Peach, B.N. and Horne, J. 1899. The Silurian rocks of Britain. Volume 1. Scotland. Glasgow. HMSO for Geological Survey. Grid references have been added for GeoGuide. They should be regarded as approximate "in the region of..."
Chapter 24 V. Upper Silurian rocks north of The Tableland
Lanarkshire and Ayrshire
Within the territory occupied by the Old Red Sandstone in the midland valley of Scotland, several inliers of Upper Silurian Rocks make their appearance. They are found in. Lanarkshire in the adjoining county of Ayr, and in the counties of Midlothian and Peebles. The Ayrshire and Lanarkshire tract, since the discovery of well-preserved Eurypterids in it by the late Dr. Slimon of Lesmahagow, has become widely known in geological literature. The more important of these inliers are situated several miles to the north of the Silurian Tableland, where they have been brought to the surface by the plication and subsequent denudation of younger Palaeeozoic rocks.
One of the most interesting features connected with these Upper Silurian areas is the conformable sequence from the Upper Ludlow Rocks, into the Lower Old Red Sandstone. This conformability was clearly recognised by Murchison, Ramsay, A. Geikie and at a later date by Mr. B. N. Peach when he mapped the region between Lesmahagow and Douglas (see Geol. Survey, one-inch map of Scotland, Sheet 23, published in 1872). In the Explanation descriptive of the geology of the area included in Sheet 23, it is shown that, while the conformable portion of the Lower Old Red Sandstone contains red and yellow sandstones and conglomerates, it likewise includes certain green and red mudstones with bands of grey shale and greywacke, which, in the Lesmahagow district, yielded "Beyrichia and some obscure Pterygotus-looking crustacean remains.<ref>Explanation of Sheet 23, page 13.</ref> The occurrence in the passage-beds of certain organisms identical with those in the underlying Ludlow Rocks suggested the recurrence of marine or brackish water conditions during the deposition of the lower portions of the Old Red Sandstone.
In the course of the recent revision of the Upper Silurian inliers in Lanarkshire by the Geological Survey (Sheet 23, of the Map), special attention was paid to the fossiliferous bands in the red passage-beds overlying the Ludlow Rocks. Towards the close of 1896 they yielded to Mr. Macconochie fragments of crustaceans and certain obscure remains resembling ichthyolites. The search was resumed in 1897, and after a prolonged examination of the various exposures by Messrs. Macconochie and Tait, it resulted in the discovery of a fauna which in some respects is remarkable; for, in addition to representatives of all the genera of Eurypteridae found in the underlying Ludlow Rocks, there is likewise an assemblage of fishes, comprising five genera, four of which and seven species are new. Dr. Traquair, in whose hands the collection of fishes has been placed for determination, has kindly furnished the following notes embodying the results of his researches:;
"The only fishes which have occurred in the 'Ludlow' beds of the Lesmahagow district are Thelodus scoticus (Traq.) and Thelodus planus (Traq.), besides a fragment of another form, which is too imperfect for description. The list from the 'Passage' or Downtonian beds is larger, and includes the following:
Thelodus scoticus (Traq.)
Lanarkia horrida (Traq.)
Lanarkia spinosa (Traq.)
Lanarkia spinulosa (Traq.)
Ateleaspis tessellata (Traq.)
Birkenia elegans (Traq.)
Lasanius problematicus (Traq.)
"All these are new to science.
"The geological interest of this discovery lies in the fact that a new fish-fauna has turned up in the Upper Silurian rocks of Scotland, two genera and several species of which belong to a family, the Coelolepidae of Pander, which, in strata of approximate age in England and other parts of the world, have hitherto been known only by scattered scales. A few isolated scales of Thelodus (Th. tulensis, Rohon.) have also occurred in the Upper Devonian of Russia, and the entire fish from the Lower Old Red Sandstone of Forfarshire, named by Powrie Cephalopteras Pagei, is now proved to belong to the same genus. Four other genera of striking novelty, Lanarkia, Ateleaspis, Birkenia, and Lasanius, also form part of the same fish-fauna, but it is remarkable that as yet no trace has been found in these Lesmahagow rocks of the Onchus-spines, of the Pteraspidians, or of the Cephalaspidians which have been long known from the Upper Silurian strata of the west of England and elsewhere.
"Zoologically the interest of these fishes is extreme. The Coelolepidae have, from their shagreen-like scales, been considered by many palaeontologists to be sharks, and to them have also been referred the undoubted selachian spines known as Onchys, and even certain selachian-like teeth such as Monopleurodus of Pander. But the recent 'find' of the Geological Survey puts a different face on the matter.
