Establishment, history and internal architecture of Kolabacken, the former beacon at Falsterbo, southern Sweden

Sammanfattning

Kolabacken is a small grass-covered mound near the southern tip of the Falsterbo Peninsula in southern Sweden. The site is known as the former beacon at Falsterbo, where a so-called tilting beacon helped ships navigate along the shallow and dangerous coast before the present lighthouse was built in 1795. Although Falsterbo’s importance for maritime traffic and trade during Medieval times is well known, the internal structure and historical development of Kolabacken have not previously been studied in detail. The study is based on hand-auger sampling to depths of 1 – 6 m at twenty points along two crossing transects through Kolabacken. The sediments were described in the field and later analysed using grain-size analysis, microscopic analysis, portable OSL profiling, quartz OSL dating and radiocarbon dating. Historical maps and written sources were also used to compare the sediment record with the known history. The results show that Kolabacken is a complex structure situated on a former beach ridge close to the early Medieval coastline. The natural beach sand is overlain by a succession of mainly wind-deposited fine- to medium-grained sand, with varying amounts of residues from beacon operation and other human activities. Six sedimentary units were identified: from bottom to top, brown beach sand with gravel, grey sand with charred wood remains and brick fragments, black sand with incompletely burnt coal, yellowish-white aeolian sand, grey sand with coal slag, and greyish-white sand with coal residues and a soil horizon. Four radiocarbon dates on charred wood from the lower unit with wood remains yielded ages in the range of 1025-1260 CE. Considering the inherent age of the trees used as fuel, these dates broadly confirm historical records stating that King Valdemar II allowed the construction of a beacon site at Falsterbo sometime during 1225-1241 CE. A peat fragment from the underlying beach sand was radiocarbon dated to 645-775 CE, showing that the shoreline was located close to the present site of Kolabacken several hundred years before the beacon site was established. The grey sand with charred wood remains represents Medieval beacon activity, when wood, probably from local woodlands, was used as fuel, while the brick fragments may derive from some form of foundation or building at the site. The overlying black sand represents a subsequent early phase of coal burning when large fragments of unburnt or partly burnt coal were left in the deposits. According to historical sources, coal burning probably began during the 1620s. Higher in the stratigraphic sequence, the grey sand with coal slag suggests a later phase of more complete burning, regular cleaning, maintenance, and more efficient beacon operation. Between these coal-rich units is a layer of aeolian sand from the late 1600s. It is unclear whether this layer resulted from a longer period of sand drift or a single storm event, but in combination with war-related interruptions in coal deliveries it probably affected beacon operation. Complementary luminescence dating results provide general chronological support, although dating of this type of mixed coastal and anthropogenic deposits is challenging. Cleaner units of aeolian sand give more reliable results, while the mixed lower units were affected by reworking, partial bleaching and later disturbance. Optimal chronological control is therefore obtained by combining the dating results with sedimentology, microscopic observations and historical records. Potential future studies of the origin and history of Kolabacken could include wood-anatomical analysis of charred wood remains in order to clarify the age and origin of the fuel. Refined luminescence dating of selected sediment layers, combined with comparisons with similar studies from other parts of the Falsterbo Peninsula, could improve the chronology. In addition, possible remains of the tilting beacon and surrounding structures could be mapped using ground-penetrating radar.