PhD thesis on the Geomorphology of Norse Harbours in Shetland, Iceland and Greenland
In March 2018 John Preston successfully defended his PhD thesis at Edinburgh University. The aim of his thesis was to understand the role of geomorphological change in the abandonment of Norse harbours in the North Atlantic. The harbours 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, which happened on different spatial and temporal scales, required a response from seafarers. In his thesis, John developed a conceptual framework for the formation, recovery and stability of headland-dominated sandy beaches in high-energy environments, 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 were assessed in the Shetland Islands, using numerical modelling, geomorphology and OSL dating on sand blow deposits. In the late Norse Shetland 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, the location of a late Norse farmstead, 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, despite it´s long use since prehistoric times.
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 per 500 years, the landing site became more difficult to access during the later period of Norse settlement due to the safe anchorages becoming shallower, skerries nearshore becoming obscured and key onshore infrastructure was disrupted.
Also, the possible role of terrestrial supplies of sediment in changing the viability of landing places was assessed with an evaluation of the Norse trading centre of Gásir in northern Iceland. Geomorphological mapping and analysis of fluvial connectivity indicated 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 for deeper draft boats such as cogs.
So, geomorphological forces acting on varying spatial and temporal scales have the potential to disrupt the use of landing sites. Whether these 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.
John´s thesis demonstrates that numerical sediment transport analysis, and geomorphological analysis in general, is a powerful tool in coastal archaeological research as it can illuminate processes driving observable changes in the empirical record.
Michael Brandl`s Analyses of Viking-Age “Ballast Flints“
Since February 2015 analyses of ballast recovered from selected harbor sites in the study region are currently ongoing. Numerous materials have been used as ship ballast. Because weight is a crucial factor, it is more than conceivable that stone was always amongst the favored material groups used. A great diversity of rocks was identified within the examined ballast materials, such as quarried schist, soapstone artefacts, and Nordic (or Baltic) flint. While the sources of most of these lithic ballast materials have been identified, the provenance of the “ballast flints” remains elusive. Due to the relatively high specific weight of flint, this material constitutes an important component of the lithic ballast in the Northern Atlantic realm. The lack of natural flint sources in Norway makes the presence of ballast flints in Norwegian harbor contexts especially intriguing. Nordic flint is bound to Cretaceous formations in Northern Europe and occurs, e.g., in Northern Germany, Denmark, Sweden and on the British Isles, directly coinciding with the area under investigation. Most of the flint deposits are situated at the coast, and the weathered flint nodules could easily be collected from the beaches. In the course of geo-archaeological surveys potential primary procurement sites as well as glacial deposits close to the shore will be investigated and the collected raw materials will be used as geological comparative samples. The principle goal of this research is to reconstruct economic aspects of medieval harbor management strategies such as the procurement, use, recycling and final deposition of materials used as ship ballast. These processes will be investigated through provenance studies.
In order to establish the provenance of archaeological ballast flints, Dr. Michael Brandl (Austrian Academy of Sciences) and Dr. Maria Martinez (Post Doctoral Fellow, Department of Anthropology, University of Texas, Austin) will analyse finds from contextually significant sites together with the geological samples petrographically, mineralogically and geochemically. For this undertaking, an internationally acknowledged and approved method – in short MLA (Multi Layered Approach) – will be applied. The MLA combines visual comparative studies, stereo-microscopic analyses of micro-fossil inclusions and geochemical trace element analyses using LA-ICP-MS (Laser Ablation Inductively Coupled Mass Spectrometry). The geochemical data are subsequently evaluated by CODA (Compositional Data Analysis), affording optimal group assignment of the datasets from the archaeological ballast materials to potential geological source areas. Extensive studies have established the Multi Layered Approach as one of the most powerful analytical methods for chert and flint source provenance studies available.
There is legitimate reason to believe that the planned investigations will for the first time allow the characterization of specific source areas on a large scale and afford the separation of particular deposits within the Northern European flint zone. This goal was to date unachieved by any other project, which is mainly due to the fact that there was never the possibility to investigate large sample amounts in a systematic manner applying the best combination of analytical methods. Therefore, our research results can eventually be used for provenance studies of any prehistoric Nordic flint artefact from throughout Europe.
Cooperating partner: Pål Nymoen, Norsk Sjøfartsmuseum Oslo.