"We have here two genera of Coelolepidae — Thelodus, whose minute shagreen-scales are of the type seen in the species which occur in the Ludlow Bone-bed and in the Upper Silurian rocks of Oesel, and Lanarkia, a new genus in which the dermal covering consists of small, hollow, pointed spines, without a basal plate. In both genera the form of the creature is the same — the head plus the anterior part of the body being broad and flattened, with right and left posterior angles in the form of fin-like expansions, the contour of which is continuous with that of the head in front but sharply marked off from that of the tail behind, which latter part terminates behind in a deeply cleft heterocercal caudal fin. There is no trace of jaws, of teeth, or of fin spines in any of the numerous specimens which have been collected, nor has the position of the eye been discovered.
"Now, although through their dermal armature the Coelolepidae appear related to the sharks, their general form, along with the absence of teeth and the apparent absence of jaws, appears rather to claim a place for them in the Ostracodermi and in the order Heterostraci, of which the only family hitherto recognised is that of the Pteraspidae.
"Ateleaspis tessellata is the type of a new family, and presents us with a form in which the body has the same general shape as in the Ccelolepidaa, blit the dermal covering consists in front of small minutely tuberculated polygonal plates, while the tail is provided with rhombic scales, ornamented with delicate tubercles and ridges. It remind us strangely both of Cephalaspis and of Psammosteus, but more especially of the latter, as the presence of orbits on the upper aspect of the head is doubtful. It is unfortunate that Ateleaspis is so exceedingly rare that only one example showing the head and body has as yet been found, nevertheless what we do see of its structure leads us to the conclusion that the Psammosteidae of the Devonian epoch are, as Reis has already indicated, also Heterostraci, and allied to the Pteraspidae. And wherever Psammosteus goes there also must Drepanaspis of the German Lower Devonian find its place. And it may also be said that although the head and body of Drepanaspis are covered with plates instead of with minute scales or spines, the general coincidence of the shape of the fish with that of Thelodus or Lanarkia is exceedingly striking and suggestive.
"The recent 'find' therefore very greatly enlarges our conception of the Heterostraci, and indicates that, though we cannot consider them as actual Selachii, they probably had a common origin with the primitive Elasmobranchs. And it may here be note that Rohon, though he calls the Coelolepidae 'extinct selachians,' and their scales genuine 'placoid-scales,' remarks that, as these scales in many respects show a simple condition of histological structure, they must be regarded as placoid scales in a lower stage of development.
"Birkenia elegans is a very peculiar as well as beautiful form, of elegant fusiform shape, with the head and body covered by narrow tuberculated scutes. There is a completely heterocercal bilobate caudal and a rudimentary dorsal fin, but no trace of paired fins, shoulder girdle, cranial bones, jaws, or even of distinct orbits. It is also the type of a new family, but it is doubtful if it can be included in the Heterostraci; more probably it represents a separate division of Ostracodermi.
"Lasanius problematicus is the name which I have applied to a fish whose remains show some very remarkable features. Apparently of an elongated form, the only hard parts which it possessed were a median row of small scutes with backwardly directed thorns, and on the opposite side in front a peculiar gridiron-like arrangement of about eight parallel bony rods on each side, whose extremities are, at the margin of the body, bent at an angle inwards to meet their fellows of the opposite side. If the row of scutes is dorsal, then the rods may have had to do with the support of the branchial apparatus. Traces of the body are often seen in the form of a carbonaceous film, and in one or two instances this film seems to terminate behind in an acutely bilobate caudal fin. The affinities of this form are, as the specific name implies, very problematical, though I feel no doubt that it is an Ostracoderm, and almost as little that, in spite of its deficiency in dermal covering, it is allied to Birkenia.
"It may, in conclusion, be truly said that this recent discovery of Silurian fishes by the Geological Survey of Scotland has opened out to us a new vista in the field of palaeozoic ichthyology".
The interest attaching to this group of rocks in Lanarkshire has been augmented by the recent discovery of ichthyolites in the admittedly Tipper Ludlow Rocks of the neighbourhood of Lesmahagow. In 1896 a species of Thelodus was collected from one of the upper zones by Mr. James Young, Lesmahagow, and in 1897–8 two new species of the same genus were obtained by Messrs. Makconochie and Tait from the same horizon. It was from this band also, that a fine example of a true scorpion (Palaeophonus) was collected, which was exhibited to the Geological Society of Edinburgh by Dr. Hunter, Carluke, and described by Mr. B. N. Peach.
