Saturday, 12 May 2012

The importance of geology

The mortaria workshop in Lincoln was very interesting and reminded one of a few basic realities in archaeology. First of all, the history of early mortarium production is little known. Mostly because the early production took place in the area were the castle and the cathedral stand and that naturally are off limits of most of invasive archaeology. This is a common problem in archaeology since later Medieval towns often cover the Roman and earlier centres in central and southern Europe.

One of the later kiln sites lie in the area where there were also kilns during the 10th, 12th and 14th century. The reason for this longevity became clear from the presentation of the local geological map where there was a narrow strip of a clay formation poking to the surface by the river valley. This outcrop was used for millennia. This reminds us of longevity of the exploitation sites of key resources. This has its implications on the knowledge of the earlier use since often the signs of earlier exploitation have disappeared or are in danger due to modern extraction.

The geology presentation

The type of clay available at any of the mortaria sources defined the outer appearance of the products of different mortarium workshops. Some of them have orangey tint due to the iron in the clay whereas the white clays that lacked the iron resulted with whitish vessels. This was also the difference between the local deposits at Harthill on one hand and Nene valley and Oxford on the other and helps to recognize the products of these most important workshops.

The later industries, copying the appearance of the Nene valley types can be separated from each others by observing their temper. Some areas are flint-poor whereas some had peat iron ore and this iron production fed to the related pottery production as well. The careful analysis of minerals and rocks used may allow a more detailed characterization and actually outdo the expensive scientific methods that promise much but cost a lot.

Geology defines other productions as well. The stones used to erect Stonehenge have been the object of study for a long time and may continue still even if Dr Bevins, keeper of geology at National Museum Wales, and Dr Rob Ixer of Leicester University, suggested the area in Pembrokeshire as the source of sarsens.

During even earlier times it was the occurrence of flint that was the defining feature. In the areas without flint the lack of the best source material for stone tools often resulted with brisk trade at some point with other regions with this material. Similarly, the circulation of obsidian in the Mediterranean is well-documented.

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