At low tide, the Lang Dyke (also referred to as the Long Dyke) is a prominent feature, running for a considerable distance along the length of the River Clyde. When it is completely submerged at high tide, its course can still be picked out by means of the beacons on top of it.
It is a familiar sight from the northern shore of the Clyde; see, for example NS4273 : River Clyde at Milton
, NS4273 : The Lang Dyke
, and the background of NS4474 : Hill of Dun
, where this section of the wall can be seen, isolated in mid-river, as a dark line connecting some beacons. See also an aerial view which shows the entire length of the wall: NS3974 : River Leven and River Clyde from the air
; also, from the Erskine Bridge: NS4672 : River Clyde and Dumbarton Rock
[The appearance of the dyke in the third of those pictures should make clear why casually wandering out to the Lang Dyke is not a good idea. There is a very real danger of stranding, and my own one-time (and brief) visit was carefully planned with tide tables, and was conducted with an eye on the water.]
This photograph was taken from a point close to NS4273 : The Longhaugh Light
. See NS4273 : The Lang Dyke
for a view from more or less the same point, but looking along the wall in the opposite direction.
The Lang Dyke is the work of the engineer John Golborne. It is a longitudinal training wall, built to help deepen the channel of the River Clyde, to allow shipping to reach Glasgow.
Highlighting the Clyde's former lack of depth is the fact that it used to be possible to walk across the river at low tide near Dumbuck; see: NS4273 : Supposed remains of a Roman causeway
(the line of the supposed causeway leads to the point from which the present photograph was taken). Although useful for those wishing to crossing the river on foot, the shallowness of the channel would prevent ships from reaching Glasgow.
The Dumbuck Shoal was particularly problematic; as early as 1566, workers had attempted to remove this sandbank; at that time, huts were set up near Dumbuck to house a workforce from Dumbartonshire, Renfrewshire, and Glasgow. The workers laboured for weeks to remove the shoal, but their efforts evidently met with little success. After advice had been taken, in 1611, from the engineer Henry Crawfurd, a further attack on the shoal was made; though much effort was expended, the task again appears to have been beyond the means available.
Later, in an attempt to deepen the channel of the Clyde, jetties were constructed, extending out from both sides of the river; these lateral jetties narrowed the channel so that an increased rate of flow over the river bed might scour away some of the more stubborn gravelly shoals.
However, Dumbuck Shoal remained a problem. In the eighteenth century, the Lang Dyke, a longitudinal wall, was built through the shoal in order to further narrow the river's channel; its role is best summarized in the words of Golborne himself:
"The first and grand obstacle is Dumbuck Ford (12 miles below Glasgow Bridge), where, the river dividing itself into two channels, the reflowing current is greatly weakened, and the bottom, being covered with a crust of hard gravel, cannot be worn down to the proper depth; but if a jetty were extended over the south channel, to confine the current, and the hard crust of gravel removed by dredging, the reflowing current would then act with greater force, and soon grind down a deep and capacious channel" [see pp124-126 of "Minutes of Proceedings of the Institute of Civil Engineers ...", Vol XXXVI, Session 1872-73, Part II].
An account of the construction of the Lang Dyke is given in the book "Clyde Navigation" (1979) by John F. Riddell, which notes that "this wall would extend from the lateral jetty at Longhaugh Point downstream to the beacon marking the lower end of Dumbuck Ford".
Construction began in June 1773, and "all building was completed before the onset of winter. In this short space of time almost 800 yards of half-tide training wall was erected along the south edge of the northern channel to contain the current. Considering that all the stone had to be floated out on barges and placed underwater, the construction of the training wall – so aptly named the Long Dyke – was a major achievement".
The book notes that the wall was added to in later years, but that it is the only one of Golborne's dykes to survive later improvements of the river.
[Although it was described above as originally being 800 yards long, the later additions to the dyke were in the form of extensions to the west, reaching as far downriver as West Ferry (which is now gone; it was located at NS39827300
). Although there are some large gaps in the wall near its western end, the remains of the Lang Dyke presently extend for a total distance of at least 2.5 kilometres (1.5 miles). However, on the first-edition OS map (c.1860), it is shown to its greatest extent; there, it reaches from what is now called Garrison Perch Light Beacon (at NS39657389
, north of the former site of West Ferry), to the beacon that marks its present-day eastern termination (at NS43277333
), a distance of about 3.7km (2.3 miles).]