The Dunes of Sabloney near Arcachon

Abstract

THE largest sand dunes in Europe are on the shore of the Bay of Biscay. Those in the neighbourhood of Arcachon are particularly interesting as one is a hill 300 feet high, which is changing its shape as well as its position. This part of the country was practically waste at the end of the eighteenth century, when trees were planted on a large scale, as it was seriously feared that the belt of sand dunes along the coast would advance inland and engulf important towns. The forests thus formed have stabilized the dunes. But at the place where they have grown to their maximum height they are quite bare and appear to be engulfing the surrounding forest. It is obvious that some movement is taking place, but no measurements appear to have been made to ascertain the extent and speed of this movement, so I have taken some photographs and have cut marks in the trees during the last few years, in order to obtain a rough indication of what is really happening. No map exists showing accurately the configuration of the land, so it is necessary to explain that the forest is about 50 to 100 feet above sea-level. From it rises the sand hill called the Dune de Sabloney. On its eastern face the hill has a bare and precipitous slope extending from the top to the base without any break. On the western and southern sides the slope is much less abrupt. On the northern side there is a plateau at an elevation of about 200 feet reached by a steep ascent from the forest (Plate 3). From this plateau to the top of the hill the ascent is less steep (Plate 1). On the plateau are three parallel ridges extending from east to west. The absence of fully grown trees here indicates that the sand has not been stationary long enough for the trees to grow up before being engulfed. To ascertain exactly what movement is taking place here, I cut a mark 5 feet above the ground-level in the trunk of a tree standing somewhat apart from the others. The following year I revisited the spot on 24 June 1925, and took a photograph (Plate 1); the mark then was completely covered by the sand, but the tree can be seen on the left-hand side of the picture. Plate 2, taken three years later (18 September 1928), shows that only the topmost twigs of this tree remain exposed. They can be seen in front of the standing figure. The twigs in front of the reclining figure comprise the tops of the trees seen in the centre of Plate 1. To determine the movement of the hill on its northern and north-eastern faces two spots were selected. The former, shown in Plate 3, is at the end of a coupe feu, a wide lane cut to arrest forest fires; while for the latter was chosen the highest point in the forest, where there is a large clearing, probably due to a comparatively recent fire. To mark the base of the hill on its northern side on 24 October 1924, I cut a mark at eye-level in a tree. On returning to this spot eight months later, I found that the base of the hill had advanced 4 yards northwards. But in the course of the succeeding three and a quarter years the base advanced only another 7 yards. Thus the advance averaged 3 yards per year for the entire period of observation. Plate 4 was taken in September 1928 at the north-eastern side of the hill. Here the advance averaged 6 yards per year from October 1924 to September 1928. On the