Organic petrology in the service of archaeology

Abstract

The techniques of organic petrology have been used to study the nature and provenance of 81 ornaments ranging in age from the Celtic (dated as late Hallstatt-early La Tène tims, 500-300 B.C.) to Roman (1st–4th Century A.D.) periods, which have been recovered from graves and settlements in Germany and Switzerland. The ornaments were mainly black or dark brown armlets but also included beads, buttons and medallions. The most commonly used source material was jet (22 objects) which is derived from bituminized drift woods found in Liassic oil shale, probably mainly of English provenance. Eleven objects were made from Carboniferous cannel coal and four from boghead coal, both of unknown geographic provenance. Only one object proved to be made from Posidonia shale, a liassic oil shale from southern Germany. The identification of two distinctive sapropelites, used mainly for the production of armlets, is of particular interest. These sapropelites are the “Schwarte” from the top of the Kounova Seam (Stephanian) of northern Bohemia (15 objects) and the Kimmeridge “coal” of Dorset, England (19 objects). The recognition of these organic materials was made possible by the study of fresh rock samples. All the armlets made from “Schwarte” were excavated in the Celtic oppidum at Manching in southern Germany; the armlets made from Kimmeridge “coal” were found in Celtic and Roman graves. These discoveries suggest the existence of early trade routes crossing the English Channel and passing south to Switzerland, probably along the River Rhine. A very few armlets were made from dark tuff (probably of Bohemian origin), black glass or dark brown bone. These materials were probably all used as substitutes for jet and/or sapropelites. It is interesting to note that all these dark armlets were thought to possess magic properties, which may explain their frequency.