are as smooth as polished ivory; others are rough and fissured. Large expanses are covered with lichens, moss and small-leaved vines." The area is dotted with small streams, waterfalls and natural springs, and many caves and rock shelters are formed in the exposed sandstone. There are two named streams flowing from the summit: Trestle Hollow and Happy Hollow. Due to its geographic isolation (despite its relatively low topographic prominence, it can be considered a minor sky island), the Fountain Bluff has been described as a "refugium for an assemblage of uncommon plants and animals". The northern harebell (Campanula rotundifolia) has been recorded to grow here, far south of its normal range; it may have become established here during the Wisconsinian glaciations. Mohlenbrock (1975) recorded a total of 991 taxa of plants. The major plant communities on Fountain Bluff are found in floodplain woods/swamps, "limestone woods" (established on shallow soil over limestone), mesic woods, "hill prairies" at exposed summits, and cliff faces. Geology Although it is located east of the Mississippi River, Fountain Bluff was once part of the west bank of the Mississippi valley. During the Wisconsinian Ice Age period from 85,000–11,000 years ago, the Cordilleran Ice Sheet advanced over much of North America and blocked the main channel of the Mississippi River. Swollen with glacial meltwater, the Mississippi River was forced to rise and carve a new channel to the west. When the ice sheet retreated, Fountain Bluff was left as an isolated "island" in the middle of the valley. During periods of heavy glacial melt, Fountain Bluff would have been a true island surrounded with water on all sides. After the ice was gone, the Mississippi may have changed course multiple times to flow around the west or east sides of Fountain Bluff, although it is currently locked in the deeper western channel. The Big Muddy River flows through the original Mississippi valley to the east as an underfit stream. The west and south edges of Fountain Bluff consists of exposed rock bluffs created by erosion from the Mississippi River. The hills are composed of Upper Chesterian limestone overlaid by Lower Pennsylvanian (Caseyville Formation) sandstone. Sandstone comprises most of the exposed (visible) strata. A smaller sandstone formation known as the "Devil's Bake Oven", which includes a natural arch, is located downstream and was formed from the same rock and glacial action that created Fountain Bluff. Two fault zones,