Ruby Falls' Geology
Lookout Mountain is widely known for its many unusual geological features. The most unique of which being Ruby Falls, a 145-foot underground waterfall located more than 1,120 feet beneath the mountain’s surface. The fascinating story of the formation of Lookout Mountain, the Ruby Falls cave and Ruby Falls itself is told by the rocks that formed each of these wonders.
Formation of Lookout Mountain
Most of Lookout Mountain is made of limestone rock. About 240 million years ago, a shallow sea covered the eastern Tennessee area, and it was in this sea that the limestone of Lookout Mountain was formed. Limestone is a sedimentary rock formed by successive layers of shale, sand, and pebbly sand. The layers of sediment hardened over time to form limestone rock.
Millions of years ago, the North American and African plates collided in a collision that lasted for several thousand years. The collision was felt even as far inland as the Chattanooga area. These tectonic forces produced a series of earthquakes that pushed and bent the hardened rock to form mountains.
The bending and folding of the brittle limestone and sandstone layers caused a series of cracks and crevices to form. The rock layers slid past each other along what geologists call faults. These faults were the very beginning of Ruby Falls cave.
Formation of Ruby Falls Cave
The Ruby Falls Cave is located deep in the heart of Lookout Mountain. It is a limestone cave (sometimes called a Solution Cave). The process that forms a limestone cave can be broken down into a few steps:
- Carbon dioxide in the air is absorbed by rain water falling through it. The rain water also absorbs carbon dioxide that is in the soil after it hits the ground.
- Water breaks carbon dioxide down into a very weak acid called carbonic acid.
- The weak acid comes into contact with limestone rock when subterranean streams find their way through the cracks in the limestone rock that have been produced by tectonic forces.
- The acid solutes (or eats away) the limestone (which is made of calcium carbonate or calcite) causing the cracks to become larger and caves and passages to form. This process is called chemical weathering.
At one time, the water table was much higher and the Ruby Falls Cave was at the top of it. Rushing water flowed through it for many years washing out the cave and enlarging the faults and cracks. The water table has since lowered and that explains the lack of water in the cave.
Formation of the Waterfall and Falls Room
After the formation of the Ruby Falls Cave, a surface stream entered it and left deposits of sand and gravel along the walls. During this time a sinkhole developed near the head of the stream and allowed great amounts of water to enter. The presence of sand and clay along the sides of the cave confirm the presence of a sink hole at one time.
The water hollowed out the dome area that Ruby Falls is located; this area is sometimes referred to as “Solomon’s Temple.” By way of erosion the waterfall carved out this area. The progress of this natural process can be observed by studying the smooth sides of the limestone walls.
Formations in the Cave
Over the years, the water level in the cave lowered and air entered the Ruby Falls Cave. When that happened, the conditions were right for the formation of speleothems, or cave formations. The formations process takes a very long time even though it is a very simple process – water moving through the soil absorbs the minerals in the limestone. As the water evaporates or drips, it leaves minerals behind.
Stalactites form as water drips from the ceiling leaving minerals behind. The minerals build up very slowly to make the stalactite. All stalactites start out as capillary tubes (also called soda straws). A capillary tube is a hollow stalactite. They are very thin and fragile. When the hold on the end of a capillary tube is closed off, it becomes a stalactite and begins to widen. Capillary tubes are very common in the Ruby Falls Cave, especially in the Hall of Dreams.
Stalagmites are formed by minerals deposited on the floor when the dripping water hits and leaves behind minerals. Stalagmites almost always form underneath stalactites.
Columns are formed when a stalactite and a stalagmite grow into each other.
Drapery formations (also called curtains) are formed as water works its way through the small cracks and crevices in the ceiling and evaporates before it has a chance to fall.
Flowstone is formed from flowing water instead of dripping water. The calcite is deposited in thin layers. These thin layers will first take the shape of the cave floor or bedrock beneath it. The flowstone builds up to become more rounded as it gets thicker.
Helictites are a type of stalactite. Helictites grow in any direction on the cave ceiling. Their twisted shape is attributed to air currents and the arrangement of the crystals of calcite.
Caves often provide homes and shelter for a variety of animals. Since Ruby Falls cave has no natural entrance, we are unlikely to find critters living here. Generally speaking, cave animals are divided into three categories:
- Cave Dwellers (troglobites) are those who spend their entire life in caves (ex. Blind cave fish or crawfish)
- Cave Guests (troglogenes) are those who use caves for shelter or hibernation but usually live above ground (ex. Bats, bears)
- Cave Lovers (troglophiles) are those that live most of their lives in caves. They are usually found near the entrance where some light filters in. They move back and forth from the outside to the inside as they search for food. (ex. Salamanders, Cave crickets)
Calcite: A mineral, calcium carbonate (CaCO3); the major component of limestone and nearly all formations.
Capillary Tubes: The first stage of growth of a stalactite, a narrow fragile formation that is hollow.
Cave: A natural underground chamber or series of chambers usually open to the surface.
Chemical weathering: A process which breaks down rocks by the action of chemicals.
Column: A pillar-like structure formed when a stalactite and stalagmite have met and joined together.
Curtain: A thin translucent sheet of calcite formed when water flows down the inclined ceiling of a cave (drapery).
Erosion: The process by which weathered materials are carried away by wind, water, or glaciers.
Fault: A crack or parting in a rock often occurring in sets or parallel groups.
Flowstone: A surface coating or a calcite layer that has been deposited by a descending film of mineral-charged cave water.
Helictites: A distorted and twisting stalactite that grows with a seemingly disregard of the pull of gravity.
Limestone: A sedimentary rock composed chiefly of the mineral calcite. It is easily dissolved by acidic ground water. Most of the world’s caves are formed in limestone.