USA > Alabama > Hand-book of Alabama. A complete index to the state, with map > Part 34
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In places, particularly in the region south of the Cahaba field in Bibb county, the uppermost beds of this formation, above the purer limestone mentioned, are calcareous shales and shaly limestones, often full of the fossil forms known as graptolites. Where these thin bedded shaly limestones form the surface they make cedar glades.
The valley between the Cahaba and Coosa coal fields shows a wide belt of Trenton limestone, which is particularly pure and well developed near Pelham and Silumia, in Shelby county, and southwards. Near Pratt's ferry, on the Cahaba, and, stretching thence northeastward, there is another great belt of it, containing some fine marbles, which have, in a small degree, been worked at Pratt's ferry.
The Clinton or Red Mountain Formation .- This is the third and uppermost of the divisions of the Silurian which we make in this State. The mass of the rocks of Red mountain are sandstones and shales, which show a great variety of color, yellow, red, brown, chocolate and olive green, in this respect resembling the Montevallo shales. Along with these are some calcareous and ferruginous rocks, the latter passing into beds of red iron ore, made up of small flattened nodules, shell casts, etc., of ferric oxide. In many places where mining has pene- trated the ore bed beyond the reach of atmospheric agencies, the ore is seen to be quite calcareous ; in fact, a kind of highly ferruginous limestone, which, when used in the furnace, often contains lime enough to flux itself. At the outerop the ore is seldom calcareous, though often sandy. So far as I know, there has been no very satisfactory explanation of the mode of formation of this ore. It is of a very variable thickness, up to twenty feet, and is in more than one bed. It is a remark-
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able fact that, while near Oxmoor the ore is some twenty feet in thickness, just across the Cahaba coal field, in the Cahaba valley, about six miles distant, the Red mountain, or, rather, its representative, contains no ore at all in the greater part of its length, nor does it seem, except at intervals, to contain any of the Clinton rocks, but only those of the Devonian and sub- carboniferous. As is well known, this formation furnishes the greater part of the material used in our furnaces. In places, the ferruginous limestone of this formation would make a fine building stone, and the same is true of the sand- stones. It would be difficult to give the average thickness of the Red mountain rocks proper; 100 feet might perhaps be a fair average, for the Red mountain, as a topographic feature, is made of the rocks of different ages-Trenton, Clinton and sub-carboniferous, together with the usually very thin black shale of the Devonian.
The thickness of the whole Silurian, as usually given at about 5,000 feet, is only an estimate. The true thickness it would be very difficult to determine, especially in the case of the most important member, the Knox dolomite, since it is in great part made up, so far as surface materials are concerned, of loose fragments of chert in which the bedding planes are seldom to be seen. A greater part of the area of our valleys is held by this formation than by any other, and its impor- tance is still further enhanced by the fact that it is the chief source of the brown iron ores of the State. Many of the noted " big springs " issue from this formation.
The Devonian .- The only representative in Alabama of this system of rocks, which in the States further north is of great thickness and importance, is a thin bed of black shale, averaging perhaps ten or fifteen feet, but being apparently absent altogether in some places. A few fossils have been found in it in the valley of the Tennessee in North Alabama, which serve to fix its position as a member of the Devonian. The shale, being soft and somewhat easily eroded, is usually covered and concealed by the debris of the adjacent rocks, so that it does not commonly come under notice even where it is present. It is of importance chiefly, perhaps, as being the source of some of our best known sulphur springs. The shale usually contains a large amount of pyrite in the form of
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nodules or kidney shaped concretions, the decomposition of which supplies the sulphur of these springs. In north Ala- bama the thickness of the black shale, as this formation has been called, may go up as high as 100 feet, but so extreme a thickness is rarely seen further south.
The Carboniferous. - This we conveniently divide in Alabama into two parts-a lower, or sub-carboniferous, and an upper, or coal bearing part, the true coal measures. The thickness of the latter is placed by Mr. Squire at 5,525 feet, and the former at 1,200 feet, making a total of between 6,000 and 7,000 feet.
