Arbroath Cliffs
Location: Coastal area north east of Arbroath.
Access: Parking, (suitable for mini bus/coach) at 656413 (sheet 54 O.S. 1:50000). Toilets are also located here.
Main Features: Unconformity between Lower Devonian and Upper Devonian sandstone; faulting; fault guided erosion; current bedding; infilled river channels; wave cut platforms; erosion notches; fossil cliffs/arches/beaches/erosion notches; caves;arches;headlands;blow holes;sea stack;pebble beaches.
Whiting Ness
Looking east from the car park, Whiting Ness headland is clearly visible (659 412). Go onto the wave cut platform to examine this feature. The unconformity between the Upper Old Red Sandstone (Upper Devonian), and the Lower O.R.S. (Lower Devonian) is visible. The lower Devonian consists of a fine grained sandstone, dipping 16 degrees S.E. The Upper Devonian is a conglomerate dipping 10 degrees E.S.E. A capping of fluvio-glacial sands and gravel's rests on top of the Upper Devonian.
The wave cut platform is extensive at low tide. Rock debris eroded from the cliffs is strewn over it. The platform is not smooth enough in profile, as it contains ridges of Lower Devonian rock which contain casts of concretionary limestone, making the rock resistant to wave erosion.
Whiting Ness Foreshore
It is possible to walk along the foreshore at the base of the cliffs N.E. of Whiting Ness for about 500 metres. This will allow closer examination of the wave cut platform and numerous geological features, such as, the unconformity, current bedding, infilled channels and fault displacements.
Standing on the wave cut platform looking N.E. you will notice another headland at 664414 (see diagram 7). This is where the unconformity stops, apparently vanishing into thin air! Erosion has worn down the headland, and with it, this boundary between the two rock types, which at one time would have continued upwards for about another 90 metres.(see page 51 Geology of Perth and Dundee District - British Geological Survey) This Marks the N.E. side of a valley cut into the Lower Devonian rock over 350 million years ago. It was then infilled during a renewed period of deposition during the Upper Devonian which ended 345 million years ago (see diagrams two and three). The S.W. side of this valley is represented by Whiting Ness, where the unconformity can be seen to dip upwards.
The Lower Devonian rocks of this area (Lower Old Red Sandstone) were uplifted and eroded during the Middle Devonian period. A valley, 100 metres deep, was cut into the O.R.S. before deposition resumed at the start of the Upper Devonian. Angular debris eroded off the sides of the valley were deposited first, and in places lie just above the unconformity, but most of the Upper O.R.S. conglomerate is composed of Highland rocks such as quartzites and schists brought to this area from the Caledonian Mountains by river.
In time the valley becomes filled with sediment. Each layer of the conglomerate sediment represents a former surface layer of the desert this once was. Distinct layers containing larger conglomerate represent significant flood sequences, as flood water from the Caledonian Mountains spilled out onto this desert plain area. Infilled channels can be located (see map 2), representing rivers which migrated and abandoned their former courses.
Rock Texture
The red colouration of both rock types is due to the cement which binds the individual sand/pebble particles together. Mineral rich water perculates into the sediment as it is laid down. The water is later squeezed out as the sediment becomes compacted, leaving the minerals behind to cement the grains together. Ferrous Iron Oxide is one of the minerals. The desert sands, which covered this area between 395 million and 345 million years ago, had no cover soil or permanent water. The sand grains were therefore exposed to atmospheric oxygen and rapidly changed from ferrous (Fe2+), to the characteristically red, ferric (Fe3+) oxide.
The red colouration therefore is an important piece of evidence in the palaeographic reconstruction of this area. (i.e. It strongly suggests that the area had a desert environment during the Devonian period.
There are numerous faults to observe along the cliff coastline between Whiting Ness and Carlingheuch Bay. Fault displacements range in size considerably. This one, at 662412, with a displacement of two metres represents one of the larger faults found in this area. In general, this fault tends to run parallel to the coastline, and has little impact on coastal erosion unlike many of the other faults which are at right angles to the coast.
Several teaching points can be made at this location............
(1) The Headland has been differentially eroded by wave action. This is due to the more effective cementing in the finer grained sands in the Upper Devonian, making it more resistant.
(2) The fault itself, can be seen to cut through both the Lower and Upper Devonian rocks. This means that the fault occurred after the deposition of the Upper Devonian (probably during the Carboniferous period when the area began to subside around 340 million years ago.)
(3) It is possible to examine the Upper Devonian conglomerate and identify the numerous rock fragments contained within it.(e.g. shists, metaquartzites, phyllites, granites etc.) The nearest source of these rocks are the Grampians, (which are eroded stumps of the Caledonian Mountains which existed during the Devonian.) This indicates that the rivers during the Devonian originated in the Caledonian Mountains flowed south, depositing the conglomerate on the flatter desert plain (where Arbroath is now located).
The conglomerate is fairly rounded, indicating that it has travelled a considerable distance in water. Beds of larger conglomerate represent periods of flooding after heavy rainstorms in the Caledonian Mountains. Some of these are almost boulder size, and are more angular than the smaller conglomerate. The finer sands represent periods during the Upper Devonian when the river were very low, and consequently were unable to carry such a large load.