Sediments, sediments, sediments – what on earth are they and what to do with them? 

For many students of archaeology and the more general public, soils and sediments might just as well be one and the same. After all, most archaeologists go mindlessly through the layers and, at most, note down the changes in colour or texture only after being forced to do so by the supervisor – a dreaded moment for most beginners. Yet, often we do not know why we are recording any of this information and expect a specialist to magically tell us how the landscape looked at that particular point in time and space. Moreover, it is important to know how the artefacts ended up where they were discovered, and knowing the type of deposition is key to this information. In the process to recognising the mode of deposition, the geoarchaeologist is constantly considering a wide variety of possible scenarios based on the deposits’ qualities. For instance, the size of the grains or stones in a layer depends on the energy involved in the deposition, while the roundedness or angularity of the material in the deposit will reveal whether the material was deposited by gravity or water. The geoarchaeologist then uses this information to answer whether or not a certain layer is a soil or a sediment, a knowledge of which makes the world of difference, as discussed in the next section. 

But what then makes a soil and what is considered a sediment? How does a specialist differentiate between the two? Well, the short answer is that a sediment is the material that accumulates on top of the bedrock through various modes of deposition, while soils are formed in the sediment through chemical, physical and biological processes acting upon the earth’s surface. Among the specialists, sediments are commonly known as the parent materials for soils. Soils represent periods of inactivity in the deposition of material, and the length of time that the landscape has remained unaffected by outside forces can be inferred from the various stages of soil formation. For us archaeologists, identifying the formation stage at which the soil formation was interrupted by human activities, such as cultivation, can be a useful indicator of the kind of environment in which certain behaviours, including agriculture, first appeared. Furthermore, it is possible to go into more detail about the way in which the land was managed. For instance, it is possible to differentiate between slash and burn versus annual cultivation, as the more sedentary the agricultural community becomes, the more they have to depend on fertilising the soil as it becomes depleted from nutrients without fertilisation. 

Typical anthrosol, or human induced soil formation.

What about distant past?

In Palaeolithic archaeology, the importance of soils is slightly different from the archaeology dating to the Holocene. Soils still formed during the Palaeolithic, but today they are often either eroded or overlain by new sediments that subsequently formed soils. The soils that are still preserved and below a sedimentary deposit are called palaeosols. These are very important layers in Palaeolithic archaeology, as they represent stable conditions during which sedimentation processes halted, and thus allow us to infer that no vertical movement of artefacts have occurred. In addition, palaeosols are very good chronological markers. However, because palaeosols may have taken several thousand years to form, any artefact assemblage within a palaeosol is formed during that time period. What this then means is that in palaeosols, the differentiation between occupations is not possible, as the artefacts were simply dropped on the surface, resulting in large accumulations without any layering in between the various visits. In contrast, due to the continuous nature of sedimentation processes, if an artefact assemblage of ten artefacts with a vertical distance of ten centimetres from each other is found in, say, one metre deep sedimentary deposit, each of those individual artefacts may represent individual instances of deposition and therefore represent ten visits to the site. However, these ten artefacts might also represent either a single episode within a reworked deposit, or a secondary deposition of the artefacts by water or colluvium, depending on the mode of sedimentation. These hypotheses would have to be confirmed through micromorphological or fabric analysis. It is therefore easy to understand that recognising the presence of palaeosols and sediments in Palaeolithic sites is of importance, as the interpretation of whether the site was visited, or if the artefacts derive from somewhere else is highly dependent on the identification of sediments. 

 

Cave sedimentation

First, it is important to realise that there are two types of caves: exogene and endogene caves. Exogene caves are commonly known as rock shelters, and they are formed in a different manner than endogene caves, which are caves that have passages and/or chambers. Both types of caves act as natural depositional traps, in which the deposits are mostly protected from subaerial weathering and erosion. Because of this quality, caves represent longer time sequences than open air locations, they preserve human and faunal activities better, and they hold a higher quality climatic record. Studying several well dated cave sedimentary records may reveal changes in regional climate, while changes in local environments are discernible in practically all records. This allows us to reconstruct entire landscapes with or without human presence during the Palaeolithic. 

There are three main types of material contributing to cave sediments: clastic detritus, organic detritus, and precipitated carbonates. The term clastic detritus refers to all rock rubble, cave earth, and water-lain material within a sediment, while organic detritus refers to all organic matter within the sediment. Precipitated carbonates are essentially speleothems, or flow- and dripstones, which may cover entire walls and floors, protecting the artefacts from re-deposition. The proportions in which these three materials are represented in the cave sediment is related to the rock type, size of fissure, groundwater regime, as well as topographical, geological and geographical context. All of these materials can be analysed to reconstruct very fine detail palaeoenvironments and record climatic changes. Their origin is often exogenous, but faunal and human remains may also originate from the individuals who once occupied the cave, whilst the speleothems are always local formations. Organic deposits rarely make up the most of a sedimentary layer, although bone and artefact beds do occur. 

1. Breccias formation under a pitfall trap 2. and 3.Waterlain silts;  4. Den accumulation of bones;  5. Accumulation of bat remains beneath roosting area in the cave roof and accumulation of small mammals beneath owl roosting/nesting area in the cave roof; 6. Water transported mud from further inside the cave;  7. Speleothems; 8. Roof fall; Accumulation of bones of cave bear, died during hibernation in the deeper regions of the cave; 9. Former lower chamber of the cave that acted as a natural trap/ accumulation room of amalgamated debris flows. - See more at: http://historyofgeology.fieldofscience.com/2010/10/if-you-gaze-long-into-abyss-abyss-will.html#sthash.Baj7tbmO.dpuf

1. Breccias formation under a pitfall trap
2. and 3.Waterlain silts; 
4. Den accumulation of bones; 
5. Accumulation of bat remains beneath roosting area in the cave roof and accumulation of small mammals beneath owl roosting/nesting area in the cave roof;
6. Water transported mud from further inside the cave; 
7. Speleothems;
8. Roof fall; Accumulation of bones of cave bear, died during hibernation in the deeper regions of the cave;
9. Former lower chamber of the cave that acted as a natural trap/ accumulation room of amalgamated debris flows.
Taken from: History of Geology Blog - Taphonomy of Cave Environments

 

My research

In my research, I look at the differences between interglacial and glacial deposits. My research asks how do sedimentation processes vary according to the environment in south-western France? Are there any discernible differences between interglacial and glacial deposits? If so, what are they? In trying to answer these questions, my focus is not solely on micromorphological differences, but also macroscopic differences. This knowledge can then be used to determine whether taphonomic factors affect the interpretations of archaeological remains. Moreover, this information may reveal settlement patterns across the landscape if several sites with similar sedimentation processes and timeframes are noted. 

 

Further reading

Farrand, W.R., 2001. Sediments and Stratigraphy in Rockshelters and Caves: A Personal Perspective on Principles and Pragmatics. Geoarchaeology, 16(5), 537-557.

Karkanas, P., and Goldberg, P., 2013. Micromorphology of Cave Sediments. In: John F. Shroder (Editor-in-chief), Frumkin, A. (Volume Editor). Treatise on Geomorphology, Vol 6, Karst Geomorphology. San Diego: Academic Press, 286-297.

Lowe, J., and Walker, M., 2015. Reconstructing Quaternary Environments. 3rd ed. London: Routledge.

History of Geology Blog - Taphonomy of Cave Environments

White, W. B., 2007. Cave sediments and paleoclimate. Journal of Cave and Karst Studies, 69(1), 76-93.