The Millersville Limestone Member thickens from its normal 5 to 10 m to as much as 15 m in central Illinois. At least part of the increased thickness correlates with algal hanks in the Millersville. Two superposed banks are exposed in quarries along the Christian-Montgomery County line. Development of the banks was controlled by pre-existing topography: the first bank was built upon a drowned delta platform and a later bank developed on top of the first. The red alga, Archaeolithophyllum , was the primary biotic contributor to the banks and also trapped carbonate sediment. The banks were not observed to be elevated significantly above the surrounding sea floor and distinct flanking facies did not develop where presently exposed. Constructionally-maintained circular depressions, here called pools, originated early in bank formation and provided a distinctive microhabitat during deposition of the first bank. The sequence of development of the Millersville banks is as follows: 1) marine transgression on a delta platform, 2) initial colonization of the platform by crinoids and brachiopods, followed by Archaeolithophyllum , 3) growth of the first bank, 4) influx of deltaic material that temporarily halted bank sedimentation, 5) growth of another bank on the site of the first bank, 6) advance of deltaic sedimentation that buried the banks.