Sub- Carboniferous .- Professor Safford divides this forma- tion in Tennessee into an upper or calcareous member, and a lower or siliceous one. This division will also apply equally well to that part of Alabama north of the Tennessee river, but to the south, and everywhere in the narrow anticlinal valleys of the State, this distinction cannot well be made.
In the Tennessee valley the siliceous member of the sub- carboniferous consists of a great series of cherty limestones somewhat analogous to the Knox dolomite, but with the lower part more cherty than the upper. This lower part (lower siliceous of Professor Safford ; probably the Keokuk of the western geologists) gives rise to rather poor siliceous soils, and the region of its occurrence, both in Alabama and Ten- nessee, is known as the " barrens." The upper part of the siliceous member (upper siliceons of Safford ; St. Louis group of the western geologists) is more calcareous, and the soil derived from its disintegration is a red loam of more than ordinary fertility, well known in the Tennessee valley as making the best farming lands in that section. Here again there is an analogy to the Knox dolomite, which affords, on the one hand, rich. red loam soils and, on the other, poor, cherty ridges.
The chert of the sub-carboniferous is in general very similar to that of the Knox dolomite, but differs from it in being usually very highly fossiliferous, containing the casts or moulds of shells that have been leached or dis solved out.
This character of the sub-carboniferous chert and the presence of the rhombohedral cavities in the chert of the
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Knox dolomite enable us in almost every case to distinguish between the two.
Now, in the antielinal valleys south of the Tennessee river we find it impossible to carry out this two-fold division of the lower or siliceous member of the sub-carboniferous, for the entire member shows, upon the surface at least, little else than chert, which appears in a mantle of angular fragments, cover- ing usually one side of all our Red mountain ridges.
We have called this the Fort Payne chert, and it is prob- ably the representative of both sub-divisions of the lower sub-carboniferous or siliceous group of north Alabama and Tennessee, as long ago conjectured by Professor Safford. Its thickness is not very great, as compared with that of the upper member .*
The upper calcareous member is variable in composition In north Alabama it is chiefly limestone --- called mountain limestone, from the fact that it forms the flanks of most of the mountains in that section that are capped with coal measures. Within this limestone there is interbedded a layer of sand- stone of variable thickness-perhaps 100 feet, at a maximum, in the Tennessee valley, while the over and underlying lime- stones are many times that. As we come southward the sandstone becomes more important, and the lower section of the limestone (below the sandstone) appears to give way to, or to be replaced by, a series of black shales closely resembling those of the Devonian, but many more times massive. In many places in the anticlinal valleys, and especially the further south we go, the upper limestone also appears to be wanting or to be replaced by the shales and sandstones above named.
The limestone which comes next below the coal measures is well exposed at many places, as at Bangor, Blount Springs and Trussville, where it is very extensively quarried for use as a fluxing material in the furnaces, as it is in part a very pure limestone. But south of the latitude of Birmingham it is very rarely seen, and in its stead we find the black shales mentioned. These shales are often interstratified with dark
* In the anticlinal valleys there may usually be sven between the Red mountain ridges and the rim of the valley a narrow sub-valley with very good reddish loamy soils, nearly always in cultivation. This soil may be in part derived from the rocks of the St. Loni- group of the siliceous, though possibly from the shales of the Oxmoor horizon. We have found no fossils to determine this point.
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colored limestones, and sometimes with tolerably pure lime- stones, but these are unimportant in thickness as compared with the shales and sandstones. The greater part of Shades valley is based upon these sandstones and shales, though the limestone appears in several places, and southwestward, be- yond the end of the Cahaba coal field, these rocks make a very prominent topographie feature.
The sandstone which in north Alabama lies between the two beds of mountain limestone has a very close resemblance in texture and other characters to the lowermost rocks of the coal measures, commonly called the millstone grit, and it makes its appearance in that part of the State either as a bench along the sides of the Cumberland mountain spurs or else as the capping and protecting rock of a detached ridge separated from the Sand mountain (coal measures) by a narrow valley of erosion. In the anticlinal valleys further south, this sandstone with the lithological characters above named appears commonly as a distinct ridge running parallel to the escarpinent of the coal measures, with a narrow valley of shales between. It appears to best advantage on one of the detached ridges above spoken of, near Tuscumbia, at the site of the old college town of LaGrange, and we have often used the name LaGrange sandstone to designate it; but the name LaGrange has been used to denote an entirely different formation, which has caused us to replace it by the name Oxmoor. where the rocks are also well exposed and where the shales are more conspicuous than at LaGrange.
