On the Geology and Palæontology of Forfarshire
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3°2 ON THE GEOLOGY AND PAL~ONTOLOGY OF FORFARSHIRE. By GEORGE HICKLING, D.Sc., F.G.S. HE Old Red Sandstone of Forfarshire was recently described T in the Geological Magazine (1908, Hickling), and as the account there given is readily accessible we may here endeavour principally to supplement it. TECTONIC STRUCTURE. The Highland Boundary Fault is the master feature govern ing the rocks of the district. In the centre of Forfarshire the throw of this fault cannot be much less than 10,000 ft. The folding of the rocks is strictly parallel with the fault; alongside of it the rocks form a deep syncline, of which the northern limb is steep, the southern more gently inclined. To the south-east the rocks roll over again, forming a symmetrical anticline, of which the Sidlaw and Ochill hills are the denuded remnant. In addition to this principal folding there is a much slighter warping about axes, which run N.W. and S.E. Two anticlines of this series affect the area under consideration-one, of very slight effect. has an axis running across the syncline north of Forfar j the second axis lies wholly under the sea, but is doubtless not far east of Stonehaven. This fold was much more pronounced and is responsible for bringing the base of the Old Red to the surface in Kincardine. Apart from the highland fault, there is no evidence of any faulting of structural importance. The admirable coast sections reveal large numbers of such fractures, but they are mostly small and tend to counteract one another. THE LOWER OLD RED SANDSTONE. In the paper to which allusion has been made the following provisional subdivision of the Old Red, based on the local litho logical facies, was suggested: It. Edzeil shales .. 1,000 Arbroath sandstone 1,200 Auchmithie conglomerate 800 Red Head Series . 1,50 0 Cairnconnan Series 2,000 Carmylie Series . 1,000 Dunnottar Conglomerate 5,000 Stonehaven Beds . 1,50 0 14,000 THE GEOLOGY AND PALlEONTOLOGY OF FORFARSHIRE. 303 It was shown that only the Cairnconnan and Carmylie series are definitely fixed in age by palreontological evidence. More recently, Dr. Campbell has announced the discovery of Down townian fossils in the Stonehaven beds (191 I, Campbell), which must therefore be transferred to the Silurian system, leaving the Dunnottar conglomerate as the base of the Old Red. Restricted as above, the Old Red Sandstone may be described as consisting of a great basal conglomerate succeeded by a series of sandstones, flagstones, and shales. In southern Forfarshire the only notable conglomerates above the Dunnottar group are those which resulted from the denudation of the volcanic cones which were piled up during the deposition of the overlying rocks. There is one noteworthy exception to this, viz., the Auchmithie conglomerate, near Arbroath. In this, the pebbles are mainly quartzite, are beautifully rounded, and occasionally reach 12 inches in diameter. It would seem to imply that "highland" rocks were still exposed within reason able distance, notwithstanding the fact that 10,000 ft. of Old Red sediments had already accumulated, in addition to the Silurians. The whole of the Old Red deposits become much coarser in character as the highland border is approached. This is well illustrated by the contrast in the character of the Cairn connan series on the two sides of the Sidlaw anticline. In its southern outcrop, between Arhroath and Carmy lie, it is essentially a series of grits and sandstones, with only insignificant con glomerates. To the north, as at Turin Hill, coarse conglomerate is largely developed in it. I suspect that these conglomerates were mistaken by Powrie for the Dunnottar series, and so led him to believe that the fish-bed of Turin Hill was on the same horizon as those of Canterland Den and Farnell, and to assume the presence of a large fault north of the anticline, for which no evidence seems to exist (186 I, Powrie). Along the highland border the great fault cuts out a steadily increasing amount of the Old Red from Stonehaven, where its base is exposed, to the North Esk, where all below the Auchmithie series appears to be thrown below the surface. In this region only the bright red Edzell shales are free from conglomerate. There is a peculiarity of colouring which deserves attention In the lower portion of the system the sandstones are usually dull red or grey, the shales red or blue, and the colour in all cases very even. Towards the top of the series, red becomes almost completely dominant and much brighter in hue, while a curious mottling makes its appearance. This is most con spicuous in the Edzell shales, which are frequently mottled with small circular