On the Geology of Bilston Burn, Near Loanhead

ON THE GEOLOGY OF BILSTON BURN, NEAR LOANHEAD.

By A. MACCONOCHIE, of H. M. Geological Survey.

HE post-glacial gorge of Bilston Burn exposes on e of the T finest sections of the Carboniferous Limestone strata in Scotland. It occurs on th e west ern limb of the " very perfect syncline that forms the Midlothian coalfield." Ab out a mile long, it trav erses a vertical thi ckness of more than 3,000 ft. of strata. The beds dip at a high angle-s-ao" to 48 °-to the S.E. Commencing 350 ft. below the Gilmerton Lim estone, which has been adopted by the Geological Survey as the base of the Carboniferous Limestone Series of the Midlothian ba sin, it cuts in succession through the topmost beds of the Calciferous Sand stone Series, the Lower Lim estones (700 ft.), Edge Coals (1,000 ft.), Upper Lim est ones (800 ft.), and 280 ft. of the lower part of the Millstone Grit (Ro slin Sandstones). As stated above, th e Gilm erton Limestone is chosen as the base of the Lower Limeston e Group, but does not here. as may be see n from the vertical sec tion (P late 6), mark the adve nt of the conditions which charac terise this group. From its persistenc e, however, and also since it has been lon g known in the district, it is the best and most convenient datum-line. Underlying it are at least th ree marine limestones, of which the lowest, a cream coloured encrinital band, crops out in the stream near to Bilston Inn (see map, Plate 5). This is succeeded by some th in oil-shales, an argillaceous limestone (the " ceph alopod lim eston e "), a series of shales and sandstones containing a 2-ft. coal, a marl y fireclay, and some dark blue Lingula-shales, int er leaved in which is a 4-in. seam of soft crumbling shale crowded with Lingula (mostly fragments). scales, teeth , and other fish remains. About a foot ab ove th is " bone-bed " is a hard nodular calcareous shale, which yields abundant specime ns of Schizo phora (Or/his) resupinata. These two bands have been help ful as " index " beds for correlatio n purposes, and will be referred to later on in this connection. This Or/his-shale is succeeded by a limestone 10 ft. thick. Then follow more sandston es, shales, fireclays, and a thin coal, which should bring on th e Gilmerton Limestone, normally 60 ft. thi ck, but it is here represented by an ochre bed 2It ft., th e lime having been dissolved out. Near Pathhead, a few hundred yards along the strike, th e un altered lime stone is known to occur. It was also cut through in minin g operations . H alf-way between the Gilmerton and the North Greens A. MACCONOCHIE ON

Limestones (see Plate 6) a thin limestone is seen on the right bank and in the bed of the burn. This is the fifth marine limestone in the upward succession; 150 ft. of arenaceous and shaly beds succeed, leading up to the North Greens coal (4 ft. 9 in.). This coal has been worked. A small part of the section here has consequently been obscured, hiding a thin black-band ironstone, which rests on the North Greens coal, and which has yielded the remains of many species of fishes and labyrinthodonts. The late Dr. Traquair states that the fishes "indicate estuarine conditions, and resemble those which are got from the Wardie shales and oil-shales, but differ entirely from the fauna of the marine limestones." * The North Greens Limestone is 90 ft. thick, but varies in purity throughout its thickness. It is very fossiliferous. Resting on it is a thick deposit of red, grey and green, mottled, rather coarse-grained sandstone-the North Greens Sandstone. It re sembles very closely and is probably on the same horizon as the red and grey false-bedded sandstone-the Seafield Tower Sand stone-which lies directly on the Chapel Limestone exposed on the north shore of the Firth of Forth near Kirkcaldy. One hundred feet above the North Greens Sandstone a coal occurs, and, about an equal distance higher in the sequence, the two Vexhim limestones, with the Vexhim coal between, crop out. Higher still the Bilston Burn (Hosies) Limestone, 50 ft. thick, forms a well-marked feature in the glen and, like the associated shales, is very fossiliferous. Fine specimens of polyzoa can be got here. Many of the weathered surfaces of the limestone are covered with Spirophyton cauda-gaili, A few feet higher a limestone (I ft. 8 in.) resting on a yellow sandstone occurs. It is the tenth, and last, of this lower series. The Edge Coal Group succeeds. Here this group is about 1,000 ft. thick. It contains many good and valuable seams of coal: For the relative position and local names of the best known of these, see vertical section (Plate 6). No fewer than 26 seams above one foot thick, aggregating 100 ft. of coal, occur in this section, in addition to seams of black-band and clay-band ironstone. Marine deposits are absent. The Upper Limestone Group (800 ft.) has four limestones: the Index, the lowest; then the Extra; the Arden, or Calmy ; and at the top the Castlecary. The two lower are characterised by the abundance of Productus latissimus. Several coal seams are intercalated in this series. In various localities they are of economic importance. Two hundred and eighty feet of the Millstone Grit is cut through before the stream loses itself in the River Esk. The

* H Distribution of Fossil Fish Remains in the CarboniferousRocks" (Trans. R QY. Soc., Edinburgh, '903). Proc:Geol:Assoc: 10l. xxv. Plate S.

