Arthur's Seat, Edinburgh - introduction
From: Lothian Geology: an excursion guide. Edited by A D McAdam and E N K Clarkson. 1996
- OS 1:50 000 Sheet 66 Edinburgh
About a kilometre from the city centre, the remnants of the long-extinct volcano of Arthur's Seat rise from the low ground on which Edinburgh is built. Part of the volcano has been lost through erosion and part has been buried under younger rocks; enough, however, is exposed to allow us to study the vulcanicity in some detail, especially as the removal of much of the superstructure has laid bare the internal parts of the volcano. The largest volcanic remnant lies within the Holyrood Park where it culminates in Arthur's Seat (251 m), the hill from which the volcano takes its name. To the north and west smaller remnants build the Calton Hill and the Castle Rock (p. 52). The volcano was active in the Dinantian, early in the Carboniferous Period, and the products of its first eruption are taken to mark the top of the Cementstone Group. The lavas are covered by the oldest sedimentary member of the succeeding Lower Oil-Shale Group, the Abbeyhill Shales.
The first eruption of the Arthur's Seat Volcano was made into shallow water in which the rocks of the Cementstone Group had accumulated but early in the activity, the higher parts of the cone were raised above water-level and colonised by land plants. Their fossilised remains are found today in the ashes and agglomerates. The deposition of chemically precipitated limestone high on the cone in the middle stages of the activity, and the final burial of the entire volcano by waterlaid sediment indicates that the greater part of the cone was submerged during most of the volcano's life; this contention is supported by the presence locally of well-bedded ashes between most the lava flows. Thus, although the lavas were erupted subaerially, much of their descent of the cone was made below water. No trace of pillow structure, however, has ever been observed, but some of the higher lavas have been partially albitised and carbonated and are now transitional between normal basalts and spilites.
As exposed today, the Arthur's Seat Volcano consists of five vents (the composite Lion's Head and Lion's Haunch vents, the basalt-filled Castle Rock and Pulpit Rock vents and the agglomerate-filled Crags Vent), three portions of the cone (the Whinny and Calton Hills and an area near Duddingston) and a number of sills and dykes. The Salisbury Crags Sill and two small dykes were intruded long after the volcano became extinct.
Whinny Hill provides the most complete and accessible sequence of lavas. Lava 1, believed from petrographic evidence to have been erupted from the Castle Rock Vent, forms the Long Rowand, its northern downfaulted portion, the Haggis Knowe. Above the lava there lies a considerable thickness of mixed ash and sediment known collectively as the Lower Ash of the Dry Dam; this contains at least two bands of precipitated limestone, the lower containing irregular masses of chert. The ash, most probably derived from the Lion's Head Vent, is covered by Lava 2 which was erupted from the same orifice within which its feeding conduit is preserved. There followed the formation of a further bed of intermingled ash and sediment in which volcanic bombs are prominent - the Upper Ash of the Dry Dam. After the accumulation of the ash, a parasitic vent - the Pulpit Rock Vent - some distance down the northern slopes of the cone, emitted Lava 3. Later Lava 4 was erupted from the Lion's Head Vent at the apex of the cone and descended normally until diverted around the obstacle formed by Lava 3. Lava 4 is only seen on the southern part of Whinny Hill today, its northern continuation having been diverted out of the present plane of exposures by Lava 3. Lava 4 was the last flow to be erupted from the Lion's Head Vent for the residue of the flow remaining in the vent blocked the orifice on consolidation. All further activity of the Arthur's Seat Volcano was focused on the Lion's Haunch Vent from which the remaining nine lavas (5 to 13) of Whinny Hill were erupted. These flows lie in normal succession, one above another, the contacts of the flows being rarely marked by any considerable ash bed.
The remnant of the cone of the Arthur's Seat Volcano exposed on the northern shores of Duddingston Loch differs extensively from the remnant which forms Whinny Hill and, apart from Lava 1, the successions cannot be correlated with any certainty. The Calton Hill succession (Locality 32) shows a general resemblance to that of the Whinny Hill.
Of the five vents of the volcano, that of the Lion's Haunch is by far the largest and most complex. It is filled chiefly by a red agglomerate consisting of a fine-grained red matrix of decomposed basaltic ash in which lie basaltic and sedimentary blocks up to 3 m in length. Within the agglomerate there occur at least seven small lava flows, of basalt or mugearite, which were erupted and confined within the crater walls. A common associate of these flows is a bedded red tuffaceous sandstone, pointing to the existence of temporary crater lakes during periods of quiescence in the vulcanicity. A mass of Dunsapie basalt forms the summit of the Lion's Haunch and partly rests on and partly cuts across the underlying agglomerate. This mass is the remnant of a one-time lava lake which probably once filled and blocked the Lion's Haunch Vent and brought the surface activity of the volcano to a close.
Several basaltic intrusions lie in the Lion's Haunch Vent. In the cast there occurs the marginal intrusion of Dunsapie Hill, the type locality for Dunsapie basalt. In the west of the vent there crops out at Samson's Ribs an intrusion, again of Dunsapie basalt, which ascended along the wall of the vent and extended in a number of irregular tongues into the crater infilling.
The other three vents seen in the Holyrood Park are simpler in constitution. The Lion's Head Vent, now partly truncated by the later and larger Lion's Haunch Vent, appears to have been originally cylindrical in form and largely infilled by a fine agglomerate, through which penetrated the feeding conduits for Lavas 2 and 4; these conduits are now filled by Craiglockhart and Dalmeny basalt respectively. The Pulpit Rock Vent - the orifice of Lava 3, is a small plug of Craiglockhart basalt. The Crags Vent is filled with fine-grained agglomerate containing fragments of basalt similar to Lavas 1 and 2 of Whinny Hill. No higher flows have contributed fragments and it is probable that this vent ceased its activity shortly after the eruption of Lava 2. The intrusions associated with the vulcanicity, other than those in the vents, include a sill, which has been divided into three portions now forming the St. Leonard's Crag, the Dasses and the Girnal Crag. The probable feeder of the sill is situated at the Dasses where some dyke-like contacts may be seen. A second sill, known as the Whinny Hill Intrusion, occurs between Lavas 6 and 7.
Two intrusions of later date than the Arthur's Seat vulcanicity occur in the Holyrood Park. The larger is the well-known teschenite sill of the Salisbury Crags of Namurian age. To the north of the Cat's Nick the sill is cut by a later (Stephanian) quartz-dolerite dyke which contains a large strip xenolith of the sill rock.
After the final extinction of the volcano it was covered by thousands of feet of sediments. Earth-movements folded the strata and imparted to the buried Arthur's Seat Volcano a general eastward dip of between 20 and 30 degrees. Many faults cut the sediments and the volcanic rocks. The sedimentary cover of the volcano was removed by prolonged denudation which culminated in the distinctive erosion caused by the Pleistocene ice-sheet. The ice moved from west to east across the area; the hard rocks of the Arthur's Seat Volcano were left as high land while the soft surrounding sediments were more extensively planed away. The easterly dip of the volcanic rocks caused the ice-sheet to produce the present-day topography of westward facing cliffs backed by gentle easterly slopes. The famous crag and-tail structure of the Castle Rock and the High Street is the best known of these phenomena, but similar land forms have been produced at the Salisbury Crags and the Calton Hill. A fine roche moutonée has been preserved in the Queen's Drive and glacial striae can be observed at several localities.
Four excursions are described in detail. These, with their approximate duration, are:
- Salisbury Crags 2.5 hours
- Whinny Hill 3 hours
- Lion's Head and Lion's Haunch Vents 3 hours
- Calton Hill 1 hour
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