Glaciation-Induced Formation of Illinois' Longest Cave

Geologists examine Foglepole CaveGeochemists from the Illinois State Geological Survey (ISGS) and Department of Geology at University of Illinois at Urbana-Champaign are currently investigating the origin and evolution of large, branchwork-type caves in southwestern Illinois. Caves are defined as openings in bedrock that are large enough for a human to enter and long enough for a visitor to experience total darkness. The large caves of southwestern Illinois, Fogelpole Cave and Illinois Caverns, are typically 100 feet below the surface and up to 15 miles long; these caves contain stream-filled passages over 25 feet in diameter. The questions of when the caves began to form and how they evolved are difficult to answer because the scientists are attempting to date and characterize an open space. Currently, ISGS scientists are taking an eclectic approach to investigating these caves and their deposits. Stalagmites and fine-grained sediments have been used to identify paleoclimatic and paleoflooding in the study area, and stalagmites are currently being used to explore earthquake activity as the area of the caves is in close proximity to the New Madrid Seismic Zone.

During the course of their work, ISGS scientists realized that deposits of surface-derived sediments, stalagmites, and flowstone located throughout the caves at a variety of different levels act as markers for cave development and downcutting after its initiation. Carbon-14 and uranium-series dating techniques were used to identify the ages of these materials at four different and far removed locations within Fogelpole Cave. The heights above the floor of the cave and the measured ages of these materials were used to calculate downcutting rates. Incision rates by the cave streams range from 0.032 to 0.048 cm/yr (about 1.25 to 1.9 inches per 100 years). Using these downcutting rates, it was determined that the caves were initiated between 140,000 and 170,000 years before present. That means that the caves were initiated near the end of the Illinois Glacial Episode and the beginning of the Sangamon interglacial interval. This timing suggests that, as the Illinoian glacier began to melt, the cold, chemically aggressive meltwaters began infiltrating into fractures and bedding planes in the limestone and dissolved pathways through the rock. Once these pathways were established, infiltrating rainwater and snowmelt subsequently used them as conduits to migrate through bedrock and discharge downgradient to the surface, again forming springs and streams. The continuous flow of water through these limestone conduits resulted in additional dissolution of rock that continues today. Flowstone ages near the ceiling of the cave suggest that the cave was only about three feet high about 110,000 years ago; the cave ceiling is about 25 feet high today.