OR/14/063 Site assessment - ELC 5: North Berwick Shore

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Whitbread, K, Ellen, R, Callaghan, E, Gordon, J E, and Arkley, S. 2014. East Lothian geodiversity audit. British Geological Survey Internal Report, OR/14/063.
ELC_5: North Berwick Shore
Site Information
Location and Summary Description:

The site spans a 2 km section of coastline at North Berwick, extending from Partan Craig in the east, to the North Berwick Bay west. Cliff and coastal platform sections along the coast at the site expose dominantly volcanic and some sedimentary strata of Lower Carboniferous age.

National Grid Reference:

Mid-point: 356026, 685471
West-end: 355204, 68549
East-end: 356860, 685547

Site type:
  • Natural section/exposure
  • Natural landform
  • Natural view
Site ownership: Crown Current use: Open Country
Field surveyors: Rachael Ellen and Eileen Callaghan Current geological designations: North Berwick Coast (GCR ID: 1375); Part of Firth of Forth SSSI
Date visited: 27/08/2014 Other designations: Firth of Forth SPA and Ramsar, North Berwick Conservation area
Site Map
Figure 10    North Berwick Shore Location Map. The site comprises rocks exposed in shore platforms with intervening areas of beach. The exact area of bedrock exposure (blue hatched areas) is likely to vary in time due to changes in the beach morphology. Areas of geological significance include a viewpoint at the east edge of the site which overlooks the historic harbour of North Berwick to the west.
Site Description
Background

The site is located along the shore to the north of the popular seaside town of North Berwick. Historically, North Berwick was a fishing port, and its harbour was built around 1170. Red tuffs from Milsey Bay were quarried for building and oven lining in the late Middle Ages, and volcanic rocks (phonolite) from nearby North Berwick Law were quarried for building stones used in some of the buildings in the town.

Volcanic Rocks
The majority of the rocks exposed along the North Berwick shore are volcanic rocks belonging to the Garleton Hills Volcanic Formation (forming the lowermost unit of the Strathclyde Group). The Garleton Hills Volcanic Formation at this site comprise a sequence of plagioclase-macrophyric basalt, mugearite, plagioclase-olivine-clinopyroxene-macrophyric basalt, trachybasalt, basaltic tuff, volcanic breccia and olivine basalt.

Four basalt lava flows of slightly differing composition and mineralogy (resulting from evolution of chemical composition, in particular Si, Na and K, within the source magma chamber over time) are well exposed in the vicinity of North Berwick harbour and the Scottish Seabird Centre. A prominent ridge of massive grey-red basalt (formerly termed ‘Markle Basalt’), some 17 m thick, contains abundant phenocrysts of feldspar, with rare pyroxene phenocrysts and pseudomorphs after olivine (ELC_5 P4). Toward Plattcock End, the top of the lava flow is more porous (due to brecciation of the lava surface during cooling), and containing many calcite filled vugs and amygdales (ELC_5 P5).

Underlying the basalt is a fissile, dark grey-purple, fine-grained mugearite lava flow (ELC_5 P6) riddled with fractures, some of which are locally iron stained. Stratigraphically below the mugearite lava flow lies a lava flow of ‘Dunsapie’ type basalt, containing phenocrysts of feldspar and pyroxene (ELC_5 P7). This basalt is fairly massive in nature at its contact with the Scottish Seabird Centre (in the middle of the flow), whereas towards its base on the shore, is highly vesicular — a feature typical of the base of a lava flow due to a higher content of gas in the original molten flow. A 4 m thick trachybasalt lies below the ‘Dunsapie’ basalt. The trachybasalt has a reddened top with abundant calcite amygdales, with a grey-purple flow interior. The base of the trachybasalt is irregular where it overlies bedded tuff units to the east (ELC_5 P8).