The recent palaeontological discoveries in these passage-beds of Lesmahagow, prove the presence of so many normal Upper Silurian organisms in shales intercalated in the overlying red and yellow sandstones as to suggest the desirability of grouping these conformable red strata with the Upper Silurian system. Certain data connected with the lithological characters and stratigraphical relations of some of the zones overlying the true Ludlow Rocks seem to favour this view. For example, throughout the Douglas and Lesmahagow districts, two prominent beds of conglomerate have been of great service in working out the geological structure of that region. The lower one, which almost immediately overlies the fish-band above referred to, is composed mainly of pebbles of quartzite derived from the Highlands, while the higher one, which is separated from the lower by red and yellow sandstones and shales, is readily distinguished by the abundance of greywacke pebbles obtained from the Silurian Tableland of the Southern Uplands. Not, indeed, till we reach the horizon of this greywacke-conglomerate do we find conclusive evidence of the predominance of materials in the passage-beds that have been derived from the Silurian Tableland.
The greywacke-conglomerate preserves its distinctive features, as a well-marked horizon along the northern border of the Silurian Tableland from the Pentland Hills to Ayrshire. It forms the base of the sediments which herald the great volcanic . series of the Lower Old Red Sandstone, and is to all appearance intimately associated with the terrestrial movements which preceded or accompanied those manifestations of volcanic activity. For both in the Pentland Hills and in Ayrshire this greywacke-conglomerate denotes a marked unconformability; in the former region, as we shall point out in the sequel, it rests on the upturned edges alike of the Wenlock, Ludlow, and passage-beds, while in the latter area it lies unconformably on Lower Silurian strata. It is remarkable, however, that in Lanarkshire and the adjoining part of the county of Ayr, where this horizon has been traced over many miles of country, no unconformability has been detected between it and the underlying sandstones. Some of the sections seem to indicate a gradual passage from the one series into the other, but even if this feature be deceptive it is at least clear that if a discordance does exist in that part of the country, it must be comparatively gentle.
In view of the palaeontological and to some extent also of the physical evidence regarding the passage-beds that overlie the Ludlow Rocks there seems ground for maintaining that they have greater affinities with the Silurian system than with the Old Red Sandstone. They may be looked upon as stratigraphical equivalents of the Tilestones, Downton Sandstones, and Ledbury Shales, which, in Herefordshire, overlie the Upper Ludlow Rocks and have been classified as forming the highest sub-division of the Upper Silurian rocks.<ref>See for example Sir A. Geikie's "Text-Book of Geology", 3rd ed., 1893, p. 753.</ref> In the following pages a similar \grouping has been adopted, and, for the sake of convenience of description, the passage-beds, ranging from the fine local conglomerate underlying the red sandstones to the base of the greywacke-conglomerate, will be referred to as the Dovvntonian series. This modification in the classification hitherto adopted by the Geological Survey is represented in the annexed table, which gives the various sub-divisions of the Upper Silurian rocks in Lanarkshire in descending order:
LOWER OLD RED SANDSTONE
Volcanic Series
3. Andesites and tuffs (volcanic zone).
2. Chocolate-coloured sandstones with Cephalaspis Lyelli
1. Conglomerate with greywacke pebbles.
Unconformability in the Pentland Hills and in Ayrshire; apparent conformability in Lanarkshire.
Downtonian
11. Chocolate-coloured sandstones-1200 feet in Lesmahagow inlier.
10. Conglomerate with large pebbles of quartzite — 100 feet in Lesmahagow inlier.
9. Green and red mudstones with bands of grey shale and greywacke' containing a band with fishes and crustaceans-100 feet in Lesmahagow inlier.
8. Red and yellow false-bedded sandstones and red mudstones-1300 feet in Lesmahagow inlier.
7. Conglomerate of local occurrence, with small pebbles of Arenig volcanic rocks, radiolarian chert, quartz, &c., resting conformably on Ludlow shales –88 to 150 feet in the Hagshaw Hills anticline.
Ludlow
6. Green flaggy and sandy greywackes, with partings of grey and red mudstone — about 130 feet,
5. Blue, grey, and green shales with sandy mudstones and greywackes, occasionally weathering with a rusty colour-200 feet (Trochus beds).
4. Hard blue and grey flaggy shales and mudstones, with occasional bands of calcareous nodules-350 feet (Pterygotus beds).
3. Hard grey flagstones and bands of hard greywacke, with bands and zones of dark fissile calcareous flaggy shales, weathering rusty brown-500 feet (Ceratiocaris beds and Ludlow fish band).
2. Grey, blue, and olive shales, with occasional greywacke bands-300 feet.
Wenlock (?)