OE.OLOGICAL MAP OF THE ROSLIN AND POLTON DISTRICT. THE GEOLOGY OF BILSTON BURN. 43 strata include red and yellow sandstones, shales and fireclays. One hundred and thirty feet above the base a remarkable fossi liferous marine band is well exposed on the right bank of the burn. This band, rich in fossils, has been found in thesame position in several localities in the midland valley of Scotland When he described and figured the larnellibranchs," Dr. Wheelton Hind pointed out "that 50 per cent. of the species were new to Europe, but resembled very closely the lamellibranch fauna of the coal-measures of Nebraska and Illinois, North America," Prothyris elegans, a shell named by Meek from specimens obtained in Nebraska.j being the most striking member of this fauna. Appended are the names of some of the fossils got from the band in the Bilston Burn section :-- Chonetes. Edmondia senilis (Phil!.). Derbya. Grammatodon tenuistriata, Meek Lingula mytiloides J. Sow. and Worthen. Orbiculoidea nitida (Phill.). .Myalina verneuillii (McCoy). Productus. Numlana attenuata (Flem.). Pugnax pugnus (Martin). Prerinopecten papyraceus (Sow.). Seminuia ambigua 0. Sow). Sanguinolites occidentalis, Meek Spiriferina cristata (Schloth). and Hayden. Aviculopecten obliquus Hind. Euphelllus d'orbignyi, Port!. " regularis" Goniatite. " 71eglectus (Geinitz). Orthoceras. Ctenodonta lo:virostris (Portl.), The fact that some of the forms described from this marine band of the Millstone Grit have since been recorded from lower horizons in the Upper Limestone Group does not diminish the in terest of the story revealed to us by Dr. Hind, nor assist in the solution of the problems it raises. Conditions ofdeposition ofthe Carboniferous Limestone Series. In the lowest 1,050 ft. of sediments exposed in the stream there are ten limestones aggregating about 230 ft. Those are marine. A coal of some kind, with a fireclay, is generaily found underneath the limestones. Shales with Lingula and Discina are not uncommon; but the main mass of the sediments-the sand stones, faky sandstones, sandy shales, shales, fireclays and coals-is probably either estuarine or freshwater. Possibly some of the coals may be marine, but so far proof is lacking. In the case of the thousand feet of Edge Coal strata, the con ditions of deposition appear to have been similar to those that obtained during the Coal Measure period, though scarcely a single species of plant or fish passes from one to the other.

* "Summary of Progress for 1905," Ment. Geol. SU'YVlY, 1906, p. 147, and" On the Lamellibranch and Gasteropod Fauna found in the Millstone Grit of Scotland," T1 ans, Roy. Soc., Edin., 1908, vol. xlvi, p. 33I. t Proc, Acad, Sci., Philadelphia, ,869. 44 A. MACCONOCHIE ON THE GEOLOGY OF ElLSTON BURN.

During the Upper Limestone period a recurrence of the Lower Limestone conditions supervened, though the inroads of the sea were probably less frequent and of shorter duration. The facies of the sediments which build up the 3,000-4,000 feet of strata of the Carboniferous Limestone Series of this district seem to imply proximity to land, but not high land; intermittent, but long-continued subsidence; irregular incursions of the sea of varying periods of duration, long intervals of conditions resem bling the Dismal Swamp, for instance, as during the deposition of the Edge Coals and associated sediments. Conelation.-ThesynclineoftheMidlothianbasin passes across into Fife, where its western limb crops out between Kinghorn and Kirkcaldy, exposing a splendid section of the highest beds of the Calciferous Sandstone Series and the Lower Limestones. Here, however, interbedded lavas and tuffs are much in evidence as has already been described in the account of "The Country between Burntisland and Kirkcaldy," (ante p. 38). In Mid and East Lothian volcanic rocks are absent from this series. If, as seems highly probable, the" bone bed" and orthrs-shale which underlie the 1St Abden limestone on the Fife shore, east of Kinghorn, are on the same horizon as the «bone-bed" and Ortlus-shale of Bilston Burn, from which ident ical suites of fossils have been recorded, then the rst Abden lime stone of Fifeand the"bone-bed"limestone of Bilston are thesame. "" We can go further and correlate the and Abden with the Gilmerton Limestone, and the Chapel Limestone of Fifewith the North Greens of Midlothian. The Seafield Tower cross-bedded, red and grey sandstone corresponds in position and lithology to the North Greens Sandstone. The thrust limestone of the Sea field shore, its weathered surfaces covered with Spirophyton cauda gallr; is probably the same as the Bilston Burn (Hosies) Limestone. NOTE. -We are indebted to Mr. Clough for permission to reproduce the map and vertical section, Plates 5, 6.

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