The basaltic tuffs which dominate most of the remaining geology along the bay from the Scottish Seabird Centre to Partan Craig consist of red and green bedded tuffs, volcanic breccias and calcareous mudstone, dipping gently toward the northwest (ELC_5 P9). It is thought the calcareous mudstone beds formed in shallow lagoons during the early stages of volcanicity. The tuffs are composed of bedded fine-grained to coarse-grained, poorly-sorted, sub-angular fragments of calcareous mudstone and volcanic rocks, e.g. basalt and trachybasalt, derived from the explosive eruptions of volcanic vents.

The prominent Yellow Craig stack (so-called for the yellow lichen which grows on the rock) lies at the high water mark, composed of a dark grey, vesicular olivine-basalt with visible phenocrysts of feldspar and augite. Yellow Craig is a small oval plug of basalt which intruded into the basaltic tuffs. A well-developed chilled margin can be traced around the edge of Yellow Craig at low tide, marked by a pale, grey glassier basalt than the interior. Good contacts can be seen between the chilled margin and tuffs surrounding this intrusion (ELC_5 P10). Thin (<20 cm) dykes extend outward from Yellow Craig, intruding the basalt tuff sequences (ELC_5 P11).

At Partan Craig, a spectacular section is exposed in the cliffs to the east of Milsey Bay. The west- facing cliff is particularly striking, where a shallow synclinal structure can be seen (ELC_5 P12). The sequence in the cliffs starts with a striking red unit containing very large blocks (<2 m) of red tuffs and tuffaceous sandstones set in a matrix of tuff. The clasts are chaotic and rotated, and are thought to be the preserved remains of a debris flow at the edge of a vent (ELC_5 P13). Above this vent, volcanic breccias are found. The breccias and debris flow contain volcanic bombs, up to 1 m in size (ELC_5 P14). Some of the bombs are composed of basanite, containing crystals of nepheline (visible with a microscope), a mineral rarely found in Scottish rocks.

Sedimentary Rocks
Cementstones (ferroan dolomite) and calcareous mudstones are poorly exposed within the intertidal zone of Milsey Bay. These rocks form part of the Strathclyde Group, and crop out with a characteristic orthogonal fracture pattern (ELC_5 P1). They form as subordinate beds interbedded with the basaltic tuff, and stand a little prouder of the surface than the tuffs. Milsey Rocks were submerged during the visit, but the Lothian Geology Excursion Guide describes outliers of massive pale sandstone interbedded with tuffs there. Red and green bedded calcareous mudstones (beds ~5 cm thick) interbedded with tuffaceous calcareous mudstones are exposed along the shoreline, with particularly good exposures found in North Berwick Bay, immediately south of North Berwick Harbour. Some of these rocks preserve discrete rippled surfaces. The brick red calcareous mudstones contain excellent examples of green reduction spots (ELC_5 P2), and rare fragments of crinoids and shells. The tuffaceous calcareous mudstones are coarse-grained, clast-supported, poorly-sorted with sub-angular clasts of creamy, altered feldspar crystals, along with volcanic and calcareous mudstone clasts (ELC_5 P3).

Structural Geology
Multiple faults bisect the strata at the site. The preferential erosion of softer fault rocks gives rise to conspicuous linear absences of rock in the foreshore. However, fault-related features are preserved in some rock platform areas and carbonate veins, slickensided surfaces and fractured deformation zones can be seen (ELC_5 P15).

An example of a syncline formed by shallow collapse of a vent is well exposed within the west-facing cliff of Partan Craig (ELC_5 P12). Small extensional, domino-block style faulting has developed within this collapse syncline (ELC_5 P16). There are excellent local deformation structures within the tuffs, presumably related to localised cryptovents — spectacular reverse faults in tuff sequences can also be identified within the wave cut platform in Milsey Bay.