1. Hard bands of greywacke with shale partings-1300 feet.
<ref>The thicknesses of the sub-divisions of the Wenlock and Ludlow Rocks given in the above Table apply only to the Lesmahagow inlier.</ref>
The various areas of Upper Silurian strata in Lanarkshire and the adjacent part of the county of Ayr (Sheets 15 and 23 of the Survey Map) may be grouped as follows according to their size and relative importance:
- The Lesmahagow inlier.
- The anticline of the Hagshaw Hills.
- The Carmichael Burn inlier.
- The small exposure south of Tinto.
Of the foregoing inliers it is highly, probable that the axial fold near Carmichael Manse (No. 3) is merely the north-easterly prolongation of that in the Hagshaw Hills (No. 2) reappearing from underneath the Carboniferous rocks of the basin of the Douglas Water. The most important, however, are the first two of the series, the anticline of the Hagshaw Hills (No. 2) being five miles and that of the Logan Water (No. 1) from seven to eight Miles distant from the northern limit of the Silurian Tableland. A careful study of the stratigraphical relations of these two inliers shows that there is a great similarity in the structure of both areas, for in each case the Wenlock and Ludlow Rocks form anticlines, now exposed at the surface by the removal of the Downtonian and Lower Old Red Sandstone strata which covered them to a great depth; each is bounded on the south-east side by a large fault, while along their north-west margins the sequence from the Ludlow to the Downtonian Rocks can be traced. A further interesting feature which illustrates the extreme plication of the Silurian and Lower Old Red Sandstone rocks in pre-Carboniferous time is the prominent inversion of the strata in the case of the Hagshaw anticline. Both limbs of the fold dip towards the north-west, and the south-east limb is truncated by a reversed fault, by means of which older are made to overlie younger rocks. In other words, we have here a repetition of over-folds culminating in reversed faults which is such a conspicuous feature of the mountain-building Of the North-West Highlands. That this plication took place in pre-Carboniferous time is evident from the fact that the Upper Old Red Sandstone and Lower Carboniferous rocks rest unconformably on the folded Downtonian and Lower Old Red Sandstone strata.
i The Lesmahagow Inlier
This area lies on the confines of the counties of Lanark and Ayr between Muirkirk and Lesmahagow. The boundary between the two counties forms there a sinuous line running along the watershed that separates the tributaries of the Clyde (Nethan, Kype, and Glengavel Waters) from the Greenock Water and its affluents, which flow into the river Ayr. Though the region lies within the midland valley of Scotland, it nevertheless forms a pastoral district, where some of the hills exceed 1500 feet in height. Excellent sections of the strata are exposed in the various streams, notably in the Logan and Greenock Waters and their tributaries, and as the beds over much of the area dip at gentle angles favourable opportunities are afforded of studying the various palaeontological zones
Wenlock and Ludlow Series
The Wenlock and Ludlow Rocks of the Lesmahagow inlier extend in a north-east direction from the Greenock Water
(1) The lowest beds (No. 1 of the Table given above and 5a of
(2) The members of the next group (Nos. 2 and 5b) consist of grey, blue, and olive shales, with occasional greywacke bands, which form a very narrow strip immediately to the north of the presumably Wenlock strata. They range from the neighbourhood of Hall, in the Greenock Water, to the north-west slope of Nutberry Hill, where, crossing the Long Burn they sweep round the east limit of the anticline towards the river Nethan, west of Eaglinside, their general dip in the latter region being easterly. At one locality, near the head of the Long Burn
Ceratiocaris papilio (Salt.)
Ceratiocaris stygius (Salt.)
Anodontopsis bulla (M'Coy.)
Anodontopsis lucina (Salt.)
Ctenodonta obesa (Salt.)
Mytilus mytilimeris ((Jour.)
Orthonota amygdalina (Sow.)
Orthonota var.
Orthonota bulla (Salt.)
Orthonota impressa (Sow.)
Orthonota solenoides (Sow.)
Pterinea retroflexa (Wahl.)
Pterinea retroflexa var. naviformis. (Conr.)
Pterinea pleuroptera ((Jour.)
Holopella obsoleta (Sow.)
Murchisonia sp.
Platyschisma helicites (Sow.)
(3) The Ceratiocaris-beds (No. 3 and 5c) are of special interest, as they contain the layer which has recently yielded the Ludlow fishes. Lithologically the sediments of this group are coarser than those of the underlying and overlying groups. They consist of hard grey flagstones and bands of hard greywacke, in which occur zones of dark fissile calcareous flaggy shales. The latter contain in remarkable abundance at certain localities remains of Ceratiocaris, which is indeed the distinctive palaeontological feature of the sub-division.
