Description
- At Birmingham the lower 100 feet (30 m) is predominantly shaly and there is a 20-foot (6.1 m) bed of thick-layered sandstone 34 feet (10 m) above the bottom. Above this shaly part lies the Irondale ore bed, which is separated from the Big Seam of iron ore by a few feet of shale and of sandstone layers that carry large and small waterworn pebbles. Next above lies the Big seam, approximately 17 feet (5.2 m) thick, above which these is about 38 feet (12 m) of red sandstone that carries small quartz pebbles, the lower 14 feet (4.3 m) of which has yellow shale about equal in amount to sandstone. About 30 feet (9.1 m) still higher is the Pentamerus-bearing bed (Hickory Nut seam), a ferruginous sandstone full of casts of the interiors of the big brachiopod Pentamerus oblongus. There is still about 70 feet (21 m) more of sandstone, largely reddish, and shale to the top of the Red Mountain formation, making a total thickness in this part of Red Mountain of about 260 feet (79 m).
- The most interesting and valuable feature of the Red Mountain formation is its iron ore, which is the chief cornerstone of Alabama's industrial structure. Although ore of good quality and of workable thickness occurs elsewhere, as at Attalla, and on the Red Mountain along the west side of Murphrees Valley, the main deposit, the Big seam, lies under Shades Valley and the part of the Cahaba coal field southeast of that part of Red Mountain which extends from a point a mile or two southwest of Bessemer to Morrow Gap about 8 miles (13 km) northeast of Birmingham.
- The Red Mountain ores are known as a red fossil ore, because originally the iron accumulated in extensive beds of fragments of fossils, principally the hard parts of crinoids, bryozoans, and brachiopods. The iron, from solution in some form, was precipitated upon and within these beds of fossil fragments and thus the ore beds are simply a particular kind of sedimentary layers inclosed in ordinary sediments, shales, and sandstones, composing the bulk of the Red Mountain formation. As the fossil fragments were composed of calcium carbonate, which is the mineral that forms limestone, the iron ore beds at depth, where they are unweathered and where there has been no condition that permitted leaching of the lime content, carry a considerable percentage of lime, so that the ore is self-fluxing. Another type of ore is oolite ore, in which the iron oxide occurs in the form of small lenticular pellets. The precipitation of the iron that forms this ore started around some minute particle like a small grain of sand or fragment of fossil and built up a small lenticular body. The two kinds of ore are more or less mixed or one or the other may predominate in a particular layer of ore. – Geology of Alabama (1926)
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