Coastal Geology

Looking North across Howdiemont Bay towards Dunstanburgh Castrle

General view from Howdiemont Sands looking northwards

Map to show the area described

A map showing the main locations mentioned.

One of the many glories of Northumberland is its coastline. In addition to its outstanding scenic qualities, it is geologically interesting and important. Much work has been done on it, and there are some detailed studies of different areas. Here we concentrated on the two mile section from Howick Haven to Boulmer village. Going from North (Howick) to South (Boulmer), the section takes in a variety of mid Carboniferous rock of the Namurian period (c326-311MA) moving from earlier to later, and ending with a Permian volcanic intrusion dating from about 295MA.
The Carboniferous period was characterised by sinking land levels associated with extensional and transtensional tectonic plate movements. This was in marked contrast to the uplift and mountain building of the earlier Caledonian orogeny which resulted from the collision of the Avalonian, Laurentian and Baltica plates. In Southern Scotland the extensional movements seem to have led to the development of alkali volcanic activity, but in northern Northumberland there was sea inundation and resulting deposition of sedimentary rocks. At the same time sea-levels seem to have been fluctuating possibly as a result of increase and decrease of ice cover in regions closer to the poles. The landmass which included Britain was on the move from the Southern hemisphere to its present position in the Northern hemisphere. Consequently, Northumberland was at this time in the tropical/equatorial region and experienced a hot, wet climate. For the first time plants had evolved sufficiently to produce sizeable trees which thrived in the warm coastal swamps, and whose remains gave rise to the rich local coal seams. The contemporary Variscan orogeny which built the Pyrenees and the mountains of South Wales, and gave rise to the granite batholiths of Brittany and South-West England, may have been responsible for lifting the slightly earlier Fell Sandstone deposits into the central Northumberland sandstone ridge which dominates the hinterland.

Howick Haven

Coal, sandstone, shale and siltstone strata on the North side of Howick Haven
Strata – north side of Howick Haven
Alternating shale and siltstone with a layer of sandstone and then a thin coal seam near the top.

The rocks on the North side of Howick Haven show a very distinctive regular narrow layering of pale and black rock with a wider sandstone stratum above them. The black rock is shale or mudstone; the paler rock is either very fine grained sandstone or siltstone. These have obviously been laid down in estuarine conditions, the mudstone in very slow moving water, and the silt in slightly faster current. The waters must have been fairly placid because there is little evidence of turbulence in the layering. The higher sandstone stratum is probably the remains of a marine sandbank. Just below the sandstone is a narrow seam of coal formed from a coastal swamp forest.


South side of Howick Haven at the headland. Tabular cross bedding.

Strata on the south side of Howick Haven showing tabular cross bedding

The same sequence as on the North side (but without the coal) can be observed on the South side of Howick Haven. The water gushing from a hole in the rock is not a natural spring. It comes out of an artificial borehole.Rounding the point leaving Howick Haven, the tabular crossbedding can be seen in the sandstone. It forms straight deposits rather like a snowdrift on the leeside of an obstacle in the path of a flowing current. It can be laid down by wind or by water. In this case, given the estuarine conditions, it was most likely to be water deposited. The sandstones on both sides of Howick Haven are yellow to brown in colour. This pigmentation is caused by the presence of ferric hydroxide. Iron is the only common element in the earth’s crust apart from carbon which gives colour to rocks. Yellow to brown indicates hydration; red indicates ferric oxide and therefore drier conditions.

There is a difference in the strike of the strata (the horizontal bearing of the strata) on either side of the haven. On the North side the strike is about 50o, whereas on the South side it is about 30o. This suggests the presence of a fault although there is no available bedrock with which to observe it. If this is the case, the Howick burn has cut its path down the fault’s line of weakness. The dip (angle of depression of the strata) remains fairly consistently at about 10 o for much of this part if the coast.

Howick to Sugar Sands

Strata showing limestone, coal, siltstone and sandstone and shale, sandstone and shale in descending order

The sequence of rock types round the headland from Howick Haven

Coal seam above the Lickar limestone

A closer view of a coal seam above the Lickar limestone

Round the point leaving Howick haven,there is asmall cove with complex and interesting geology. Dark and light strata of mud and siltstone continue into the cove. As the strata dip down to the left (facing the cliff), two layers of limestone accompanied by two thin coal seams, make their appearance. This limestone is marked on the geological survey 1” map as the ‘Lickar limestone’. ‘Ball and pillow’ structures which are caused by material from a higher stratum slumping into a lower one, can be seen in some of the strata.

Blocks of limestone resting on shale.

Limestone (Lickar) resting on black shale

lime_cracks

Solution cracks in the surface of limestone

Continuing south, there is a small reef with interesting erosion cracks on its top surface. The lower strata of the reef are definitely dark mudstone, but the top layer proved, on the application of dilute acid which caused effervescence, to be more limestone. Moving on over a rough area of boulders, there is a much more extensive belt of limestone. This is the ‘Sugar Sands limestone’ named after the wide sandy bay which lies ahead. The lime-rich cliffs here have an abundance of wild flowers in spring and summer.

Wider strata. Sugar Sands Limestone

Sugar Sands limestone

Sugar Sands to Howdiemont Sands

Trough cross bedding showing as rippling patterns in the sandstone

The rippled effects of trough cross bedding.
North side of Howdiemont Sands

Crossing Sugar Sands to the next rocky area there are some concrete blocks which have fallen onto the beach and appear to be the remnants of Second World War defensive structures. The next area is a boulder field. The large dark rocks are mainly ‘whinstone’, the quartz-dolerite of the Whin Sill. This was intruded (c295MA) after the Carboniferous period. These boulders have been washed up from the sea or out of the glacial drift.Moving over the boulder field, some bedrock is visible at low tide. It is mainly sandstone and often shows rippling patterns suggesting more turbulent conditions. These are examples of trough cross bedding which like the earlier tabular cross bedding form on the lee side of obstacles but unlike the tabular cross bedding forms curved deposits.

