Gabions as Art & Architecture

At Whitstable Harbour gabions have been put to good use in the construction of a remarkable feature. Approaching the construction I could see an intriguing pale line across the darker pebbles, very reminiscent of the pale lines of empty oyster shells that I had just been seeing on the pebble beaches between the breakwaters. Closer inspection proved the pale line to be a symbolic tide line of man-made decorated ceramic pebbles. The feature is known as the Deck at Dead Man’s Corner and is supposed to resemble the bow of a ship, the wall a pebbly beach, and the vertical timber structures are made to the same specification as the groynes along the shore. The ‘tide line’ is a key feature of white ceramic pebbles set into the face of the wall. These were made by local people during a series of public workshops and classes held at the Community College Whitstable. The whole structure comprises seating and a stage built in 2011 for gatherings and events.

Coastal erosion & defence at South Beach, Studland

All around our British coasts we can see evidence of coastal erosion. It seems to be happening at an ever increasing rate in recent years. It is especially noticeable where the edge of the seashore is composed of softer rocks or sand dunes, for example. The coastline of Dorset in England, like many other places, is vulnerable to coastal erosion. Studland Beach near Poole Harbour is a case in point.

The shore of Studland Bay is divided into three stretches: South Beach, Middle Beach, and Knoll Beach. At South Beach, we have already seen how the burrows of small marine worms help to break up the surface of the chalk that underlies the beach in an almost imperceptible process called bio-erosion. However, the physical, hydraulic, and abrasive action of high energy destructive storm waves on the soft materials of the cliff is the coastal process responsible for immediately obvious damage with a wearing-away and break-up of the rocks and other materials on the upper margin of the seashore.

This destructive action of waves is most apparent from the number of small land slips, and trees that have collapsed to the beach, as cliff material [like London and Creekmoor Clays, and Broadstone and Oakdale Sandstones] has washed away from beach level. This has caused the undermining of the cliff deposits, and the eventual fall of material from higher up. There are a number of trees lying in a horizontal position at the base of the low soft cliffs on South Beach.

Where the ground level changes from cliff top to nearer beach level, the wearing away of the soil by the waves means that the trees now seem to be growing directly from the seashore with nothing but sand and pebbles around their roots and trunks. Just a short distance north of these beach-bound trees there are numerous beach huts on the slightly higher ground behind the shore. They are in a vulnerable position. Here the soft ground on which the huts stand is protected from the destructive force of wave action and flooding by the emplacement of stone-filled wire cages known as gabions. These are stacked to form a barrier wall of harder material that is more resistant to erosional coastal processes.

Beach Stones at Corney Brook

There were no other visitors on the dull day that we turned off the Cabot Trail to look at the Corney Brook shore in the western Cape Breton Highlands. There was very low cloud cover, and it began to rain after a while, but there were treasures to be found – at least if you are like me and are fascinated by beach stones. Three main rock types are found at Corney Brook. The oldest are Neoproterozoic-Ordovician granitic pluton rocks of the Bras D’Or Terrane. Ordovician-Silurian metasedimentary rocks of the Aspy Terrane are slightly younger. And red sandstones and conglomerates belonging to the Horton Group come from the Devonian to Carboniferous Period.

The stones on the beach include all three types and probably a lot more due to the glaciation of the area. I wish I could identify and tell you the exact composition of each photographed pebble, stone or boulder – but that is tough for an amateur to determine. There is a great variety of colour, pattern, and texture to the stones which look dull when dry but amazing when wet. They include igneous and metamorphosed rocks like granite, gneiss, schist as well as sedimentary rocks like sandstone. It is possible to see just how difficult it is to not only understand the texts but also to convert into straight forward language for the non-specialist reader from the following detailed description that I discovered about the Corney Brook schist by Jamieson et al. (1987).

Comey Brook schist (unit 3d)
Medium- to high-grade pelitic and semi-pelitic schists, with minor marble and psammite, occurring on the Cheticamp River, Corney Brook, the northeastern end of Jumping Brook, and the central highlands near Calumruadh and Coinneach brooks, are referred to here as the Corney Brook schist. This unit is equivalent to the “medium grade belt” of Craw (1984). Pelitic and semi-pelitic members of the unit characteristically contain coarse staurolite, biotite, and garnet porphyroblasts, with kyanite at the highest grade, in a phyllitic to schistose matrix. Medium- to high-grade marbles, quartzites, albite schists and hornblendite recognized in the Corney Brook area (Plint et al., 1986) have not yet been identified south of the Cheticamp River. Centimetre- to metre-scale compositional layering, folded by tight to isoclinal folds, is interpreted as transposed bedding. Based on bulk compositions and rare relict primary textures, the Corney Brook schist is interpreted to have formed as a suite of clastic sediments interlayerd with felsic tuffs and minor basic flows – that is, it appears to represent the higher grade equivalents of units 3a-3c.