Coal Measures .- The strata of the coal measures are sandstones, conglomerates, shales and coal beds in many alter- nations, and at one horizon, at least, occurs a thin bedded limestone. The thickness of the measures of the Cahaba coal field is estimated by Mr. Squire to be 5,525 feet, and the esti- mate for the Warrior field, by Mr. MeCalley, is about the same. The conglomerates are found principally at the base of the measures, though a very heavy bed of this rock occurs also at the top of the measures in Shelby and Tuskaloosa counties. We have good reason for thinking that all our coal fields were at one time continuous and have been separated by subsequent changes into the three distinct fields-named by Prof. Tuomey from the rivers which drain them-the Coosa, Cohaba and
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Warrior fields. We should expect to find the succession of the coal seams and of other strata the same in all three fields. and undoubtedly such a similarity or equivalency will one day be thoroughly made out. At the present, however, our knowledge of the fields is so limited that it is not possible to correlate their seams fully, though some identifications have been very satisfactorily made.
In the reports of Messrs. Squire and McCalley on the Cahaba and Warrior fields full particulars will be found con- cerning these fields, and the reader is referred to these reports for information as to details. In the Warrior and Cahaba fields there are about forty coal seams, of which twelve to fourteen have a thickness of two and a half feet and upwards. of coal, which can be profitably mined. Much of the coal, however, which is actually mined now comes from a much smaller number of seams. In another place we shall give a few particulars concerning each of the three fields above named.
GEOGRAPHICAL DISTRIBUTION IN ALABAMA OF THE PALEOZOIC FORMATIONS.
It is a commonly received opinion among geologists, and one capable of demonstration, that the older stratified rocks of the Appalachian region of the United States, of which the Paleozoic rocks of Alabama form the southwestern termination. are formed principally out of the detritus of a previously ex- isting land area lying eastward of the present shore line of the Atlantic ocean, washed down and deposited upon the floor of the inland sea, which formerly occupied the greater part of what is the present United States. Naturally, by far the greater part of this land waste would be deposited close to the shore line, while only the finer sediments, such as silt and mud, would be held in suspension long enough to be carried out and deposited at a distance from the shore. As a matter of fact, in Alabama, in going from the northwest to the south- east, across the region made by our Paleozoic rocks, we find a
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gradual increase, not only in the thickness of the strata, but in the coarseness of the material; heavy bedded sandstones and conglomerates being much more abundant in the eastern part of this area than farther west. The maximum thickness of the Paleozoic rocks in Alabama, along their eastern border, is not less than 20,000 feet, but further westward it is probably not much more than half this amount. After the deposition of the Carboniferous strata, there followed movements of the earth's crust by which all these Paleozoic deposits were elevated above the sea, and pressed or squeezed laterally from southeast to northwest into a much narrower space than they occupied when spread out upon the floor of the sea. This compression into narrower limits could take place either by the crushing together laterally of the beds and their corresponding swelling up vertically ; or by the warping of the strata into more or less closely folded waves or wrinkles running at right angles to the direction of the compressing force : or by fracture along the same line at right angles to the compressing force, and the sliding of one part of the strata over another. It is easy to recognize in Alabama instances of all these results of compres- sion ; for we find our Paleozoic rocks seldom in their original position ; they are mostly tilted at high angles to the horizon ; the same beds are repeated many times, and sometimes strata are now found in juxtaposition which are many hundreds of feet apart in the geological column. The direction of the dip of these tilted beds is mostly towards the southeast, and the amount of the tilting and deformation is greater in the same direction, gradually diminishing in intensity as we go north- west. After the uplifting, faulting and folding above indicated, there began a gradual degradation of the strata by atmospheric agencies. The places most elevated by these movements, or most weakened y racture, would suffer most by these means. So we find, as a rule, the valleys now occupying what were formerly the crests of the folds or waves of the strata, and it is consequently in these valleys that we find exposed the lowest or oldest rocks in the series, and the depth of the exposure in proportion to the amount of the previous elevation.