Access and Additional Information
Access to the coastline is tide dependant, as most of the rocks are covered at high tide. The John Muir Way passes through the town of North Berwick, linking North Berwick Law, North Berwick and Yellow Craig plantation. Access to the coast at North Berwick is achieved by either parking at any of the numerous car parks within the town, or by taking a bus or train into the centre. A road can be followed most of the way along the shore at high tide. At the Scottish Seabird Centre there are interpretation boards describing the history of North Berwick Harbour and information about the surrounding islands. There is also an information board available overlooking the (now filled in) old harbour swimming pool, which gives a very brief geological account of North Berwick Law, and information about the town.

Stratigraphy and Rock Types
Age: Lower Carboniferous Formation: Aberlady Formation
Rock type: Sandstone, siltstones, calcareous mudstones, limestones, ferroan dolomite
Age: Lower Carboniferous Formation: Garleton Hills Volcanic Formation
Rock type: Basaltic tuff, trachybasalt, plagioclase-olivine-clinopyroxene-macrophyric basalt, mugearite, plagioclase-macrophyric basalt
Age: Carboniferous Formation: Southern Scotland Dinantian Plugs and Vents Suite
Rock type: Tuff and breccia
Age: Carboniferous Formation: Midland Valley Carboniferous to Early Permian Alkaline Basic Dyke Suite
Rock type: Olivine-basalt
Assessment of Site: Access and Safety
Aspect Description
Road access and parking Park either at Tantallon Castle, or with permission, at the farmstead at Castleton.
Safety of access Easy access to the shore but all visitors should be aware of the tide times when planning a visit, as most of the exposures are only visible at low tide.
Safety of exposure The rocky exposures have an uneven surface and are often slippery with seaweed. Stout footwear is recommended. The site is exposed to the open sea and the weather forecast should be checked before visits.

Some of the exposures are found below cliffs where potentially loose material may fall, therefore care should be exercised. Exposure near the harbour is restricted by a footpath and metal barrier — caution should be exercised if visiting outcrops beyond the barrier due to steep drops.

Access Access along the foreshore/beach.
Current condition The rocks can be covered in barnacles and seaweed. Rocks exposed at the high water mark are mostly free of vegetation, but contain patches of lichen which cover discrete features.
Current conflicting activities Part of the section of walkway around the harbour was closed during the visit due to construction of a new pier.
Restricting conditions Tide: many of the geological exposures are located in the intertidal range and are therefore covered at high tide.
Nature of exposure Intertidal and beach exposures, low cliff exposures.
Assessment of Site: Culture, Heritage & Economic Value
Aspect Description
Historic, archaeological & literary associations North Berwick harbour dates back to at least 1177, used as a fishing port and ferry port for pilgrims headed to Fife. Historically there was a large open-air swimming pool at the north of the harbour, which closed in 1995.
Aesthetic landscape Coastal landforms and historic town.
History of Earth Sciences The John Muir Way passes through North Berwick.
Economic geology Red tuffs in Milsey Bay were quarried for building and oven lining in the Middle Ages.
Assessment of Site: GeoScientific Merit
Rarity Quality Literature/Collections Primary Interest
Lithostratigraphy Regional Good
Sedimentology Local Poor
Igneous/Mineral/Metamorphic Geology Regional Excellent X
Structural Geology Local Moderately good
Palaeontology
Geomorphology
Site Geoscientific Value

The site comprises a sequence of extrusive lavas and volcanic tuffs, allowing interpretation of the volcanic environment during the Carboniferous. The interbeds of calcareous mudstones in the basalt tuffs provide additional environmental indicators during the Carboniferous in Scotland, representing shallow lagoons which formed during early onset of volcanism.
North Berwick Shore provides an excellent example of a Carboniferous volcanic sequence and related vents, and has regional significance.

