On a well-marked Horizon of Radiolarian Rocks in the Lower Culm Measures of Devon, Cornwall, and West Somerset
Authors: Hinde George Jennings , Fox Howard 
Published: October 2009

Summary and Conclusions

The main fact brought out in this paper is the presence of a series of organic siliceous rocks, some of a very hard, cherty character, others platy, and yet others soft, incoherent shales, which are developed in the lower portion of the Culm Measures of Devon, Cornwall, and West Somerset. These rocks can be traced along, and a short distance within, the line of the boundary between the Upper Devonian slates and shales and the conformably succeeding Culm shales from a few miles west of Barnstaple, North Devon, to Ashbrittle, near Wellington in West Somerset, where the Culm Measures disappear beneath the New Bed rocks. On the southern margin of the synclinal trough of the Culm Measures the same rocks are shown from near Boscastle, on the coast, up to and beyond Launceston, Cornwall; at Tavistock, and as far south as Painter's Cross, near St. Mellion, Cornwall, where a small outlier is exposed. On the east of Dartmoor they are shown at Ramshorn Down, near Bovey Tracey, also near Chudleigh, and at Bishopsteignton. These rocks, throughout, are characterized by radiolaria, occasionally associated with sponge-spicules. These organisms in some instances are thickly crowded in the beds, in other cases they are sparsely scattered in a siliceous groundmass. Detrital materials are, as a rule, absent in the Radiolarian Beds, but in some instances they occur as microscopically minute fragments of mica and quartz. Dark carbonaceous and ferrous materials, crystals of pyrites, and rhombohedral crystals or negative crystals of carbonate of lime or dolomite are also abundant in these rocks.

The radiolaria occur both in the hard, cherty beds and in the soft, friable shales; they are not well preserved, and are now mostly in the condition of siliceous casts. Forms belonging to 23 genera have been recognized; they are included in the following orders:— Beloidea, Sphæroidea, Prunoidea, and Discoidea, In addition to the radiolaria, a scanty but significant fauna of corals, trilobites, brachiopods, and goniatites is present in some limited thin shaly beds in the Barnstaple district. Nearly all the forms are diminutive; of the 23 species which have been determined, several are only known elsewhere from the Lower Culm of Germany, others are common to the Carboniferous Limestone of the British Isles and Belgium. From the other divisions of the Lower Culm Measures below the radiolarian rocks about 36 species are known, and 8 of these occur in the Radiolarian Beds.

No definite thickness can as yet be assigned to these beds. In common with the other strata of the Lower Culm they have been considerably disturbed, and continuous sections showing their relations to the other beds of the series are rarely shown. That they are of no inconsiderable thickness is proved by sections in quarries where a series of beds from 50 to 190 feet in vertical thickness are exposed.

Radiolarian deposits of a similar character to these fossil-beds are now known only from great oceanic depths, and we are justified in regarding these Culm radiolarian rocks as laid down in the deep water of an open sea at some distance from a shore-line. Such a conclusion must needs very considerably modify the current ideas as to the character and origin of the Culm Measures in the South-west of England, which have been regarded as a continuous series mainly of detrital deposits of shallow-water origin. Thus we find them described in recent text-books as a thick series of well-bedded grits, sandstones, and occasionally thin limestones, which pass down conformably into Upper Devonian strata and contain plants resembling those in the Calciferous Sandstone Series in Scotland, as well as animal-remains which point to a position low down in the Carboniferous system.

The facts now ascertained as to the real nature of the Lower Culm Measures show that the series of rocks included under the general term of Culm Measures, instead of being, as at present supposed, an uniform series of shallow-water detrital deposits, really consist of two groups of rocks, which in their nature and origin present the widest possible differences. In the Lower Culm Measures, the basal
Posidonomya-
beds and the Waddon Barton Beds with
Goniatites spiralis
consist of fine shales and occasional thin limestones which may have been formed in a moderately deep sea, but not beyond the reach of the finer sediments brought from the land; and overlying these are the Radiolarian Beds, mainly made up of these organisms, and formed by the gradual accumulation of their remains in a deeper sea too far removed from a land-surface for any but the finest detrital particles to be carried out and mingled with them.

In the Upper Culm Measures, on the other hand, which are of great thickness and extend over the greater part of the Culm area, the beds consist of conglomerates, grits, sandstones, and shales, with occasional thin bands of the Culm itself and plant-remains, which show with sufficient clearness that this Upper Division has been formed under shallow-water conditions.

We are at present quite in the dark as to the nature of the changes which set in after the deposition of the Radiolarian Beds; the conglomerates overlying these latter in certain areas give evidence of an elevation and partial denudation of the Radiolarian Beds at an early period in the history of the Upper Culm Measures.

The additional fossils (excluding radiolaria) which we have found in the Radiolarian Beds tend to confirm the view that these and the Lower
Posidonomya-
and Waddon Barton Beds are the representatives and equivalents of the Carboniferous Limestone in other portions of the British Isles; not, however, in the at present generally understood sense that they are a shallow-water facies of the presumed deeper-water Carboniferous Limestones, but altogether the reverse, that they are the deep-water representatives of the shallower-formed calcareous deposits to the north of them. Regarded in this light, the occurrence of these Lower Culm Measures in Devon and Cornwall, within about 30 miles of the great development of Carboniferous Limestone in the Mendips to the north-west, no longer presents such an anomaly as seemed to be the case when they were believed to be of shallow-water formation. Under this impression as to their character, Mr. A. J. Jukes-Browne considers that they indicate a muddy sea which received the sediments brought down by large rivers draining continental land, which probably existed south-west of these areas, and the shore-line lay not very far south of Devonshire. The picture, however, that we can now draw of this period is that while the massive deposits of the Carboniferous Limestone—formed of the skeletons of calcareous organisms—were in process of growth in the seas to the north, there existed to the south-west a deeper ocean in which siliceous organisms predominated and formed these siliceous radiolarian rocks. These are of much less thickness than the equivalent Carboniferous Limestones, but this may arise from the fact that the siliceous organic deposits are of far slower accumulation than calcareous beds, and they may well have occupied in their formation an interval of time as great as that taken by the formation of the limestones.

It is worthy of note that the deep-sea character of the Lower Culm of Germany, which corresponds with our Lower Culm Measures, had been maintained by Dr. Holzapfel, even before the discovery of radiolaria in the beds of Kieselschiefer furnished such strong evidence in support of this view.

Much field-work has to be done before a satisfactory knowledge of the ‘British Culm Measures’ can be obtained, and the present paper only professes to be an introduction to the study of the one division of the Radiolarian Beds. The Upper Culm Measures, which in our opinion should include the beds above the Radiolarian rocks (with perhaps the exception of the shales with fishes and goniatites at Instow), have only incidentally been considered, and the question of their relative age can be determined only by their plant-remains. We have, further, not referred to the igneous rocks which in Central Devon and Cornwall are of not infrequent occurrence in the Lower Culm Measures.