The Geological Map (Sheet 23) shows that the members of this group extend along the northern limb of the anticline from Linburn
At various points in the Logan Water the fossiliferous bands of this subdivision (No. 3 and 5c) are admirably displayed; indeed, owing to the gentle inclination of the beds, large surfaces of the more important seams are laid bare for investigation. Though fragments of the characteristic crustaceans are found on different horizons throughout the group, one band is of special importance from the abundance of species of Ceratiocarisin it, and from the occurrence of Ludlow fishes, and a well-preserved scorpion. The group is exposed in a small gorge in the midst of a plantation about three-quarters of a mile to the north-east of Logan House. Here the stream-course makes a marked loop at a place known locally as Shank's Castle
Worm tracks.
Ceratiocaris laxa (Woodw. & Jones.)
Ceratiocaris longa (Woodw. &Jones.,
Ceratiocaris papilio (Salt.)
Ceratiocaris stygius (Salt.)
Ceratiocaris telson, like Murchisoni (M'Coy.)
Slimonia acuminata (Salt.)
Above the wood a large alluvial flat extends up stream for half a mile to Logan House, where operations are now in progress for the formation of a large reservoir. Not far to the west of Shank's Castle, where excavations are now being made for an embankment
Worm tracks
Myriapods ? (impressions of).
Ceratiocaris sp.
Dictyocaris Ramsayi (Salt.)
Pterygotus bilobus (Salt.)
Slimonia acuminata (Salt.)
Thelodus scoticus (Traq.)
Though the remains of Eurypterids are numerous, only two fragments of the Ludlow fish, Thelodus scoticus, were here obtained, which, however, are sufficiently distinct for determination.
As the Ludlow fish-band is followed westwards its outcrop is affected by two normal faults. The first, trending in a northwest and south-east direction not far to the west of Shank's Castle, shifts the outcrop to the north side of the valley; the second, trending E.N.E. and W.S.W., produces a still further displacement towards the north-west. Near Logan House, certain members of the Ceratiocaris-group — though on a slightly lower horizon than the fish-band — appear at the foot of the Long Burn
Spirorbis sp.
Beyrichia Kloedeni (M'Coy.)
Beyrichia Kloedeni var. torosa (Jones.)
Ceratocaris.
Dictyocaris Ramsayi (Salt.)
Pterygotus bilobus (Salt.)
Lingula minima (Sow.)
Modiolopsis Nilssoni (His.)
Orthonota sp.
Platyschisma (Trochus) helicites (Sow.)
About Logan House
Archidesmus loganensis (Peach.)
Ceratiocaris longa (Jones & Woodw.)
Ceratiocaris Murchisoni? (M'Coy.)
Ceratiocaris papilio (Salt.)
Ceratiocaris stygius (Salt.)
Slimonia acuminata (Salt.)
Physocaris sp.
Pterinea retroflexa (Wahl.)
Platyschisma (Trochus) helicites (Sow.)
Thelodus scoticus (Traq.)
Thelodus planus (Traq.)
Fish fragment undt.
In the valley of the Greenock Water, another exposure of these beds is to be seen at Linburn
(4) The Pterygotus-beds, the strata so successfully explored by Dr. Slimon of Lesmahagow, though traceable from end to end of the anticline as a narrow belt lying to the north of group No. 3, are best exposed in the Logan Water and its tributaries. At Dunside
Ceratiocaris papilio (Salt.)
Neolimulus falcatus (Woodw.)
Eurypterus lanceolatus (Salt.)
Eurypterus obesus (Woodw.)
Eurypterus scorpioides (Woodw.)
Pterygotus bilobus (Salt.)
Pterygotus bilobus var. accidens (Woodw.)
Pterygotus bilobus var. inornatus (Woodw.)
Pterygotus raniceps (Woodw.)
Slimonta acuminata (Salt.)
Stylonurus Logani (Woodw.)
Lingula minima (Sow.)
In a small tributary of the Logan Water from the north, at a spot about 250 yards west from Dunside
In like manner, in the Blackberry Burn, at a point about 100 yards up stream from the alluvial cone where it joins the Logan Water
On the south-west side of the county boundary, and close to the watershed, the representatives of this sub-division reappear near the head of the Lease Burn, where they consist of blue-grey flaggy shades, dipping to the W.N.W. at angles varying from 20° to 30°, and are traversed by thin basalt dykes. Some of the seams freely yield fragments of Eurypterids, the following fossils having here been collected: Beyrichia Kloedeni, Ceratiocaris papilio, C. stygius, Pterygatus bilobus, and Slimonia acuminata.
(5 and 6) Along the northern limit of the Upper Ludlow Rocks numerous excellent sections show the two highest subdivisions (5e and 51 of
Some of the same characteristic fossils are likewise found in tributaries of the Logan Water from the north, viz., in the Kip Burn
The most highly fossiliferous locality in the basin of the Greenock Water is to be found in a small streamlet about half a mile E.N.E. of Waterhead
Spirorbis Lewisi (Sow.)
