To the south, the overburden materials are generally thicker and less permeable. Where the overburden is 30 to 200 feet thick, sinkholes are numerous and two types are prevalent, cover-subsidence and cover-collapse. Where permeable sands are predominant in the overburden, cover-subsidence sinkholes may develop gradually as the sands move into underlying cavities. Where the overburden contains more clay, the greater cohesion of the clay postpones fail-ure,and the ultimate collapse tends to occur more abruptly.
In the southernmost part of the region, overburden materials typically exceed 200 feet in thickness and consist of cohesive sediments interlayered with some carbonate rock units. Although sinkhole formation is uncommon under these geologic conditions, where sinkholes do occur they are usually large diameter, deep, cover-collapse type.