By Thilanka Siriwardana, PhD candidate, University of Groningen, the Netherlands
Every problem in archaeology
starts with a problem in geoarchaeology…….
Well said Sjoerd!. I have formally started my PhD
programme in November 2020 with the University of Groningen. At the very beginning of my field research
orienting toward the topic, I'm stuck in nowhere. Holocene beaches, inland Pleistocene
palaeo coasts, coastal erosion, new dunes, rapid coastal changes…..! OMG, where
now? At the right moment, a three-day workshop on global geoarchaeology: theory
and practice gave me many solutions and showed many paths to solve some issues
I had. Thank you Professor Ian Sympson
and Professor Sjoerd Kluiving.
A toolbox was nicely presented during three days session, and I tried my best to order it below.
Knowing the soil family
Current COVID – 19 situation
teaching us many lessons and providing many opportunities. Though we missed the
physical meetings and interactions as well as get to know each other, the three-day
session with the resource persons and participants from several regions of the
world proven even the virtual interactions can make a change, and at the end of
three days, I felt like in a family.
And so does the soil. Knowing the
Sri Lankan soil to some extent but not beyond, Ian and Sjoerd's introduction to
the types of soils, their characteristics, archaeological values, and matters
to be concerned clarified soil's global commonalities. Soil family! The World
reference base for soil resources introduced by the Food and Agriculture
Organization of the United Nations (FAO) provides an excellent base to understand
these globally. However, why we need to know these?
We recently studied a megalithic
burial complex in the Dry zone of Sri Lanka, which has Red-Brown soil and low-humic-gley
soil. The land uses slash-and-burn farming, and farmers use individual small
land plots to plant different crops such as black gram, corn, finger millet,
rainfed paddy, sesame, and other crops, but why? On the other hand, the
settlement sites were not visible to the surface, but recent tilling uncovered
an artefact deposition ca. 20-30cm below the surface. Identifying these were
useful to interpret the anthropological and archaeological data recovered from
the study. Geoarchaeology assists us to understand the site and preservation
process. Geology and geomorphology hold many secrets. The Anthropogenic
diagnostic horizons such as plaggic, terric, hortic, irragic, anthraquic, hydragric
and other soil types introduced by Ian will help analyse these.
Identifying the potential soil
formations worth to study archaeology is a large issue in Sri Lankan archaeology.
Except for the cave deposits, some recent Holocene shell beds, and lithic scatters
on the hilltops, most prehistoric and early historical open-air habitation
floors are not known widely yet. Sjoerd's hand-drilling and cross-sections study
in Egypt was able to provide an insight into these issues.
Meeting CleOPaTRa
Soil forming is a process. Pedogenesis
(soil formation) is different from Geogenesis (sedimentation), and soil is
forming in the upper part of the earth’s crust, which is a space-time continuum.
Soils are different in each place, and there are stimulating factors for the
changes. They are,
-
Climate
-
Organisms/Vegetation
-
Parent materials
-
Time
-
Relief/Drainage
The stimulant factors of soil
formation have regional variations. In north-western Europe, the climate has the
least impact comparing to the other very important factors. Climate is a critical
factor in tropical countries as Sri Lanka, where the annul flooding and surface
runoff causative of the soil formation process. The drainage impact is visible
in the central highlands and Sri Lanka's peneplains produced by long periods of
weathering and erosion. The Red-Brown earth is unique to the dry zone of the
latter is archaeologically significant. Siran Deraniyagala widely discussed the
archaeological value and RBE formation conditions in Sri Lanka in his ‘’prehistory
of Sri Lanka’’ (1992). The high or low positioning, availability of podzols, and
erosion can determine the archaeological value of a place, but not necessarily;
these factors define archaeological materials' availability. However, it aids in
decision making.
Sight beyond sight….
Layers… usually, they are visible to the naked eye. However, when we talk about the microlayers and
 |
| Figure 1 Thin sections analysed from the Batadombalena prehistoric cave (Perera and others, 2011, fig. 4) |
events in shorter intervals, the eagle's eye may not be sufficient. The thin section analysis will open a new path to larger-scale investigations and explanations through its microscopic capacities. The colouration, structure, grain orientation, texture, chemical composition will provide an insight into the depositional process and the soil. The human interferences and natural phenomenon leave filmy traces of such activities. Preparing a section of this film is a laborious process, but each slide will bring a story recorded over the years. Types of grains, structure, organic and mineral particles have their script. However, the archaeologist is responsible for making them talk and narrate the story! Combining the view with Scanning Electron Microscope/Energy Dispersive Using X-Ray (Analysis) (SEM-EDX) and X-ray fluorescence analysis (XRF) makes it more profound narration, with their capacity to determine the elemental composition of materials. The thin section analysis has been used in Sri Lankan archaeology for few prehistoric caves and to the Bellan Bandi Palassa Open habitation site.