Beyrichia Kloedeni (M'Coy.)
Dictyocaris sp.
Slimonia acuminata (Salt.)
Goniophora cymbaeformis (Sow.)
Modiolopsis complanata (Sow.)
Modiolopsis Nilssoni (His.)
Orthonota impressa (Sow.)
Orthonota rotundata (Sow.)
Orthonota solenoides (Sow.)
Platyschisma helicites (Sow.)
In the prolongations of this group towards the south-west of the Greenock Water, some of the characteristic fossils have been found in the Lann Burn
The members of the highest sub-division of the Ludlow rocks, consist of green flaggy and sandy greywackes with partings of grey and red mudstones, that have hitherto proved unfossiliferous. Indeed, they merge gradually into the red and fellow sandstones which form the base of the overlying Downtonian series.
Downionian Series
On the north-west side of the arch of Wenlock and Ludlow rocks, in the Lesmahagow inlier, there is a broad area occupied by the overlying strata forming the Downtonian group. Indeed, over much of that region the beds lie at low angles; and sometimes form gentle undulations. In this respect the geological structure of the Downtonian area on the north-west side of the Lesmahagow inlier differs widely from that met with in the case of the FIagshaw anticline.
Though there are various sections showing the gradual sequence from the Upper Ludlow rocks into the conformable series of red and yellow sandstones, yet the local conglomerate (Sub-division 7) has nowhere been detected on the north-west limb of the Lesmahagow inlier. In the sequel we shall point out that it is typically developed in the Hagshaw anticline and near Carmichael Manse.
(8) Red and Yellow Sandstones (6b in
Still further to the south-west, in the Lease Burn, one of the tributaries of the Dippal Burn, in the adjoining county of Ayr, the grey flaggy sandy greywackes (No. 6) pass upwards into red flaggy sandstones and shales which are inclined to the W.N.W. at angles varying from 10° to 20°. These are succeeded by false-bedded yellow sandstones, which are extremely friable, and likewise dip to the W.N.W. at an angle of about 10°, though in some instances gentle undulations are observable. In the Dippal Burn above and below the mouth of the Lease Burn
(9) The Fish-beds. — The group of strata which includes the Downtonian fish-band (6c in
The Dippal Burn, from the shepherd's house at Dippal
Again, near the head of Dippal Burn, about half a mile to the south-east of the crest of Goodbush Hill
Fucoid-like markings.
Pachytheca sp.
Parka, new sp.
Ceratiocaris sp.
Eurypterus dolichoschelus (Laurie.)
Lanarkia spinulosa (Traq.)
Lanarkia horrida (Traq.)
Lanarkia spinosa (Traq.)
Thelodus scoticus (Traq.)
Birkenia elegans (Traq.)
The various exposures of the fish-band just described occur along the south-east margin of the belt formed by the members of group 9. In the Slot Burn, one of the tributaries of the Greenock Water near Seggholm, an important outcrop of this fossiliferous zone lies considerably to the north. It may, nevertheless, be on the same horizon, for the beds are inclined at gentle angles, and in some cases are almost flat, while the outcrop of the fish-band is shifted by a normal fault. This view is strengthened by the fact that, on the crest at the watershed east of Seggholm, an outlier of the overlying quartzite conglomerate (group 10), appears about half a mile to the south of the normal outcrop of the band. The Slot Burn outcrop appears about 200 yards up stream from the house at Seggholm (now in ruins)
Plant stems.
Pachytheca sp.
Dictyocaris sp.
Sponge?
Ceratiocaris taxa (Jones and Woodw.)
Eurypterus dolichoschelus (Laurie.)
Stylonurus ornatus (Laurie.)
Myriapod.
Lanarkia spinulosa (Traq.)
Lanarkia horrida (Traq.)
Lanarkia spinosa (Traq.)
Thelodus scoticus (Traq.)
Birkenia elegans (Traq.)
Lasanius problematicus (Traq.)
Ateleaspis tessellata (Traq.)
A short distance up the Slot Burn Mr. Tait recently detected a second fish-band on a slightly higher horizon, which yielded several species of fishes, together with a myriapod; and in a streamlet about half a mile to the south of this locality he also obtained specimens of Glauconome in sandy shale not far below the outcrop of the quartzite-conglomerate.
It is highly probable that other exposures of the fish-band may yet be found along the northern limb of the Lesmahagow inlier, as that region has not yet been thoroughly explored since the recent discovery of fish-remains.
