Twenty metres deep! The Mesolithic period at the site Yangtze Harbour in the Rotterdam Maasvlakte, the Netherlands. Early Holocene landscape development and habitation.

Introduction In November 2011 archaeologists of City of Rotterdam Archaeological Service (BOOR) conducted underwater research in the Yangtze harbour, Rotterdam Maasvlakte, The Netherlands. The research was carried out by order of Port of Rotterdam Authority and supervised by the Cultural Heritage Agency of the Netherlands. The results of geological, botanical, zoological and archaeological analyses of the retrieved material generated new information on the occupation of a relatively high river dune by prehistoric hunter-gatherers, and on the development history of the surrounding landscape ca. 9,000 years ago. Methods Rather than employing divers the underwater investigations were carried out on board a vessel using a wire-operated, horizontal closing grab. Three small trenches (total area ca. 375m²) were excavated in layers in a fairly controlled fashion. Underwater excavations cannot achieve the same level of precision as is possible on land, but the many soil core samples taken in the project’s preliminary phase allowed detailed descriptions of the geomorphological stratigraphy. The excavation resulted in 316 bulk bags of soil. All soil was sieved on land, using sieves with mesh sizes of 10 and 2mm, after which archaeologists and volunteers carefully sorted the residues, documenting a total of ca. 46,000 finds. Results Plenty of Mesolithic occupation remains were retrieved at all three grab locations, from depths ranging between 17 to 21m below modern MSL. The finds span the age range from ca. 8400 to 6500BC, when the site transformed from dryland (an inland dune) to wetland (drowned delta subsurface). At the foot of the inland dune, the depositional conditions allowed for excellent preservation of bone, charcoal and plant material as well as stone artefacts. The site provides an unusually rich and detailed body of evidence on environmental conditions and the Middle Mesolithic palaeo-economy. The landscape ecotones around the site yielded an abundance of food while gradually being transformed, due to rising sea levels, from a valley containing the rivers Rhine and Meuse into the mouth area of those rivers. At 6500 BC, the site was finally transgressed: drowned in an estuary and swallowed up by the sea. Conclusion The Rotterdam Yangtze Harbour research project demonstrates the preservation of Mesolithic sites along the river Rhine, at depths in nowadays coastal and offshore areas. Furthermore, it demonstrates the feasibility of archaeological investigation of such submerged sites, even at depths of 18 to 20 m beneath sea, lake and harbour floors. Never before had such a submerged site been excavated at such a great depth. The scientific report (in English) will appear in the autumn of 2014, providing a full description of all finds as well as their landscape context.