Assessment of Site: Current site usage
Community The easy access to the shore and the shore walkway is used regularly by locals. The John Muir Way passes through Yellow North Berwick which attracts visitors from further afield. The Scottish Seabird Centre and ease of access to Bass Rock is a significant tourist attraction.
Education The site displays a variety of features suitable for amateur geologists to study a sequence of igneous rocks representative of a series of volcanic eruptions. This site is an excellent locality for educational fieldwork. The geodiversity of this site could be further promoted by a series of on-site interpretation boards, geo-trail and distribution of geological leaflets.
Assessment of Site: Fragility and potential use of the site
Fragility Weathering/erosion; development of coastal defences may affect the geodiversity.
Potential use On site interpretation, on site geo-trail, school and higher education, research.
Geodiversity Summary
This site contains a good variety of geological features especially associated with volcanic strata. It exposes a sequence of the Lower Carboniferous Garleton Hills Volcanic Formation, along with a small section of the sedimentary Aberlady Formation within the sequence. The volcanic rocks seen allow interpretation of the emplacement of each formation, how they differ from each other and how different phases of volcanism and therefore eruption types represent the type of rock deposited. The coastline is attractive and has easy access. There are possibilities for adding geological interpretation to this site, potentially adjacent to an interpretation board already in place on Castle Hill.
Site Photos
Photo ELC_5 P1:    Cementstones (ferroan dolomite) and calcareous mudstones interbedded with basaltic tuff in Milsey Bay. These interbedded sedimentary rocks are little more than 10 cm thick, and are recognizable in the field by their characteristic orthogonal fracture pattern. The deposition of these sediments during the Carboniferous would have occurred between volcanic eruptions (mostly ash fall), in shallow tropical lagoons. In the photo the town of North Berwick is visible on the skyline. Photo looking west. © BGS, NERC.
Photo ELC_5 P2:    Perfectly circular pale-green reduction spots within red calcareous mudstones. Reduction spots are thought to form due to the reduction of Fe3+ to Fe2+, caused by the presence of organic particles in the original geological deposit. The dark centre of the larger reduction spot in this photo is likely to be the remains of an organic particle around which reduction occurred. © BGS, NERC.
Photo ELC_5 P3:    The calcareous mudstones contain tuffaceous layers, representative of ash-rich volcanic eruptions over the shallow lagoonal environments forming the calcareous mudstones. These tuffaceous layers are clast–supported, and composed of sub-angular clasts of creamy, altered feldspar crystals, along with volcanic and calcareous mudstone clasts. © BGS, NERC.
Photo ELC_5 P4:    The grey-red basalt protecting the north-west wall of North Berwick harbour contains abundant mineral crystals, namely plagioclase feldspar phenocrysts (<5 mm in size) with occasional 1 cm euhedral labradorite feldspar phenocrysts set in a fine grained groundmass. Occasional <1 mm sized phenocrysts of pyroxene and olivine pseudomorphs can be identified within the face, such as the large crystal being pointed to in the photo. The presence of large crystals set in a fine grained groundmass are indicative of two phases of cooling in this lava flow — a slow, initial cooling forming larger crystals (probably within the magma chamber) and a fast, rapid cooling during eruption which formed the fine grained groundmass. © BGS, NERC.
Photo ELC_5 P5:    The image shows a particularly fine example of calcite infilling a large vesicle in basalt, where all three of the calcite crystals natural cleavage planes can be seen The relatively porous top of the grey-red basalt forming the north-west wall of North Berwick harbour represents the flow top of an ancient lava flow. The tops of lava flows are typically very vesicular and gas bubble rich due to gas release from the bulk of the flow below, and its interaction with the open air during eruption. This increase in porosity allows groundwater in more easily in this part of the lava flow, allowing in some cases the deposition of Ca-bearing fluids and precipitation of calcite in these ‘vugs’. © BGS, NERC.
