Geomorphology of Viking and medieval harbours in the North Atlantic
Preston, John Ian
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The aim of this thesis is to understand the role of geomorphological change in the abandonment of Norse harbours in the North Atlantic. Nodes of maritime activities that were established by Norse settlers during the Scandinavian Viking Age often developed into important towns and cities. Some of these, however, disappeared for unknown reasons. Norse harbours in the North Atlantic varied in scale. They ranged from small landing beaches used by small boats for local use through to much larger anchorages handling considerable trade and being important nodes on the transatlantic trading network. Changes in coastal geomorphology necessitated a response from seafarers. In this thesis, a conceptual framework for the formation, recovery and stability of headland-dominated sandy beaches in high-energy environments is established, based on empirical observation and on the use of the MIKE21 numerical sediment transport model. Under persistent calm climatic conditions, nearshore seabed gradient is a weak control on beach formation and persistence in embayments. However, under persistent stormy conditions, nearshore sea bed gradient becomes the prominent control. Embayments with nearshore gradients of > 0.025 m/m inhibit beach recovery on a sub-annual timescale, while gradients < 0.025 m/m promote beach recovery. These ideas are assessed in the Shetland Islands, using numerical modelling, geomorphology and OSL dating on sand blow deposits. In the late Norse era beach landing sites in Unst became prone to depletion and destruction because of increased storminess. Numerical modelling (MIKE21) supports the idea that the recovery time of different sandy beaches on Unst is dependent on average nearshore slope. The beach at Sandwick has shallow nearshore gradients and recovers quickly in the face of storminess, but beach stability at Lunda Wick is more uncertain, and thus Lunda Wick represents a more problematic landing place. The Norse harbour of The Bishop’s seat at Garðar in the Eastern Settlement of Greenland is assessed to evaluate the impacts of gradual long term geomorphological change on coastlines that lack soft-sediment. A high resolution, near shore bathymetric survey shows that, due to relative sea level rise of 1 m/500 years, the landing site became more difficult to access during the later period of Norse settlement and key onshore infrastructure was disrupted. The possible role of terrestrial supplies of sediment in changing the viability of landing places is assessed through an evaluation of the Norse trading centre of Gásir in northern Iceland. Geomorphological mapping and analysis of fluvial connectivity indicate that the delta on which Gásir is located is prone to aggradation from large, irregular pulses of sediment derived from landslides in the catchment. Written sources and geomorphological mapping indicate geomorphological changes around the same time that trade was shifting to the use of boats with a deeper draft. Cultural change and environmental changes would have reinforced each other in rendering the harbour site nonviable. Geomorphological forces acting on varying spatial and temporal scales have the potential to disrupt the use of landing sites. Whether environmental changes led to the abandonment of a landing site was strongly influenced by the seafarers’ competence and available technology. Higher levels of competence would enable more problematic landing sites to be used, but there are limits to this adaptation. Technological changes, such as the use of larger and deeper draft boats, would have changed the geomorphic requirements for harbour sites, and thus may have led to a passive abandonment of the site over time rather than active abandonment such as that in the face of a catastrophic change of the shoreline. Coastal geomorphology was a critical factor affecting the use of Norse harbours, as it interacted with the wider cultural and economic developments in the North Atlantic realm. This thesis demonstrates that numerical sediment transport analysis is a powerful tool in coastal archaeological research as it can illuminate processes driving observable changes in the empirical record.