In accordance with these general principles, we find that the oldest of our Paleozoic rocks have their greatest develop- ment as surface roeks along the eastern border of the region of
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their occurrence, i. e., in the Coosa valley region. Here also we find the strata most disturbed and deformed, while the rocks of the Clinton. Devonian, or Carboniferous formations occur sparingly in the Coosa valley, except along its western limit. It is only to the westward of the Coosa valley that these upper strata of the Paleozoic formations are widely distributed.
The areas of our coal measures are separated by valleys, in which the older rocks are exposed by denudation as above described. All these valleys have what is called the anticlinal structure, i. e., they have been eroded out of the crests of folds in the strata : but this structure is generally obseured by the occurrence of faults where the beds on one side of the line of fracture have been pushed up over on those of the other. These valleys are, in succession, going to the northwest : The Cahaba valley, lying between the Coosa and the Cahaba coal fields : Wills' valley, between Lookout and Sand mountains : Rom's and Jones' valley, between the Warrior and Cahaba coal fields ; Murphree's valley, which is a prolongation of Jones' valley and separates two parts of Sand mountain ; and Brown's or Big Spring valley, extending down from Tennessee, as a prolonga- tion of the Sequatchee valley, separating likewise two parts of the Warrior field. In the last-named valley, south of the Ten- nessee river, the sub-carboniferous rocks form the greater part of the surface, and the underlying and older rocks are exposed only in isolated traets. North of the river, the elevation of the fold has been greater, and the consequent exposure, by erosion of the valley, deeper. In the Tennessee valley likewise, the sub-carboniferous rocks occupy the greater part of the surface. Each of the valleys above enumerated has its peculiarities, but to go into details would lead us too far. We may, however. add a few words to what has already been said about the coal fields.
Materials of Economic Value in the Paleozoic Region. Coal .- The Alabama coal fields, occupying, as they do, the troughs between the folds or wave erests above mentioned. are more or less basin-shaped, but the axis of the basin in each case lies close to one side, usually the southeastern, instead of being in the center. and the slope or dip of most of the strata is consequently towards the southeast. In going from the north-
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east toward the southwest, we find the strata of these fields gradually sinking. Thus, on Sand mountain, near the Georgia line, the base of the coal measures is six or seven hundred feet above drainage level, especially along the elevated rim of the basin, while, at Tuskaloosa, the base of these measures lies 4,000 feet, or more, below drainage level. For convenience, we have designated those parts of the coal fields which are high above the drainage level, as the plateau region, in contradistinction to the basin region, where they are at or below drainage level. This distinction applies mainly to the Warrior field and Look- out mountain, the Cahaba and Coosa fields having measures generally below the drainage level; but a gradual southeast sinking of the measures is very clearly seen in them also.
The Coosa Fieldl .--- This has less of what are called the flat measures than either of the others. It has suffered most from denudation, and has the least thickness of coal measures, and consequently of coal seams; nevertheless, two or three seams of workable size are known in the field, and mines are in opera- tion in two or three localities-Broken Arrow, Ragland, Trout Creek, etc. The rocks and coal seams in this field appear to be more faulted than in the others.
The Cahaba Field-This field has been recently thoroughly described by Mr. Squire, and his report is accompanied by a large and detailed map. It extends from Canoe creek, in St. Clair county, to Haysop creek, in Bibb county, its form very closely resembling the human foot and leg, Montevallo being close to the heel, and the part repesenting the toe being on Haysop creek, four or five miles west of Scottsville. The field begins to widen at Helena, and opens out gradually as it con- tinues southward until it attains a maximum width of fifteen and a half miles, on a line due east and west from Blocton. The length of the field on an air line is sixty miles, and its area about 390 square miles. Mr. Squire describes eleven subordi- nate basins in this field, besides the overturned measures at the southern end of the field. In consequence of the gradual dip of the basin as a whole toward the southwest, the greatest thickness of the measures in this field is found in the south- western end. Active mining is now going on at Blocton, Gur- nee, Montevallo, Helena and Henry- Ellen, the seams worked being: At Blocton, the Thompson, or Underwood, and the
ยท
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Woodstock seams; at Gurnee, the Clark and Gholson seams; at Montevallo, the Montevallo seam ; at Helena, the Wadsworth and Helena seams; at Ilenry-Ellen, the Mammoth seam. Mr. Squire estimates that this field has eleven seams of coal over two and a half feet in thickness, aggregating forty feet of marketable coal. Coke is made in large quantity at Blocton and other mines in this field.
