Thursday 16 August 2018

The submerged prehistoric forests on Trusthorpe and Cleethorpes beaches, Lincolnshire

The aim of this post is simply to share some recent images of the underwater prehistoric forests at Trusthorpe and Cleethorpes, Lincolnshire. The submerged forest at Trusthorpe is only rarely seen, especially since beach replenishment works along the coast here; however, an unusually low tide on Monday 13th August, 2018, exposed at least two of the tree stumps and I was able to take the following pictures of these.

One of the tree stumps exposed on Monday, 13th August 2018 at Trusthorpe, Lincolnshire; click here for a larger version of this photograph (image: C. R. Green).

Top view of the submerged prehistoric tree stump exposed at Trusthorpe, Lincolnshire, in August 2018, showing its tree rings; click here for a larger version of this photograph (image: C. R. Green).

Another of the prehistoric tree stumps exposed by an exceptionally low tide at Trusthorpe, Lincolnshire; click here for a larger version of this photograph (image: C. R. Green).

The tree stumps and trunks that are revealed by such very low tides and in excavations all along the Lincolnshire coast from Immingham to Ingoldmells have their origins in a drowned prehistoric forest that once stretched out over what is now the floor of the North Sea after the last Ice Age, when global sea-levels dropped to around 120 metres below their current levels. For the early part of the Mesolithic era, beginning c. 9600 BC, the actual coastline lay a significant distance to the north-east and eastern Lincolnshire represented part of an upland district rather than a coastal zone. However, from about 8,500 years ago, this situation began to change as the inexorably rising sea-level due to the melting of the glaciers pushed the coastline ever nearer. Sometime around 6200 BC, the land bridge connecting Britain to the continent was severed, perhaps being finally destroyed by the Storegga Slide tsunami, and by approximately 6000 BC the flooding of what remained of Doggerland had advanced sufficiently that the coastline probably lay just to the seaward of its present position along much of east Lincolnshire. As this process continued, the trees that are now found submerged off the Lincolnshire coast were first subject to waterlogging as the water-table rose and were then submerged by the rising tide. The date of this waterlogging and submersion varies from site to site, depending on the elevation of the land on which the forest grew: at Immingham and Theddlethorpe the waterlogging of the prehistoric landscape has been dated to 5840–5373 BC and 6205–6012 BC respectively, whilst at Anderby Creek and Cleethorpes the trees on the foreshore were submerged in 3514–3349 BC and 2912–2299 BC, as determined by the radiocarbon dating of their wood.

The extent of Doggerland about 12,000 years ago at the end of the last glacial era, with possible reindeer migration routes shown (drawn by C. R. Green for Origins of Louth, based on Barton, 2005 and Shennan et al, 2000, with permission).

The last stages in the drowning of Mesolithic Doggerland, from the perspective of Lincolnshire and the Fens (drawn by C. R. Green for Origin of Louth, based on Shennan et al, 2000, with permission). Louth is marked to help in understanding the changes; darker blue indicates areas permanently under water, light blue the inter-tidal zone and low-lying marshland.

