond the scope of this report. Prior to its rediscovery in 2005, Daeschler et al. (1993) presented a thorough review and geologic contextualization of the Bone Cave fauna and flora.
Bone Cave is one example of the action of karst processes on the landscape at Valley Forge National Historical Park. The lack of streams in areas underlain by dolostone, such as those visible from Inner Line Drive, County Line Road, and Gulph Road, is typical of karst topography (Wiswall 1993). Karstic dissolution occurs when acidic water reacts with carbonate rock surfaces along cracks and fractures. Most meteoric water is of relatively low pH due to the reaction between carbon dioxide in the atmosphere and water. The product of this reaction is carbonic acid (Palmer 1984). Groundwater may become more acidic as it flows through decaying plant debris and soils. The acid dissolves calcium carbonate, producing soluble ions (Palmer 1984). Most of the carbonaceous limestones and dolomites in southeastern Pennsylvania are inherently porous, consisting of voids and underground drainage systems.
Many large surface karst features, including sinkholes formed by mechanical processes such as collapse, were triggered by subsurface solution. Sinkholes are produced by the collapse of overlying rock and soil into a void. They may also form where cracks in the underlying limestone are widened by dissolution, so that soil and pieces of bedrock subside into the enlarging solutional openings (Palmer 1981). They are typically funnel- shaped topographic depressions that are elliptical to circular in landscape view (Palmer 1981; Drumheller 1985). A sinkhole occurring in a streambed may capture the water flow to create a disappearing stream.
Active karst processes in the Valley Forge area include limestone dissolution, underground cavity development, spring activity, sinkhole collapse, and erosion (fig. 8). The dominant carbonate-bearing units in the region include the Cambrian-age Ledger and Elbrook formations. These units contain thick deposits of soluble limestone and dolostone interlayered with shales, siltstones, and sandstones (quartzites). The rocks record a longstanding marine basin on the subsiding margin of the continent during the Cambrian Period. Detrital sand, silt, and mud were deposited on this margin, followed by the deepening of the basin and the deposition of
VAFO Geologic Resources Inventory Report 15
carbonate chemical sediments approximately 500 million years ago (Bechtel et al. 2005).
Caves in Valley Forge National Historical Park likely formed by both vadose (above the water table) and phreatic (at or below the water table) solution. Such formation occurs when a nascent passage conducts water through the vadose zone to the phreatic zone, and solutional enlargement of the conduit occurs simultaneously in both zones. The local water table is highly irregular and discontinuous. Features characteristic of both solution types are present in a conduit described by Palmer (1984, 1990); these references could be valuable for researchers attempting to interpret the cave features at Valley Forge National Historical Park.
- groundwater: Fresh water that moves through pore spaces and fractures in soil and rock beneath the land surface.
- sinkhole: Circular hole in the ground that forms as the roof of a cave collapses.
- Groundwater erodes rock beneath the ground surface. Limestone is a carbonate and is most easily eroded.
- Groundwater dissolves minerals and carries the ions in solution.
- Groundwater erosion creates caves and sinkholes.
Use this resource to answer the questions that follow.
Kartchner Caverns State Park, Arizona at http://www.youtube.com/watch?v=uF75q4vzbAY (3:11)
- What is under Arizona?
- When were they discovered?
- When did this area become a state park?
- How long did it take the caverns to form?
- What makes caves interesting?
- How does groundwater erode rock?
- Why is groundwater acidic?
- How does a cave become a sinkhole?