THE PENTLAND HILLS.

CHAPTER IV.

THE PENTLAND HILLS.

By ROBERT CAMPBELL, M.A., D.Se.

At three successive periods during Palreozoic times the crustof the earth in the neighbourhood of Edinburgh was effected bymovements of compression, with consequent production of anti­dinal and synclinal folding, the axes of the folds in each casetrending north-east and south-west. The range of the PentlandHills, which forms the dominating topographical feature, followsthe same general direction, coinciding in the main with the trendof a great anticline belonging to the post-Carboniferous crustalmovement. The core of the denuded fold is occupied by rocksof Upper Silurian age, and these are flanked in turn by stratabelonging to the Old Red Sandstone and Carboniferousformations.

SILURIAN.

Silurian rocks occur as three separated inliers: (a) In theneighbourhood of the Lyne Water, (b) at Bavelaw Castle, and(c) at Loganlee.

(a) The Lyne Water Inlier.-This, the largest area of Silurianrocks in the Pentland Hills, is, both from the palreontological andstratigraphical point of view, by far the most interesting. The bestsections are seen in the Lyne Water and the North Esk and theirtributaries. Round the head waters of both streams there is asplendid development of greywackes, grits, sandstones, shales, andmudstones containing fossils of Wenlock and Ludlow age. Ofparticular interest is the admirable section in the GutterfordBurn, in which occur the well known "Eurypterid bands," firstdiscovered by Mr. John Henderson. These contain many newforms of eurypterids, which have been described by Dr. MalcolmLaurie. Certain associated beds have yielded well-preservedstarfishes. The section also contains a thin band of corallineand encrinital limestone-an unusual type of sedimentation inthe Pentland Silurian.

Both in the Lyne Water and the North Esk sections the Ludlowshales and mudstones pass upwards into a conformable series ofconglomerates and red sandstones, with intercalated green andgrey shales and greywackes. The discovery in the latter group byMr. D. Tait of characteristic fossils of the Glauconome zone ofLanarkshire, including Ateleaspis tesseiata, Birkenia elegans, andLasanius problematicus, shows that it must be referred to theDowntonian.

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FIG. 2.-DIAGRAMMATIC SECTION ACROSS PENTLAND HILLS FR OM BALERNO TO PENICUIK.

dS• Coal Measures. d 4• Millstone Grit. d2• Carboniferous Limestone Series. d' , Calciferous Sandstone Series. c3• UpperOld Red Sandstone. c', Lower Old Red Sandstone. Pc', Rhyoliti c Tuffs of Lower Old Red Sandstone age. a. Basic Lavas ofWarklaw Hill. b. Acid Lavas of Howden Burn. c. Basic Lavas overlooking Loganlee. d. Acid Lavas of Scald Law andCa rneihy. e. Intermediate Lavas of Woodhouselee. f. Basalts (including" Carnethy Porphyry "). b5• Upper Silurian.

L. Lamprophyre Dyke. D. Dolerite Dyke. G. Microgranite. F' . CQlinton Fault . P . Hare Hill Fault . P . LoganleeFault. P. Pentland Fault.

THE PENTLAND HILLS. 21

The Downtonian beds are overlain unconformably by thebasement grey wacke conglomerate of the Lower Old Red Sand­stone.

(b) The Bavelaw Castle Illlier.-This inlier extends fromBavelaw Castle to the western slopes of Black Hill. Goodsections occur in the quarries at Bavelaw Castle and in the smallravines along the northern face of Hare Hill. Lithologically therocks are similar to the Wenlock strata of the North Esk section,and the palseontological evidence, although not conclusive, alsosuggests the same age.

(C) The Loganiee Inlier. - The Loganlee inlier forms anelongated narrow belt, bounded on the west by the micrograniteof the Black Hill, and truncated on the east side by a fault whichbrings down a series of lavas of Lower Old Red Sandstone age.The rocks are chiefly grey slaty shales of Wenlock age.

About the end of Downtonian times important movements ofcompression and elevation affected the area now occupied by theSouthern Uplands. The Silurian rocks were thrown into a seriesof steep-limbed folds, the axes of which have a north-east south­west direction, and their disintegration supplied most of thematerials for the formation of the overlying uncomformable strata-the coarse conglomerates and sandstones of the Lower OldRed Sandstone formation.