patches of yellow, grey, or green, or more rarely have bands of the same colours. The same feature is well hown in the sandstones near Kelly Castle; Arbroath. GEORGE HICKLING ON UPPER OLD RED SANDSTONE. It was shown in my former paper that the small remaming outliers of Upper Old Red between Perth and Dundee, at Arbroath, at Boddin Point, at Montrose and at St. Cyrus are remnants of a nearly horizontal sheet, which must originally have extended along with the Carboniferous rocks, whose base it forms, over the Sidlaws and probably as far as the highland border. It is needless to describe these deposits again, beyond a state ment that they present a basement conglomerate overlain by soft light red and mottled sandstones and cornstones. The relations of the Upper Old Red show clearly that the Sidlaw anticline and Strathmore syncline had been completely formed, and that denudation had reduced the arch to pretty near its present surface level, before the Upper Old Red and Carboniferous were spread over the old land-surface and buried the ancient Sidlaws. This Carboniferous sheet is the last deposit which there is any reason to suppose has ever spread over Forfarshire, and it appears to have played the principal part in determining the present topography. During the general "sagging" of the Central Valley in Permo-Carboniferous times the sheet would doubtless acquire a gentle south-east slope, and give rise to a series of consequent rivers flowing over Perthshire and Forfarshire in that direction. During subsequent denudation the hard ridge of the Sidlaws gradually emerged again and seems to have proved a powerful check to the rivers crossing it. The Earn, however, crossing the barrier where it was broken by faulting, and being thereby greatly favoured, was able through its tributaries to capture successively the head-waters of the Almond, the Tay, the Ericht and the Isla, building up the complex stream which now flows along Strathmore. PAL£ONTOLOGY OF THE LOWER OLD RED OF FORFARSHIRE. The fauna of these rocks is of the highest interest, alike to the geologist and paleeontologist ; which makes all the more remarkable the fact that it has received scarcely any attention since Powrie ceased collecting about 1870' No complete list of the fossils has to my knowledge been published. Much more unfortunately, no adequate record exists of the exact localities from which fossils were obtained, and it is highly improbable that anyone can now supply it. An effort has therefore been made, in the table now given, to collect such information as can be found scattered through various papers. A complete list of the sources of information will be found in the bibliography at the end of this paper. It may here be noted that the most important accounts of the fauna are those of Mitchell (r861), THE GEOLOGY AND PAL-EONTOL OGY OF FORFARSHlRE. 305 Powrie (1864), and Powrie (1870). We may note at once, also, that almost the whole of the fossil localiti es are situate within narrow stratigraphical limits, in the Carmylie and Cairnconnan series. Notes on Beds and Localities.-Powrie latterly concluded that all the fish-remains came from a single bed, of which he speaks as follows (Powrie (1864), p. 414): "This bed is composed of ~ semi-calcareous shale, some bands highly calcareous and serm crystalline, and having many embedded nodul es. A peculiar band of a tenacious light-coloured clay, some 5 or 6 inches in thick ness, is very persistent in this shale, and from it the Fish-bed may be recogni sed wherever it crop s out. The colour of these calcar eous shales varies much. The thi ckness varies from 3 to 8 ft. Some of the layers are exceedin gly fissile, splitting into larnime not much thicker than writing-paper. I have now found this Fish-bed in Canterland Den in Kincard ine shire, near Farnell, at Turin Hill, in some quarries south of Forfar, in many places, in the Sidlaws, in Balruddery Den in Forfarshire and in R ossie Den in Perthshire." It seems scarcely possible that this can be true of Turin Hill; it may be correct regarding the other localities named, the bed being just at the base of the Cairnconnan series. Most of the localities named in the table are rath er vague. Balruddery Den is 5 miles N.W. of Dundee. T ealing is 5 miles north of Dundee, on the western margin of a basalt sill. Th e Myreton quarries are 1 mile east of the same sill. Pitairlie probably refers to exposures on the Pitairlie burn , 3 miles north of Monifieth, It is on nearly the sam e horizon as the Kelly Den exposure, near Arbroath, in the Red Head series, and much higher than any of the others. In Kelly Den the exposure is immediately above the grounds of Kelly Castle. The Forfar quarries are about a mile south of that town.