Magnified 200 times. Calc-silicate rock with possible Vesuvianite crystals.

Contact metamorphic calc-silicate rock altered by whinstone intrusion
Possible crystals of vesuvianite (Mag X200)

The cliffs here are composed of glacial drift which occasionally throw up interesting rocks such as andesite lava from the Cheviot and basalt from the Scottish borders. A most interesting discovery is a block of whinstone intruded into limestone. The limestone has been hardened into what is partially a calc-silicate. The surface of the calc-silicate area has minute brown pyramidal crystals which are only visible using a USB microscope at X200 magnification. Unfortunately, this sort of examination precludes the tests available for thin sections or hand specimens so the only guide to identification is by colour, crystal structure and provenance. This specimen is too delicate to be made into a thin section, so identification of content must be tentative. Given the contact metamorphic provenance, and the crystal form, the brown crystals are probably vesuvianite which is a typical contact metamorphic mineral of impure limestones. It is known in Northumberland at the contact zones of whinstone and limestone.

Upper Foxton Limestone

Blocks of limestone fromUpperFoxton limestone
North side of Howdiemont Sands.

After the boulder field, the expanse of Howdiemont Sands opens up. The 1” geological survey map marks outcrops of the Lower Foxton Limestone at the North side of the bay, and the Upper Foxton Limestone at the South side. The only sign of these outcrops is of limestone boulders at the edge of the face of the glacial drift (NU 26376 15558 and NU26905 15303). Various rocks, mainly sandstone and mudstone are exposed at low tide. At NU 26406 15609, the strike of the strata changes from North of North-east to East of North-east, and the dip of some of the rock is more severe, possibly indicating minor faulting here.

Longhoughton Steel to Boulmer

Coarse-grained sandstone

Coarse-grained sandstone
Longhoughton Steel.

Red sandstone

Red coarse-grained sandstone
Longhoughton Steel

At the south side of Howdiemont Sands, after crossing over a burn, there isa rocky area called Longhoughton Steel. In some places at low tide bed rock of mudstone and sandstone is visible. However, fairly soon, there is a marked change to a much coarser conglomerate sandstone (NU 26651 15484). Some of the larger grains are as much as half a centimetre across. Solid blocks and structures of red coarse-grained sandstone appear at NU 26815 15417. Many of the grains consist of feldspar rather than quartz. These features give clues about the rock’s formation. A mixture of large and small grains in a sandstone indicates that it was laid down rapidly with little time for different grain sizes to separate out. This is confirmed by the presence of sizeable grains of feldspar which would normally undergo chemical solution and disintegration over a longer period of time. The red colour is caused by a ferric oxide cement which is normally formed under arid conditions. It is clear that we have moved away from the estuarine and deltaic conditions which formed the rocks in and south of Howick Haven. This coarse rock used to be classified as Millstone Grit linking it with the well known so-named formations in the Pennines; however, it has now been re-classified as ‘Longhoughton Grit’. It belongs to a later period (nearer 310MA) than the previously described estuarine deposits.

Reverse or Thrust fault in sandstone

Reverse or thrust fault
Longhoughton Steel

Further on there is an obvious fault at NU 26875 15170. The underlying rock is often obscured by sand deposited by high tides but, when visible, indicates that this is a thrust fault, the south side having been pushed over the lower strata. The thrust has forced the dip to an angle of 45o. It is also clear that the strike of the two sides of the fault are more than 90o different from each other. A violent earth movement must have taken place here.

Convoluted bedding in sandstone

Convolute bedding in coarse-grained sandstone
Longhoughton Steel

Approaching Boulmer village the red rock gives way to a much paler, yellow sandstone which is sometimes but not always finer-grained than the red rock. It is characterised by splendid examples of convolute bedding and slumping (NU 26778 14848). This is evidence that the deposition was very rapid and vigorous. This caused the sand to undergo liquefaction where the grains are separated from each other by the violent action of gravity and/or water, and then re-consolidated in convoluted layers and structures. The dip of the strata becomes rather variable in this phase.

The Whin Sill at Boulmer

Whin Sill (quartz-dolerite) intrusion at Boulmer

There is a small intrusion of the Whin Sill more or less in a line with the Boulmer lifeboat station. It is only visible at low tide. Its stratification is in line with the strata of the country rock. This indicates that it is a sill intrusion rather than a dyke which would have been forced against the prevailing strata. This sill dates from the Permian era at about 295MA, and consists predominantly of Clinopyroxene and plagioclase feldspar (oligoclase to labradorite in composition). It contains smaller quantities of magnetite, ilmenite, quartz and orthopyroxene.

REFERENCES

Northumberland Coast Rocks! Helen Page (published by Howick Heritage Group)

Earth Materials. Klein and Philpotts. Cambridge U P. ISBN 978-0-521-14521-3

Northumberland Coast Area of Outstanding Natural Beauty and European Marine Site GEODIVERSITY AUDIT AND ACTION PLAN. Produced on behalf of the Northumberland Coast AONB Partnership and Berwickshire & North Northumberland European Marine Site Partnership

Earthwise – British Geological Survey http://earthwise.bgs.ac.uk/

Geological Structures, John L. Roberts (Macmillan Field Guide) ISBN 0-333-66295-4

No vestige of a beginning, – no prospect of an end

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