LOWER OLD RED SANDSTONE.

In a study of the Pentland Hills the rocks of Lower Old RedSandstone age, alike in extent and in features of generalgeological interest, must always occupy the first place. TheSilurian rocks, as we have seen, occur only as three small inliersin the core of the anticline j the relatively soft overlying strata ofUpper Old Red Sandstone and Carboniferous age have beenalmost entirely removed j the hard, resistant igneous rocks andconglomerates of the Lower Old Red Sandstone, on the otherhand, like the rocks of similar age in the Sidlaws and Ochils,have withstood the forces of denudation, and remain as a groupof picturesque hills, which add greatly to the beauty of the centralvalley of Scotland.

Mainly through the researches and writings of CharlesMaclaren, Sir Archibald Geikie, and Dr. B. N. Peach, the LowerOld Red Sandstone succession is now well known to geologists.It consists of coarse conglomerates and sandstones, with whichare intercalated contemporaneous igneous rocks. The con­glomerates show their greatest development at the base of theseries. The boulders in the lowest conglomerates consist chieflyof greywackes, along with which may be noted cherts and jaspersfrom the Arenig belt of the Southern Uplands, Haggis rocks ofCaradoc age, fossiliferous limestones, probably of Upper Silurian

22 ROBERT CAMPBELL.

age, and Highland quartzites and schists, derived, in all pro­bability, from the local Downtonian conglomerates. Boulders ofthe volcanic rocks of Old Red Sandstone type are absent. Fromthe character of the boulders themselves, and from the mannerof imbrication of the flatter pebbles, it is certain that the materialswere transported from the high ground of the Southern Uplandsthat had been elevated in late Downtonian or post-Downtoniantimes. Conglomerates occurring at higher horizons often containa considerable percentage of pebbles derived from the contempo­raneous denudation of the lavas.

The coarse conglomerates are accompanied by thick accumu­lations of lavas and tuffs, and there can be little doubt that therewas a causal connection between the movements of elevation andcompression, and the marked outburst of volcanic activity in thePentland area. The extraordinary alternation of different typesof lava was recognised hy Sir Archibald Geikie, and is wellbrought out in the following table of the Lower Old Red Sand­stone succession by Dr. B. N. Peach:

"(g) Carnethy and Hillend group of basalts and basicandesites.

(f) Woodhouselee and Braid Hills group of acid andesitesand trachytes.

(e) Caerketton group of rhyolitic lavas and tuffs.(d) Allermuir group of basic andesites and basalts, with one

flow of porphyritic rhyolite near the base, and thinintercalations of sediment.

(c) Bell's Hill and Howden Burn group of rhyolites, withsuhordinate conglomerates.

(b) Bonally group of basalts and basic andesites.(a) Basement conglomerates and sandstones resting uncon­

formably on Upper Silurian rocks."

The lavas are all of calc-alkali or Pacific type, comprisingrhyolites, trachytes, acid and basic andesites and basalts. In anadmirable chapter on the petrology of these rocks Dr. Flett hasdrawn attention to the highly decomposed character of nearly aUof them, and has shown that they fall into three groups, whichmay be readily recognised in the field:

1. The more basic rocks, basic andesites and basalts, arerichly charged with iron oxides, mainly hsernatite.Consequently the screes and weathered outcropspossess a characteristic dark-red colour.

2. The rhyolitic types are silicified, and their flinty character,and the pale colour of the scree material afford asure means of identification.

3. The lavas of intermediate composition have decomposedto "claystoens," usually grey, lilac, or buff in colour.

THE PENTLAND HILLS. 23

Since the volcanic series attain s its maximum developmenttoward s the north, it seems likely that the main centres oferupt ion are conc ealed under youn ger rocks somewh ere towardsthe north end of the range-whether to east or west is uncertain.A group of minor necks occurs in the neighbourhood of Swanston.and the felsite of Bell's Hill may represent another orifice. Thelaccolite of the Black Hill, which has been intruded along theunconformabl e jun ction between the Silurian and Lower OldR ed Sands tone, probably represents an aborti ve attempt to set upa vent. Other hypab yssal intrusions of presumably Old R edSands tone Age are seen in several highly-decomposed lam pro­phyre dykes and sills, while the coarsely crystalline " diorite" ofthe Lyne Wat er in all likel ihood represents the plutonic phase ofthe same magma.