The softer sandstone cliffs are being eroded back by the sea. This has implications for the ground higher up and the roadway further back from the shore. For this reason a sea defence structure has been emplaced to protect the base of the cliffs. This is a gabion made of wire cages full of large beach stones and boulders that are stacked up into a wall, positioned at the most vulnerable part of the shore.

Erosion of the tombolo at Dogs Bay

View of Dogs Bay, Connemara, Ireland

Looking northeast from the offshore island towards the white calcareous sandy beach and dune system, known as a tombolo, that connects the island to the mainland at Dogs Bay in Connemara on the west coast of Ireland.

Dogs Bay is a famously beautiful sandy beach in Connemara on the west coast of Ireland. The beach is actually part of a geomorphological feature known as the Dogs Bay/Gorteen Bay tombolo. A tombolo is a spit or bar of sand or gravel connecting an island to the mainland or another island. In general terms, persistent winds from the southwest have meant that waves meeting the island and wrapping around it, slow down as they converge on the northeast and landward side of the island, where force and speed of the waves decreases and they deposit their load of sediment. Over time the sediments gradually accumulate to such an extent that they rise as a bar above the water. The sediment bank eventually stretches all the way from the island to the mainland, and the connecting bar is termed a tombolo.

The sand has become stabilised by the growth of vegetation; and at the present time is a very special and rare type of habitat known as machair. Machair only forms on calcareous soils. At Dogs Bay the sand is  composed mainly of minute fragments of the carbonate skeletons of marine animals such as sea urchins and their spines, sponge spicules, bryozoa, seashells, snails, and most remarkable of all, the intricate microscopic skeletons of one-celled creatures called Foraminifera. Dogs Bay is one of only a few beaches in the world with predominantly Foraminiferan sand.

Machair, and the surface of the Dogs Bay tombolo, is unlike many of the coastal dune systems that I have visited in England, where the dunes are full of peaks and troughs, and where marram grass dominates. Marram is often a major initial factor in the stabilisation of the loose grains. Here, however, a grassland vegetation of low species diversity is encouraged to grow in a moist, cool, windy, oceanic climate on the fairly level and compacted alkaline soil of a mature sand dune system, and grazing by animals is vital to the maintenance of the habitat. Useful information about the features in this area is available from an on-line field guide produced by the Irish Quaternary Association. Click here for details of tombolo formation and machair habitat (pages 13-17).

Arriving at Dogs Bay, it was clear to see the impact of the earlier winter waves. Storms in the first few months of the year had ripped up and washed away the road and the car park at the entrance to the beach. A sign post now lying on the shore showed where it had been. On scrambling down to the beach, a close inspection of the wonderful curve of the dunes at the top of the shore revealed that the leading edges had been sliced away leaving hanging sheets of machair turf and huge clumps of vegetated dune material on the shore. Wooden posts, perhaps fencing from the top of the dunes delineating boundaries and preventing grazing animals from falling over the edge, were lying loose on the beach below, sometimes apparently supporting  the dune-top hanging mats of vegetation.

The tombolo is a vulnerable feature of the landscape. It seems that there is a history of natural damage to it in this location. In pictures 4 and 5 of the gallery below, the cross-section through the eroding dune shows a narrow horizontal dark brown band about half way down the vertical surface. This is a richly organic ancient soil level (palaeosol) that is associated with archaeological remains such as shell middens and the remains of a settlement, showing that people in the past used the site and exploited its marine resources to augment their diet. The palaeosol  is present at each end of the line of dunes but is absent in the central part. Its absence in this part could indicate that either the dunes at that time in that place were not stabilised by vegetation, or that the centre of the tombolo has been severely eroded in the past and recovered from the damage.

Apparently, about ten years ago, local people feared that the tombolo would be totally breached. Maybe it was at that time that steps were taken to prevent destruction of the spit. It is clear from the presence of rock-filled metal gabion cages on the beach that conservation measures were in place prior to last winter’s storms that battered much of the coastline around Ireland and Britain. However, at the time of my visit on a cold, wet, mostly dull day at the end of March 2014, it was evident that more of these measures might be required in future to prevent further damage.

Gabions made from lobster pots on Grand Manan

Sea defence gabions made with pebble-filled old lobster pots

Wooden stilted houses cling to the slopes at the foot of basalt scarps in Dark Harbour on Grand Manan Island in New Brunswick, Canada. To prevent waves washing away the foundations, sea defence gabions made from pebble-filled old lobster pots have been stacked on the shore, together with other barriers built from massive tree trunk driftwood that accumulates in vast quantities on the massive pebble bank separating the harbour from the open sea.

Sea defence gabions made with pebble-filled old lobster pots

Sea defence gabions made with pebble-filled old lobster pots

Sea defence gabions made with pebble-filled old lobster pots

Shore line at Dark Harbour

Sea defence structure built with driftwood timbers

Detail of a sea defence structure built with driftwood timbers

Detail of a sea defence structure built with driftwood timbers

Driftwood timbers and fishermen's hut on a pebble bank

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