With 8.1 miles of surveyed passage, Blanchard is the second longest cave in Arkansas and the largest in volume. The limestone rock from which the caves and their formations developed was laid down in an ancient sea more than 350 million years ago. The cave is in middle Ordovician to lower Mississippian rocks and extends through six stratigraphic formations. The cave has shown over 5 levels of passage development but the upper two levels have eroded away as deepening valleys on the surface cut into them. The cave's formation was largely phreatic in nature (formed below the water table) and passages have elliptical cross-sections typical of these formations. During the cave's development, active streams have been pirated from one level down to another without much vadose erosion occurring. The present stream currently rises from the cave at Blanchard Springs itself, at the same temperature as the cave, a constant, year-round 58 °F (14 °C).