CULM, in geology, the name applied to a peculiar local phase of the Carboniferous system. In 1837 A. Sedgwick and R. I. Murchison classified into two divisions the dark shales, grits and impure limestones which occupy a large area in Devonshire and extend into the neighbouring counties of Somerset and Cornwall. These two divisions were the Upper and Lower Culm Measures, so named from certain impure coals, locally called “culm,”1 contained within the shales near Bideford. Subsequently, these two geologists, when prosecuting their researches in Germany and Austria, applied the same name to similar rocks which contained, amongst others, Posidonomya Becheri, common to the phase of sedimentation in both areas.
The Culm measures of the Devonshire district are folded into a broad syncline with its axis running east and west; but within this major fold the rocks have been subjected to much compression accompanied by minor folding. This circumstance, together with the apparent barrenness of the strata, has always made a correct interpretation of their position and relationships a matter of difficulty; and for long they were regarded as an abnormal expression of the Lower Carboniferous, with the uppermost beds as doubtful equivalents of the Millstone Grit of other parts of Britain. The labours of W. A. E. Ussher and of G. J. Hinde and H. Fox have resulted in the differentiation of the following subdivisions in the Devonshire Culm:—(1) Upper Culm Measures or Eggesford grits; (2) Middle Culm Measures, comprising the Morchard, Tiverton and Ugbrooke lithological types overlying the Exeter type; (3) Lower Culm, the Posidonomya limestone and shale overlying the Coddon Hill beds with radiolaria. Ussher’s subdivisions were introduced to satisfy the exigencies of geological mapping, but, as he pointed out, while they are necessary in some parts of the district and convenient in others, the lithological characters upon which they are founded are variable and inconstant. More recently E. A. N. Arber (1904-1907) clearly demonstrated that no palaeontological subdivision of the Upper Culm (Middle and Upper) is possible, and that these strata, on the evidence of the fossil plants, represent the Middle Coal Measures of other parts of the country. Wheelton Hind has called attention to the probability that the Posidonomya limestone and shale may represent the Pendleside group of Lancashire, Derbyshire, &c. The Coddon Hill beds may belong to this or to a lower horizon. Thus the English Culm measures comprise an Upper Carboniferous and a Lower Carboniferous group, while in Germany, Austria and elsewhere, as it is important to bear in mind, the Culm, or “Kulm,” stage is shown by its contained fossils to belong to the lower division alone.
The typical Carboniferous limestone of the Franco-Belgian area changes as it is traced towards the east and south into the sandy, shaly Culm phase, with the characteristic “Posidonia” (Posidonomya) schists. This aspect of the Culm is found in Saxony, where there are workable coals, in Bohemia, Thuringia, the Fichtelgebirge, the Harz, where the beds are traversed by mineral veins, and in Moravia and Silesia. In the last-mentioned region the thickness of the Culm formation has been estimated 618 by D. Stur at over 45,000 ft. In the east and south of the Schiefergebirge (a general term for the slaty mountains of the Hundsrück and Taunus range, the Westerwald and part of the Eifel district), the Culm shales pass upwards into a coarser deposit, the “Culm-grauwacke,” which attains a considerable thickness and superficial extent. Culm fossils appear in the Carnic Alps, in the Balkans and parts of Spain, also in Spitzbergen and part of New Guinea.
The most characteristic fossil is of course Posidonomya Becheri; others are Glyphioceras sphaericum, Rhodea patentissima, Asterocalamites scrobiculatus (Schloth), Lepidodendron veltheimianum, Gastrioceras carbonarium.
See E. A. N. Arber, “On the Upper Carboniferous Rocks of West Devon and North Cornwall,” Q.J.G.S. lxiii. (1907), which contains a bibliography of the English Culm; E. Holzapfel, Paläont. Abhandl. Bd. v. Heft i. (1889); H. Potonié, Abhandl. preuss. geol. Landesanst., Neue Folge, 36 (1901); D. Stur, “Die Culm Flora,” Abhandl. k.k. geol. Reichsanst. viii. (Vienna, 1875).(J. A. H.)
1 This word is possibly connected with col, coal; distinguish “culm,” the stem of a plant, Lat. culmus.
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