The photographs of submerged trees at Trusthorpe included above were taken at approximately 14:30 in the afternoon, when the tide was at its lowest point of 0.4 metres above chart datum, equivalent to around 3.35 metres below Ordnance Datum. Unfortunately, this wasn't quite low enough to expose more than a handful of tree stumps, especially after beach replenishment works along this coast, although wading a short way out beyond the shoreline revealed a number of additional tree stumps lying just below the water's surface. A number of photographs are available online of the more dramatic exposures in the Mablethorpe to Huttoft area visible in previous decades, especially those in 1984 and 1992, although none of these in turn seem to approach those recorded in previous centuries, leading to the suggestion that the drowned forest remains have been subject to recent erosion as well as being covered up by beach replenishment schemes. In particular, the outcrop of exposed forest seen in 1796 by Sir Joseph Banks and Joseph Correa de Serra was around 1 mile wide just to the south of Trusthorpe at Sutton-on-Sea (something also apparent on Robert Mitchell's 1765 coastal sailing chart, where the forest 'islets' are marked as a wide belt of 'Clay Huts' between Sutton and Anderby Creek), whereas in 1923 it was only 150 yards wide. According to A. J. Clapham, 'even allowing for the shifting pattern of the sand covering the foreshore and the fact that the tides might not have fallen as low in 1923 as on the 1796 visit, this is evidence for considerable erosion of the outcrop in a century and a quarter'.(1) With regard to the 1796 exposure, it is worth quoting Joseph Correa de Serra's 1799 description of the 'submarine forest' at length as an indication of what was visible in the eighteenth century:
It was a common report in Lincolnshire, that a large extent of islets of moor, situated along its coast, and visible only in the lowest ebbs of the year, was chiefly composed of decayed trees. These islets are marked in Mitchell's chart of that coast, by the name of the clay huts... In the month of September, 1796, I went to Sutton, the coast of Lincolnshire, in company with the Right Hon. President of this Society [Sir Joseph Banks], in order to examine their extent and nature. The 19th of the month, being the first day after the equinoctial full moon, when the lowest ebbs were to be expected, we went in a boat... and soon after set foot upon one of the largest islets then appearing. Its exposed surface was about thirty yards long, and twenty-five wide, when the tide was at its lowest. A great number of similar islets were visible round us, chiefly to the eastward and southward... These islets, according to the most accurate information, extend at least twelve miles in length, and about a mile in breadth, opposite to Sutton shore... The channels between the several islets [representing the eroded lines of drainage from wave backwash], when the islets are dry, in the lowest ebbs of the year, are from four to twelve feet deep.(2)
Banks and De Serra examined the composition of these 'islets' on the 19th, 20th and 21st of September, 1796, and concluded that
they consisted almost entirely of roots, trunks, branches, and leaves of trees and shrubs, intermixed with some leaves of aquatic plants. The remains of some of these trees were still standing on their roots; while the trunks of the greater part lay scattered on the ground, in every possible direction. The bark of the trees and roots appeared generally as fresh as when they were growing; in that of the birches particularly, of which a great quantity was found, even the thin silvery membranes of the outer skin were discernible. The timber of all kinds, on the contrary, was decomposed and soft, in the greatest part of the trees; in some, however, it was firm, especially in the knots.... The sorts of wood which are still distinguishable are birch, fir, and oak...
     The soil to which the trees are affixed, and in which they grew, is a soft, greasy clay; but for many inches above it is entirely composed of rotten leaves, scarcely distinguishable to the eye, many of which may be separated by putting the soil in water, and dextrously and patiently using a spatula, or a blunt knife. By this method, I obtained some perfect leaves of Ilex Aquifolium [holly], which are now in the Herbarium of the Right Hon. Sir Joseph Banks; and some other leaves which, though less perfect, seem to belong to some species of willow. In this stratum of rotten leaves, we could also distinguish several roots of Arundo Phragmites [common reed].

Robert Mitchell's 1765 coastal sailing chart of Lincolnshire, showing 'clay huts' (islets of exposed submerged forest separated by deep eroded backwash channels) extending significantly offshore from Sutton to Anderby Creek.

Whilst only the tops of a few tree stumps were visible at Trusthorpe as a result of the unusually low tides this August, rather more was visible of the submerged forest at Cleethorpes on 14 August 2018 (when low tide was only 0.1 metres higher than on the previous day) and some pictures from this visit are shared below as a comparison. As was noted above, the forest at Cleethorpes is perhaps a thousand years younger than that further south at Mablethorpe–Anderby, being probably drowned in the Late Neolithic era, and both this and the lack of intensive beach replenishment as seen elsewhere on the Lincolnshire coast may explain why significantly more trees are visible here. In any case, as can be seen from the pictures below, a variety of fallen tree trunks, stumps and roots were easily to be seen on Cleethorpes beach without having to venture too far out, many well-persevered due to a layer of marine crustaceans overlying them.

A tree stump from the Late Neolithic drowned forest on Cleethorpes beach; click here for a larger version of this photograph (image: C. R. Green).

Two fallen trees from the Late Neolithic drowned forest on Cleethorpes beach; click here for a larger version of this photograph (image: C. R. Green).

Another tree trunk from the Late Neolithic drowned forest on Cleethorpes beach; click here for a larger version of this photograph (image: C. R. Green).

A tree stump from the submerged prehistoric forest on Cleethorpes beach; click here for a larger version of this photograph (image: C. R. Green).

Another piece of the drowned Late Neolithic forest visible on Cleethorpes beach; click here for a larger version of this photograph (image: C. R. Green).


1.     See A. J. Clapham, The Characterisation of Two Mid-Holocene Submerged Forests (Liverpool John Moores University PhD Thesis, 1999), pp. 62–4, for a brief discussion.
2.     This and the following quotation are taken from J. C. de Serra, 'On a submarine forest, on the east coast of England', Philosophical Transactions of the Royal Society of London, 89 (1799), 145–56.

The content of this post and page, including any original illustrations unless otherwise stated, is Copyright © Caitlin R. Green, 2018, All Rights Reserved, and should not be used without permission.