Causeways
Causeways are essentially banks of material built across often shallow waterways to connect an island to the mainland, or two sides of an estuary. They usually incorporate a small bridge or culvert somewhere along the length to allow for tidal movements. However, one of the more famous causeways does not really conform to this description. The Lindisfarne Causeway in Northumberland is a road built essentially on the beach which gets covered at every high tide. It does have a bridge to cross the South Low creek, but there is no substantial bank to lift the road above high tide. Similar causeways can be found in the Thames and Essex estuaries, linking some of the islands back to the mainland.
Construction
The 'standard' causeway can be built up from all sorts of material. The Churchill Barriers in Orkney are constructed on top of large pre-cast concrete blocks which were dumped into the channels between the islands. This probably relates to their original purpose being to safeguard the harbour of Scapa Flow, rather than as traffic routes between islands. More recent causeways, such as those linking the islands together in the Western Isles, are more likely to use large blocks of quarry stone as a base. Whether this is down to cost or resistance to salt water, or indeed some other consideration is not certain. The same basic design can also be used to create a ledge along the shore on which a road can sit, as is seen on the A851 as it approaches Armadale on Skye, where a 'causeway' has been constructed along the cliff base to provide a wider and safer road alignment.
Types of Causeway
Coastal Causeways
The majority of Causeways can be found on or around the coast, whether carrying a road to an island, such as Lindisfarne or Eriskay, or crossing an estuary area or sea inlet such as the Morvich Causeway in the Scottish Highlands. Older causeways are generally built of local materials piled into the shallow water and then a road surface formed over the top. Many of these, particularly in the Scottish Islands, have subsequently silted up to some extent, disguising the original construction, further hampered by modern rock armour and stabilisation works designed to protect the causeway both from tidal action, and also modern traffic volumes.
Newer coastal causeways (with the exception of the Churchill Barriers) are generally built largely of rock armour in the first place. This is then topped off with smaller stones and / or concrete on which a road surface can be formed. Unlike older causeways, which were not really designed as such, modern structures are engineered to survive differing tidal and storm forces without being breached or otherwise damaged. Modern causeways generally cross deeper water than the older ones, perhaps several metres rather than just a few inches at low tide. The deepest parts of the channel are often spanned by bridges, such as that at Morvich.
Inland Causeways
In addition to coastal, or island causeways, there are also a number of causeways found inland. The majority of these are comparatively modern, carrying a realigned road across a section of a lake or loch instead of detouring around the shore. One such example is the causeway carrying the A838 across Loch na Thull in the Scottish Highlands. Here, a new road cuts across several sections of the loch, twice cutting off an area of water (although one is now largely dry) and then a third section carries the road along the shore of the loch. However, this is not just a new innovation, and causeways such as the Loch Bee Causeway on South Uist appear to date back centuries. On older inland causeway which can be fairly accurately dated is the Loch Dochfour Causeway on the A82 at the northern end of Loch Ness. This was built by Thomas Telford in the 1810s to carry his new road across the loch with the water level due to rise when the Caledonian Canal and the weir across the River Ness was completed.
There is one final type of inland causeway to consider, and this is in many ways more an embankment than a causeway. In some cases, wide river valleys or flood plains are crossed by embanking the road to protect against flooding. The basic construction methods of embankments and causeways have, in the past at least, been very similar, making it really a matter of semantics as to what a particular structure is called. In general, a causeway should cut across or be built into a lake or loch, but there don't appear to be any hard and fast rules. On the A82 on Rannoch Moor, for instance, the road is carried on a continuous, apparently level embankment from the Tulla Bridge to the shore of Loch Tulla, approximately half a mile in total. There is a short section at the southern end where the road has quite clearly been built across a corner of the loch beach, and comparison with old maps suggests that the beach once extended into the hollow on the opposite side of the road. But is this a causeway or an embankment, or perhaps (despite being a single structure) a bit of both?
The Uists
The largest concentration of causeways in the British Isles can be found on the Uist chain of islands in the Western Isles. Here, the landscape is low lying with large areas of the islands containing more water than land. Causeways are therefore a way of life for travellers, with almost every road on the Uists crossing a causeway at some point. Some of these are inland causeways (see below), but many are coastal, cutting across inlets or estuary areas, even when the sea appears to be a long way away on a map. The two most famous are, of course, the North Ford Causeway and South Ford Causeway, connecting Benbecula to North and South Uist respectively. These were built, in their current forms, in the 1950s and early 1980s. However, elsewhere on the islands many of the shorter, easier causeways show up on the early OS maps from the 1880s and 1890s, and are doubtless much older in some cases.
The oldest causeways often don't have any obvious culvert or pipe to allow the water to flow through, and while it is pure speculation, it is perhaps possible that the loose construction of the causeway allows the water to filter through sufficiently. The silting up of some of the cut off areas of the lochs certainly suggests that any original pipe or culvert has long since been choked.
