(LEVELS E & F)
The phreatic tubular passage from Great Relief Hall leads down to River Hall at Mammoth Cave's level E. This room provides the easiest access down to see the modern underground rivers at the water table (level F). River Hall is the lowest point on the “Historic tour”. The "River Styx tour" continues down from River Hall to see the water table (albeit from a distance).
River Hall (looking N) - the walls & ceiling of this room have prominent scallops (phreatic flow-dissolution features). This room is at the junction of two tributaries of an old underwater river system. One tributary brought water from the River Styx area, and the other brought water from the passage leading from Great Relief Hall. The ceiling & uppermost walls consist of basal Fredonia Member limestones (Ste. Genevieve Ls.). The middle to lower walls and floor are all upper St. Louis Limestone ( Meramecian Stage, Middle Mississippian), specifically the Horse Cave Member of the St. Louis Ls.
River Hall - beautifully scalloped ceiling of River Hall, formed when this room was flooded.
River Hall - dissolution features along the walls of River Hall have complex geometries. They are the result of complexly intersecting small & large solution pockets. The rocks are Horse Cave Member limestones of the St. Louis Ls.
Dead Sea - this pool represents the level of the modern water table in Mammoth Cave (level F). During rainstorms or winter snow melting, the water table here can be much higher. After such events, the water table lowers again, but leaves mud behind. Mud covers the rocks & passages around the Dead Sea and the River Styx. The tourist trails in this vicinity are frequently slippery from wet mud. Sometimes, River Hall (see above) has been close to being flooded.
A waterfall can be heard from this overlook at the Dead Sea. The sound of rushing water comes from the nearby Charon's Cascade.
Dead Sea - rills (vadose dissolution features) are present along the edges of Dead Sea. These are either mud deposits or mud-covered limestones. The bedrock in this area is part of the Horse Cave Member of the St. Louis Limestone.
Scallops - beautiful small-scale scallops developed on a St. Louis Limestone bed (trailside, Dead Sea-River Styx area). The wet mud coating indicates relatively recent flooding at this level.
Scallops - interestingly dimpled St. Louis Limestone walls in the Dead Sea-River Styx area.
Solution Pocket - scallops, solution pockets, and other karst forms are abundant along passage walls in the Dead Sea-River Styx area. The rock is Horse Cave Member of the St. Louis Limestone.
Mud-Filled Solution Pockets - the water table at level F fluctuates up & down with the passing of surface rainstorms and wintertime snow melting. Here's some solution pockets in the St. Louis Limestone that have been secondarily filled with modern fluvial mud. The mud is cohesive enough to remain in the cavities. Some of the mud filling has been subsequently sculpted (partially eroded) by more recent water movement during floods.
Karstified St. Louis Limestone - wet mud coats these interestingly-shaped dissolutional features called pendants in the Dead Sea-River Styx area. They are similar to those in River Hall - complexly intersecting solution pockets. Some surface karst on Bahamian islands has a similar chewed-up, “swiss-cheese” appearance.
Rapier's Pit (looking from below) - a nice domepit along the southwestern side of the River Styx. Domepits are vadose dissolution features. The rocks are still St. Louis Limestone (Horse Cave Member).
Echo River is a moderately long, somewhat snaking underground river at Mammoth Cave's level F. Its top surface represents the elevation of the modern water table. Early tours routinely included boat rides on Echo River (see photos from old postcards below). Such experiences are a thing of the past (sad, sad). Rocks along the walls & ceiling of Echo River are Horse Cave Member limestones (St. Louis Ls.). A fossiliferous chert horizon (Lost River Chert Bed) also occurs at or below the water table here (I've not seen it, so I can't comment on its nature from first-hand observation).
Echo River - early boat tour at level F in Mammoth Cave. The background (ceiling & distant walls) has been artistically enhanced (= fake).
Some unusual organisms live below the water table in Mammoth Cave. They evolved from surface-dwelling ancestors into blind (eyeless) creatures with translucent bodies (no pigment). They include cavefish (Amblyopsis spelaea and Typhlichthys subterraneus) and various arthropods (Orconectes pellucidus, Palaemonias ganteri, Caecidotea spp., Crangonyx spp., Stygobromus spp.).
Waters in the Echo River and River Styx drain through flooded passageways and emerge at the land surface at two springs on the southeastern side of the Green River.
River Styx Spring Echo River Spring