Warrior Field .- The area of this field is estimated at about 7,800 square miles, and the thickness of the measures varies with the localities, it being only a few hundred feet in the northeast part of the plateau region, and nearly 5,000 feet at the southwestern end of the field. The number of coal seams varies in the same sense. Mr. McCalley gives about forty seams in this field, fourteen of which are two and a half feet and upwards in thickness. The localities where most mining is done are: Pratt Mines, Mary Lee mines, and others near Birmingham; Blue Creek mines, below Bessemer; several mines at Newcastle and Warrior, in Jefferson county ; at Cor- dova, Ilorse Creek, Carbon Hill and Corona and other localities in Walker county; Brookwood and Coaling, in Tuskaloosa county. For details concerning the field, the reader is referred to the report of Mr. McCalley on the Warrior basin of the platean region. Coke is made in the largest quantity from the coal of the. Pratt, Blue Creek, Mary Lee and Brookwood mines.
Red Iron Ore .-- The Clinton or Red Mountain formation carries a varied thickness of red fossiliferous ore. This forma- tion occurs on the ridge on each side of the anticlinal valleys above named, and in each of these, at one or more points, the ore is of quality and quantity which make it of commercial importance. The great bulk of this ore mined in Alabama comes from Red mountain ridge, along the eastern side of Jones valley, from Reeder's Gap to Gate City. At one place the ore is twenty feet, or more in thickness. On the west side of Murphrees valley, along the western border of the Coosa valley above Springville, and at Attalla near the base of Look- out mountain, and along the eastern foot of Lookout at inter- vals up to Round mountain, the ore is also extensively mined.
Brown Iron Ore .- This ore is usually associated with the strata of the lower Silurian ( Knox dolomite) and the upper
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part of the shale formation in all the valleys above enumer- ated. In the Coosa valley it is mined extensively near the Shelby Iron Works, and also in Talladega, Calhoun and Cher- okee counties. In the Cahaba valley, near Briarfield ; in Roups and Jones valley, near Woodstock, and again at points near Tannehill station ; in Murphrees valley, in Blount county, above Oneonta; in Wills valley, there are several mines be- tween Attalla and the Georgia line. In Franklin county, near Russellville, the ore is associated with cherty limestone of the sub-carboniferous formation. Near Vernon, in Lamar county, it occurs with pebbles and bodies of a much later geologic period.
Limestone .- The mountain limestone, which comes directly . below the coal measures of the plateau region, is usually very pure and well suited as a flux, and is extensively quarried at Trussville, Bangor and Blount Springs.
Among the cherty limestone of the lower sub-carboniferous formation occur heavy beds of variegated and gray fossilifer- ous limestone. The latter is extensively quarried in Colbert county, and the same stone is found abundantly in Landerdale, Franklin, Limestone and the other counties of the Tennessee valley.
The Clinton formation also furnishes a highly ferruginous limestone that will one day be used for ornamental purposes.
The Trenton limestone of the Silurian formation is now being quarried at Gate City for the furnaces, and at Siluria and near Calera for use in lime kilns. The limestone of this age occurring about Pratt's ferry forms a beautiful variegated marble, and it has been quarried and worked on a small seale. 'A fine red or variegated marble also of this age is found in the southwestern part of Jefferson county, west of McCalla.
White Crystalline Limestone or Marble-Occurs at many points along the eastern border of the Paleozoic region, in Tal- ladega, especially near Sylacauga, where it has been worked to some extent in past times.
Of less importance than the foregoing, we may mention the following minerals occurring in these formations :
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