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is a city in and the county seat of Madison County, Missouri, United States, in the northeastern foothills of the Ozark Mountains. The population was 3,928 at the 2000 census. The city is surrounded on three sides (east, west, and south) by the easternmost parcel of the Mark Twain National Forest.
The location of Fredericktown at the border of the northeastern edge of the Ozark Mountains provides a sharp contrast in geography as one travels from east to west. The eastern edge of Madison County consists largely of gentler, more rolling hills ridges of sedimentary dolomite, while the western edges of the county consist of steep-sided, round-profile hills consisting largely of far more ancient Precambrian rhyolites (heavily compacted and heat-fused volcanic minerals) and beautiful pink granites.
By driving west from Fredericktown on Highway 72 and looking further west, one can witness a rare geological oddity: the fossilization not just of a plant or animal, but of an entire very ancient landscape. As viewed from that vantage point, the round-profile rhyolite and granite hills of the western side of Madison County resemble an archipelago of heavily weathered islands, with the shallow tree-filled valleys taking the place of shallow seas. The resemblance is not a coincidence. These particular hills are in fact fossil islands from the Precambrian period. Even more remarkably, the remnants of the shallow sea in which they once stood can still be seen in the form of sedimentary dolomite deposits that still lap up gently against the sides of the much harder igneous and metamorphic rocks of the hills.
The hills were originally islands that developed over an unimaginably long period of gradual erosion, during a geological period when change took place much more slowly than it does today. The islands were subsequently submerged under an ocean, and in time buried under several thousand feet of much softer dolomite deposits. When the Ozark Mountains plateau was later pushed upward again, possibly due to an intra-continental mantle plume that may also have been responsible for some of the abundant ore deposits of the region, the softer dolomite was weathered away to leave the original Precambrian islands largely intact. Since juniper prefers a more alkaline soil, it is sometimes possible to locate the margins of the ancient sea from a distance by looking for the distinctive light green color of junipers growing on the more alkaline oceanic dolomite deposits between the island hills.
Another remnant of the ancient sea can be found in road cuts along Highway 72, where one can find small, rounded granite boulders that once rolled off of the ancient islands and into shallow sea surrounding them. Unlike more familiar forms of granite, these boulders are so soft that it is often possible to peel onion-like layers from them using only one's hands. The softness of these unusual stones reflects the very slow pace of events in the ancient archipelago, since such deep surface-inward weathering requires that the original rounded granite rocks remain immobile and undisturbed over immense lengths of time. Our much more dynamic modern world breaks such soft layers from rocks long before they can accumulate, so that modern granite rocks age not by getting softer, but by getting smaller.