OR/14/063 Site assessment - ELC 5: North Berwick Shore
|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|
|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
|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|
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.
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.
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
|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|
|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|
|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|
|Structural Geology||Local||Moderately good|
|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.
|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.|
|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.|