(10) Quartzite-Conglomerate. — The outcrop of this remarkable conglomerate (6d in
(11) Chocolate-coloured Sandstones. — Next in order chocolate-coloured sandstones (6e in
Having described in the foregoing paragraphs the relations of the Downtonian rocks along the north-west margin of the Lesmahagow Tipper Silurian inlier, we may now refer to their development on the east and south-east sides of that tract, where, as already indicated, the Wenlock and Ludlow rocks are abruptly truncated by a normal fault that obscures the natural sequence from the Upper Ludlow to the Downtonian series. In this area the various zones from the red and yellow sandstones (Group 8) to the greywacke-conglomerate, at the base of the Old Red Sandstone, are to be seen.
The best section occurs in the Birkenhead Burn
By means of a small normal fault which crosses the Birkenhead Burn in an E.N.E. direction, the outcrops of the green mudstones, fish-bearing shales, and the overlying quartzite-conglomerate are shifted westwards on the south side of the stream. On that side the green mudstones and shales are more weathered than in the exposure on the north bank, and they more readily yield the characteristic fishes and eurypterids. They are inclined to the north-east at an angle of about 45°, and are there seen to rest on red sandy mudstones.
The total thickness of the fish-bearing band in Birkenhead Burn is about fifteen feet, which includes the associated layers of barren green mudstone. The lowest fossiliferous zone of carbonaceous shales is about a foot thick, while higher up the fish-bearing seams measure from one to six inches. The remarkable feature of this exposure is the constant association of the fish-fauna with eurypterids that are characteristic of the underlying Upper Ludlow rocks. The fossils obtained by the Geological Survey from this locality are given in the annexed list:
Plants.
Sponge.
Ceratiocaris sp.
Eurypterus sp.
Pterygotus bilobus? (Salt.)
Slimonia acuminata (Salt.)
Stylonurus sp.
Theolodus scoticus (Traq.)
Lanarkia horrida (Traq.)
Lanarkia spinosa (Traq.)
Lanarkia spinulosa (Traq.)
Birkenia elegans (Traq.)
Lasanius problematicus (Traq.)
Ateleaspis tessellata (Traq.)
Lower down the stream the quartzite-conglomerate and part of the underlying fish-band are together repeated by a strike fault with a downthrow to the east. Still further east red and grey friable sandstones (11), are followed by the greywacke conglomerate, which is visible near the foot of Birkenhead Burn
The section of Downtonian rocks in the Logan Water, though by no means continuous, resembles in some respects that in the Birkenhead Burn, for the group containing the fish-band, together with the overlying quartz-conglomerate, is repeated by means of a strike fault. Immediately to the east of the north and south fault that truncates the Ludlow rocks at Dunside
Along the south-east side of the Wenlock and Ludlow rocks of the Lesmahagow inlier, the boundary-fault gradually increases in magnitude. Hence the observer finds that southwards from the Birkenhead Burn to the Nethan Water and the Auchenstilloch Hills
ii Hagshaw Hills Anticline
To the south of the Carboniferous areas of Muirkirk, Glenbuck, and Bankend, the Ludlow and Downtonian rocks reappear along the anticline of the Hagshaw Hills, where the strata are inclined at high angles and where, in consequence, the various zones follow each other in rapid succession. Though the beds on the southern limb of this fold are inverted, the anticlinal structure is well defined by means of the various bands of conglomerate which in that region are typically developed.
From the neighbourhood of Little Cairn Table
Wenlock and Ludlow rocks
For the purpose of this work it will be sufficient to refer to one or two typical sections showing the order of succession from the base of the Downtonian series in descending order.
Perhaps the best section occurs in the Ree Burn
In the Podowrin Burn
Favosites asper (D'Orb.)
Lindstromia sp.
Glyptocrinus basalis (M'Coy.)
Crinoid stems.
Ceriopora sp.
Cornulites sp.
Beyrichia Kloedeni (M'Coy.)
Ceratiocaris papilio (Salt.)
Calymene Blumenbachi (Brong.)
Encrinurus sp.
Illaenus sp.
Proetus Stokesi (Murch.)
Slimonia acuminata (Salt.)
Athyris (Glassia) compressa (Sow.)
Orthis Bouchardi (Dav.)
Orthis (Dalmanella) elegantula (Dalm.)
Orthis polygramma (Sow.)
Strophomena (Leptaena) rhomboidalis (Wilck.)
Ctenodonta sp.
Orthonota sp.
Orthoceras angulatum (Wahl.)
Orthoceras small smooth sp.
It is not improbable that the beds containing this assemblage of fossils may belong to Group 2 of the succession in the Lesmahagow inlier, where a somewhat similar facies of organic remains has been obtained (p. 572).
