Seaton Cliffs, Arbroath: Sandstone sculpted by the sea

Skirt the tops of dramatic red sandstone cliffs and find beautiful and attractively-named coastal landforms including the Needle E’e, Mermaid’s Kirk, Deil’s Heid and The Sphinx

Seaton Cliffs, Dickmont’s Den, a geo representing a fully collapsed cave. The walls are mainly of Lower Devonian bedded sandstones but the upper left (east) is in Upper Devonian conglomerates separated by an unconformity from the Lower. Image: Mike Browne.

The Arbroath cliffs are made up of river-lain sandstones and conglomerates (petrified gravel) of two different ages – 410 million years old (Lower Devonian) and 370 million years old (Upper Devonian). During these times, Scotland was located south of the Equator in the desert belt. The climate was most likely hot and dry but with dramatic seasonal flooding when rivers flowed across the landscape. The rocks at Arbroath tell the story of two ancient rivers and what happened to the landscape during the interim period.

The older river flowed southwest, draining eroding mountains to the north in Scandanavia and dumping lots of sand, gravel and boulders in the Arbroath area. Once the sediments had solidified into rock, earth movements resulted in the horizontal beds of sandstone and conglomerate being tilted towards the southeast. Erosion by wind and water then sculpted these rocks into a landscape with hollows and considerable hills. This ancient land surface is now preserved as an ‘angular unconformity’ – where older rock has been weathered, eroded and tilted before more sedimentary layers were laid down on top of it, leaving a ‘time gap’ between the two different rocks – that is clearly visible in the cliffs north of Whiting Ness.

The younger river flowed towards the southeast and drained an uplifted, much eroded highland area to the northwest of Arbroath. The sands, gravels and boulders transported by this river are preserved as red or yellow sandstones, conglomerates and breccias – rock containing broken angular fragments of older rock. After these younger rocks were laid down, a further period of Earth movements also tilted these rock layers.

The erosive power of the sea since the last ice age ended around 15 000 years ago has dominated the development of the Arbroath coastal landforms. These powerful forces have produced high cliffs, the rocky shore platform beneath and the narrow, boulder and gravel bays rather than broader sandy beaches. Modern coastal processes are subjecting the cliffs to erosion by waves, wind abrasion and rock falls, with faults and joints being exploited by these dynamic forces to form sea caves, natural arches (Needle E’e) and stacks (Deil’s Heid). The Lower Devonian rocks are harder and more resistant to erosion than the Upper Devonian rocks that weather and erode more easily to form shapes (the Sphinx and Camel’s Back).

The different stages in the evolution of rocky coastlines can also be seen. Faults and joints erode to form everything from minor gashes to large caves (Mason’s Cave). You can see phases of cave collapse from roof blowholes (caused by compressed air during storms), collapsed cave roofs (Gaylet Pot) and natural arches. Complete roof collapse has formed elongate clefts, called ‘geos’ (Dickmont’s Den), and secluded coves.

The power of the sea and wind during storms is demonstrated by the presence of fresh sand and gravel on rock surfaces well above normal high tide. Changes in relative sea levels, which used to be about 30 metres above present level 15,000 years ago, have added to the complexity of the coastal features.

Text contributed by Mike Browne

Find out more
Arbroath to Auchmithie Geodiversity Trail Leaflet: download from www.taysidebiodiversity.co.uk/wp-content/uploads/…/Auchmithie_Trail_Leaflet.pdf

Seaton Cliffs, the Needle E’e arch; unusual in being parallel to the coast but still representing the remnant of a collapsed cave. Cut in Lower Devonian sandstones and conglomerates, the arch aperture is above modern tidal reach but storms allow access of gravel through it. Image: Mike Browne.

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