The timing of speleogenesis of two of Illinois’ longest caves (Fogelpole Cave and Illinois Caverns) located in southwestern Illinois’ sinkhole plain was investigated by integrating differential erosion and datable sediment and speleothem deposits within the caves. Five locations in the two caves, and three different methods were used to estimate incision rates and timing of the initiation of the caves. Two of the sites investigated in Fogelpole Cave have second-order channels or passages that are now filled with silty subterranean slackwater sediments deposited during dry periods of the Pleistocene and Holocene Epochs; the passages are now raised above the present cave stream bed by benches that are similar to each other in character and height. The other three sites contained speleothems that recorded the age of early cave levels. Radiocarbon dates of disseminated organic matter in the fine-grained sediments and uranium-series dating of the speleothems yielded approximate ages for the benches within the caves. The heights of the benches and cave passages, and the age of the sediments and speleothems on those benches were used to estimate incision rates and the timing of initiation of the two caves. The various techniques and locations within the two caves yielded rates of incision ranging from 0.031 to 0.050 mm/year averaged over the course of 30,000–125,000 years. Based on the rates of incision and passage dimensions, the age of speleogenesis for Fogelpole Cave, Illinois Caverns was estimated to range from 120,000 and 164,000 calendar years before present. Coeval surficial ice-marginal deposits have been mapped about 5 km south of the caves, suggesting a causal relationship between the chemically aggressive glacial melt waters of the penultimate glaciation in Illinois and the onset of speleogenesis.