Downtonian Series
The groups of the Downtonian series are typically developed along the north side of the Hagshaw Hills anticline. Within a horizontal distance of three-quarters of a mile, the various subdivisions from the local conglomerate at the base to the chocolate sandstones (11), which underlie the greywacke-conglomerate, can be examined. The relations of the Downtonian groups to the Ludlow and Wenlock strata of that region are illustrated by the accompanying horizontal section which is drawn across the southwestern part of the arch from Parishholm Hill
In the northern limb of the fold the basal conglomerate (6a) follows the Upper Ludlow rocks in normal sequence on the south side of the road at the south-west termination of the Glenbuck reservoir, where it is inclined to the north-west at an angle of 30°. Lithologically, it differs in a marked degree from the quartzite-conglomerate and greyrwacke-conglomerate that occupy higher horizons, inasmuch as it consists generally of small pebbles of Arenig volcanic rocks, radiolarian chert, and quartz, with an occasional small pebble of Silurian grit or greywacke. The matrix is composed of a greenish-grey friable grit, which readily decomposes.
Next in order come red and yellow sandstones (6b), visible on the west bank of the reservoir, and dipping in a similar direction at angles varying from 30° to 40°. These correspond to Group 8 of the Lesmahagow inlier. The south-east side of the reservoir displays a fine development of the succeeding sub-division containing the fish-band (6c). Here red mudstones with green mudstones and brown carbonaceous shales, are inclined to the N.N.W. at angles varying from 45° to 50°. At a point where a small stream enters the reservoir, ichthyolites were exhumed from these shales, referrible to the genera Lanarkia and Birkenia. The fossiliferous zone is followed on the east bank of the reservoir by the quartzite-conglomerate (6d), which there attains a thickness of 100 feet, and presents its characteristic features, viz., the abundance and well rounded character of pebbles of quartzite derived from the Highlands. Next in order come the red and grey sandstones (6e) followed by the greywacke-conglomerate (CI) of Hareshaw Hill. On the southern limb of the fold represented in
The various sub-divisions of the Downtonian series can be traced along the north side of the Ludlow anticline of the Hagshaw Hills. During 1898, Mr. Tait proved the occurrence of the fish-band at three new localities — (1) in the Douglas Water south-west of Parishholm
Sponge?
Ceratiocaris ?
Eurypterus, small sp.
Scorpion.
Thelodus scoticus (Traq.)
Lanarkia spinosa (Traq.)
Lanarkia spinulosa (Traq.)
Lanarkia horrida (Traq.)
Birkenia elegans (Traq.)
Lasanius problematicus (Traq.)
At the top of the green mudstones (No. 9 of the Table, p. 569 and 6c of
Pachytheca sp.
Sponge.
Glauconome disticha (Goldf.)
Eurypterus dolichoschelus (Laurie.)
Spirorbis sp.
Lasanius problematicus (Traq.)
Ateleaspis tessellata (Traq.)
On the southern side of the Ludlow arch only the members of the highest sub-division (6e) of the Downtonian series can be traced from end to end of the area, being followed in normal order by the greywacke-conglomerate. At one point, however, viz., in the Monk's Water, a small exposure of the quartzite-conglomerate can be seen in contact with the Wenlock and Ludlow rocks, the remainder of the outcrop being concealed by the reversed fault. It is worthy of note that the inversion of the Downtonian rocks is not persistent along the southern limb of the fold, for in the Monk's Water the sub-divisions of that series generally have a normal dip to the south-east.
iii Carmichael Burn Inlier
To the north-east of the Carboniferous basin of the Douglas Water the Ludlow and Downtonian rocks reappear along an axial fold in the neighbourhood of Carmichael Manse
The fossils occur in the Carmichael Burn, at a point about 250 yards south of the manse and about 60 yards north from a bridge across the stream manse
To the south of the exposure of Upper Ludlow mudstones in the Carmichael Burn, no solid rock is visible for a distance of upwards of half a mile, so that we have no evidence bearing on their relations to the strata in that direction. The latter consist of sandstones with conglomeratic bands, which are associated with the volcanic and plutonic rocks of Lower Old Red Sandstone age. It is highly probable, however, that in the Carmichael Burn area the geological relations of the Upper Ludlow and Downtonian rocks are identical with those which prevail in the Hagshaw Hills; or, in other words, that an arch of Ludlow and Downtonian rocks is there truncated by a reversed fault which may be the prolongation of that in the Hagshaw Hills.
iv Area south of Tinto
The south-east slope of Tinto displays a prominent exposure of the quartzite-conglomerate of the Downtonian series, at a point about a mile due north of the village of Wiston