It is contaminated…. Oh.. we are virtual..
There were many exciting tasks
and the group works assigned during the course. Frankly, in some cases, it took
time for me to make clear some of the works as such were in Dutch, but the team
was so helpful. Thank you, Simone, Ola and Roberto. Another discussion in
groups of 2 was held about our home town's natural landscape, the type of
sediments, and the depositional age. Those were quite exciting questions which
I did not think that much.
All the tasks were equally
challenging and exciting; I would like to note the Tyndrum Historical lead mine
XRF analysis. The task was to work with the table of contaminating elements
from Tyndrum Area B, compare, contrast and explain the levels of contamination
in Histosol, Technosol and Fluvisol samples and establish and discuss the
environmental significance of the contaminant levels identified.
Table 1 – All contaminants in
soil type of each sample reported from Tyndrum Historical Lead mine
|
Soil type |
Sample 1 |
Sample 2 |
Sample 3 |
|
Histosol |
0.6204% |
0.6612% |
0.5106% |
|
Technosol |
24.3065% |
25.02% |
24.0249% |
|
Fluviosol |
14.6965% |
16.9086% |
15.3907% |
Table 2 - Levels of
contaminants in soil types
|
Element |
Histosol (average %) |
Technosol
(average %) |
Fluviosol
(average %) |
|
Chromium (Cr) |
0.02 |
0.01 |
0.01 |
|
Copper (Cu) |
0.00 |
0.27 |
0.20 |
|
Zinc (Zn) |
0.31 |
4.80 |
0.82 |
|
Arsenic (As) |
0.06 |
1.11 |
0.67 |
|
Lead (Pb) |
0.21 |
18.27 |
13.97 |
I'm onto it…
Several questions come up with our study doing at the north-western and northern coasts which has
 |
| Figure 2 Fluvisol (coastal exposure at Arippu-Sri Lanka) contains shell middens and potsherds in top horizon. Deltaic deposit of Aruvi-aru river? Photo-Thilanka Siriwardana |
I
have recorded the pearl shell middens lying on top of a soil formation washed off by
erosion during my fieldworks, another pearl shell midden site lying ca. 4 feet
below the surface and continuously covering by dune formation, processes of
beach ridge formation and covering the old sites. There are various sediments as
alluvial, colluvial, windblown, marine, glacial, volcanic, organic and caves.
 |
| Figure 3 – Arenosol covering Dutch settlement phase (Talai-Mannar, Sri Lanka). The dark brown coloration shows the settlement phase Photo-Thilanka Siriwardana |
While the glacial and volcanic sediments are not available in the Sri Lankan context, other sedimentary systems are apparent. The discussed case studies showed the impact of deforestation to form parabolic dunes in the Netherlands and landscape changes throughout the Holocene in the Netherlands. Similarly, the increasing threat of coastal erosion at Silavathurai-Arippu was identified through satellite images, which could impact the recent marine sand digging off the Negombo coast, Colombo Port City and extensions of Colombo Harbour. Studying the landscape change in the study area is another step to be taken.
Following the workshop, I have started
to gather the GIS data sets of my study area relevant to its geology and
geomorphological changes. I am not sure about the applicability of some tools,
such as chemical analysis, in my study, but being open to many proxies will provide
a better insight into the study area. Knowing how, when, and where to use the
geoarchaeology toolbox at the first level of my PhD research is one of the best
things.
Further readings
Food and Agriculture Organization
of the United Nations, 2014, World reference base for soil resources: International
soil classification system for naming soils and creating legends for soil maps,
Rome
Sorcha Diskin, Vanessa Mary An
Heyvaert, Kosmas Pavlopoulos, Brigitta
Schutt, 2013, Geoarchaeology: A toolbox of approaches applied in a
multidisciplinary research discipline, Quaternary International 308-309 (2013)
1–3Figure 3 Arenosol covering Dutch
settlement phase
Perera, Nimal, Nikos Kourampas,
Ian A. Simpson, Siran U. Deraniyagala, David Bulbeck, Johan Kamminga, Jude Perera,
Dorian Q. Fuller, Katherine Szabó, Nuno V. Oliveira, 2011, People of the
ancient rainforest: Late Pleistocene foragers at the Batadomba-lena
rockshelter, Sri Lanka, Journal of Human Evolution 61: 254-269
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