Photo ELC_5 P6:    View to the north looking out across the basalt lava flows to the north of North Berwick harbour. The high standing cliff to the left of the photo with large ‘holes’ is the porous flow top to the plagioclase-macrophyric basalt, whereas the beneath it (at the same level as the grey pipe), the slightly reddened mugearite lava flow is exposed. The basalts here dip toward the west. The island of Craigleith is visible in the background. © BGS, NERC.
Photo ELC_5 P7:    Macroscopic detail of the ‘Dunsapie Basalt’, which lies stratigraphically below the mugearite lava flow at the north end of North Berwick harbour. The photo shows a large, <1 cm black phenocryst of pyroxene set in a fine-grained red-brown ground mass. © BGS, NERC.
Photo ELC_5 P8:    View toward the Sea Bird Centre, to the west. The photo shows the ‘Dunsapie’ basalt cliff below the Sea Bird Centre, which is underlain by a reddened tuff unit (between the base of the cliff and seaweed covered rock platform). The wave cut platform is composed of trachybasalt, which forms an irregular base overlying tuffs (the reddened unit between the boulder foreground and small cliff). © BGS, NERC.
Photo ELC_5 P9:    View toward the east along Milsey Bay, with bedded grey-green tuffs in the foreground. The coarser grained beds are volcanic breccia, and the finer grained beds tuff, representative of ash fall deposits during the Carboniferous. The bedding represents pulsatory jetting of material and showers of ash from a volcanic eruption.The volcanoes from which these were emplaced are preserved as vents situated along the coast, such as that of Parten Craig. The Parten Craig vent lies in the background of the photo. © BGS, NERC.
Photo ELC_5 P10:    The westward margin of the Yellow Craig basalt plug displays a well-developed chilled margin at its contact with the bedded basaltic tuff sequence. The fresh dark gray-black basalt can be seen in the centre of the image, becoming increasingly paler as it approaches the reddened bedded tuffs (to the left of the hand lens). The chilled margin formed when the hot intruding basalt plug cooled rapidly against the cold tuffs, restricting crystal growth and resulting in very fine grain sizes. Photo looking north. © BGS, NERC.
Photo ELC_5 P11:    Minor dykes radiate out from the Yellow Craig plug, composed of the same basaltic material as the plug. Here they can be seen intruding the bedded tuff units. The phonolite plug of Bass Rock is visible in the background. Photo looking toward the north-east. © BGS, NERC.
Photo ELC_5 P12:    View toward the Partan Craig cliff, where a synclinal structure, formed by shallow collapse of a volcanic vent, is clearly visible. The layers of the syncline are composed of tuff, volcanic breccia, and debris flow deposits. Photo looking east. © BGS, NERC.
Photo ELC_5 P13:    The red-grey strata above the geologist in the image is composed of chaotically orientated blocks <2 m in size, floating in a matrix of tuff. It is thought this 3 m thick unit represents the preserved remains of a debris flow at the side of a volcanic vent. The debris flow is overlain by grey beds composed of tuffs and volcanic breccias. Photo looking east. © BGS, NERC.
Photo ELC_5 P14:    The volcanic breccias and debris flows of Partan Craig contain volcanic bombs, up to 1 m in size. The bomb photographed here is composed of nepheline-basanite, a light grey and friable volcanic rock. Sagging of the beds can be seen beneath the bomb where it would have plummeted on the then unconsolidated slopes of the volcano. © BGS, NERC.
Photo ELC_5 P15:    A 10 cm displacement normal fault cutting the ‘Dunsapie’ basalt and red tuff layer is exposed in the intertidal zone below the Sea Bird Centre. The plane of the fault is near vertical in the overlying basalt, and as it dissects the tuff becomes more inclined. This ‘refraction’ of the fault plane results from the differing strengths of the rock it is cutting — the basalt is strong and tends to fault with a vertical orientation, whereas the underlying tuff is weaker and shears more easily into an inclined orientation. Hand lens for scale (circled). © BGS, NERC.
Photo ELC_5 P16:    Small extensional, domino-block style faulting has developed, accommodating movement within the Partan Craig vent as it was collapsing into its present day shallow syncline form. Photo looking to the north west.
© BGS, NERC.