P rior to the deposition of th e Upper Old R ed SandstoneSeries another movement of compression affected our area, andth e rocks of Lower Old Red Sandstone Age were bent into a greatanticline pitching to th e north. Another result of the same earthmovement was th e production of a group of important strikefaults .

UPPER OLD R E D SANDST ONE.

T o th e Upper Old R ed Sandstone system belongs a series ofred sands tones, corns tones, and green and reddi sh sandy mud­stones or marls. In th eir lithological characte rs th ese resembleth e rocks of Craigmilla r and the Qu een's Dri ve, which haveyielde d remains of H oloptychius nobilissim us. Fragment ary fishremain s were collected from the series in the grits at ClubbiedeanR eservoir.

This system has its greatest development round the northernend of th e range and in the neighb ourhood of East and WestCairn Hills in the south . Over th e intervening country alongthe west side of the Pe ntlands it res ts on a very uneven floor ofth e Silurian and Lower Old R ed Sand stone ; along the east sideit is entirely cut out by the Pentl and boundary fault.

As the basement members of th e U pper Old Red Sands toneare traced rcund the hills th ey are found to rest everywhere withcomplete unconformity on th e older rocks. Not only do th eyrest success ively on different horizons of the Lower Old R edSan dstone, but at the Bavelaw Castle inlier they are in directcontact with Wenlock strata. One must picture, th erefore, aprolonged period of denudation between the folding of the Lowerand the deposition of the Upper Old Red Sandstone Series.During that interval the anticline of the older rocks had beendenuded to the core. Indeed , as Dr. Peach has suggested, manyof the top ographical features of the Pentlands-the escarpmentsof Warklaw H ill, for example-may th en, to a large extent, have

THE PENTLAND HILLS.

assumed their present form. And this period of denudation, too,may be responsible for the highly decomposed character of thePentland lavas.

CARBONI FEROUS.

The Upper Old Red Sandstone Series passes upwards into theconformable Calciferous Sandstone Series, the lowest group of theScottish Carboniferous system. As has already been pointed out,a third great crustal movement affected the rocks of the Edinburghneighbourhood after the close of the Carboniferous period, andthe Carboniferous strata must have formed a great arch over thepresent Pentlands. Denuding agencies, however, have almostentirely removed the softer Carboniferous rocks, and now theyare found only on the extreme flanks of the range, occupying therising ground leading up to the Pentlands proper.

A noteworthy result of the post-Carboniferous movementswas the production of the Pentland fault-an overthrust separatingthe Pentland Hills from the Mid-Lothian coal basin to the east.

THE PENTLAND PASSES.

One of the most striking features in the topography of thePentlands is the occurrence of numerous conspicuous "windgaps," or transverse passes. These valleys are relics of an ancientriver system, and indicate stream capture and beheading on a veryextensive scale.

GLACIAL PHENOMENA.

The rounded contours of the hills, the widespread distribu­tion of boulder clay and erratics, and the frequent occurrence ofstriated surfaces all bear witness to the fact that the Pentlandswere overridden by an ice-sheet during the Great Ice Age. Thecharacter of the boulders in the till shows that the ice had itsorigin in the West Highlands.

Splendid examples of dry valleys, belonging to a drainagesystem initiated during the retreat of the Highland ice, occur onthe north-western flanks of the Pentlands. Evidence of theretreat of the Southern Upland ice is seen in the marginaldrainage channels so well displayed along the south-eastern slopesof the range.

Other phenomena associated with the retreat of the Highlandice, e.g., overflow channels, over-deepening of valleys, gravelterraces, and the subsequent formation of 'corroms,' are wellillustrated. An excellent account of the glacial phenomena ofthe Pentlands is given by Dr. Peach in Cochrane's "PentlandWalks."