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Learning legacyLessons learned from the London 2012 Games construction project

Environmental analysis AbstractA large number of environmental remains were recovered and studied from the Olympic Park which have helped build up a picture of the past environment of the local area and to see how the landscape changed over time. This analysis, together with the archaeological remains and finds from the site, also helped show how people were using the area of the Park in the past.

The wide range of environmental remains analysed from the Park included waterlogged plant remains, charred plant remains, wood charcoal, pollen, insects, fresh-water molluscs and land snails, ostracods and diatoms.

These remains are preserved in a number of ways, such as by being in waterlogged deposits or by being charred. The preserved remains are very different both in terms of their size and the type of material recovered.

Environmental material was recovered from sediments in a number of ways, using different sampling techniques,

sample sizes, processing methods, sieve sizes and chemicals, depending on the particular type of remains.

Some environmental remains provided information on the wider landscape, such as pollen, on the more localised environment (waterlogged plant remains and molluscs), on the quality of and salinity levels of the water (diatoms), and on specific settlement activities (charred plant remains and wood charcoal).

It was the combination of the analytical results of all the different types of environmental remains that gave the clearest idea of what the changing environment of the Park was like.

The environmental data revealed landscape changes from open wetlands some 10,000 years ago, to alder carr some 6,000 to 3,500 years ago, gradually becoming transformed into the open marshland and pasture that characterised the area before the onset of the industrial factories during the Victorian period and the early 20th century.

AuthorsChris J. Stevens,Environmental Archaeologist, Wessex Archaeology

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IntroductionAn essential component of the archaeological investigations in the Olympic Park was the recovery and analysis of environmental remains. These included the preserved remnants of plants (such as charcoal, seeds and pollen) and animals (such as insects, snails and microfossils), as well as sediments1.

Together, these provide invaluable information about the past environment of the Lea Valley, against which we can place the archaeological evidence for past human activity and settlement.

The oldest organic materials preserved on the site date from the end of the last Ice Age, some 12,000 years ago – the youngest to the Victorian period.

Since many of the organisms, whose remains were recovered, have preferences for certain types of habitat, they help us to build up a picture of the environment and vegetation from that early time up to the present, and to understand how these have been affected and modified by both natural events and human activity.

How are environmental remains preserved?Past organisms are all subject to biological decay and under normal circumstances are unlikely to survive for even a few years, let alone for millennia.

Harder materials like animal bone and shell, composed largely of calcium,

are less readily decomposed, but even they break down in acidic soils or where the soil conditions alternate between wet and dry.

Certain parts of plants, too, such as the sturdy walls (exines) of pollen and spores, are quite resilient to decay. Softer material, such as plant tissues, insects’ exoskeletons or animal tissues, however, are far more rarely preserved.

For such materials to survive through the years, from when the organism died up to their point of archaeological recovery in the present, the mechanisms that usually result in their decomposition need to be halted.

The most common form of environmental preservation occurs under waterlogged conditions, where the lack of oxygen prevents decay. Such conditions were present in the slowly accumulating sediments that were laid down in the Lea Valley flood plain; they also allowed the preservation of a wide range of organic artefacts, including worked wood and leather.

Organic materials can also be preserved where they are exposed to fire, with charring preventing decay. As such materials are commonly associated with human activity, they can provide important insights into many aspects of social and economic life.

Material dates from the end of the last Ice Age to the Victorian period.

Categories of environmental material from the Park site and sources of such material within the wider valley landscape © Wessex Archaeology

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Some organic materials can also be mineralised, where their soft tissues are replaced by hard minerals. Although no instances of mineralisation were found during the Park investigations, it is a common phenomenon found, for instance, in the cesspits of medieval London.

What is preserved?Waterlogged and charred plant remains were the main forms of organic material found on the Park site. The parts most likely to survive are their harder, more resistant reproductive bodies, such as the seeds, nuts, fruit stones and much smaller pollen, although thorns, twigs and branches were also recovered, along with occasional stems and roots. More delicate parts, such as buds and leaves, were also occasionally preserved in the finer sediments.

Given that the River Lea rises in the chalk spring lines of the Chiltern Hills to the north, the water is quite ‘hard’ (calcareous). This contributed to the survival, in many deposits, of animal bones and the shells of freshwater molluscs, terrestrial snails and marine shellfish, all of which are composed of calcium carbonate.

The magnesium calcite ‘shells’ of microscopic aquatic organisms known as ostracods were also preserved. Preservation was much poorer, however, in locations where soils had become more acidic.

The exoskeletons of insects, ticks and mites, which are made of a hard material called chitin, were also preserved in waterlogged deposits, particularly common being beetles (Coleoptera) – especially their wing-cases (elytra).

Also recovered, and similarly composed of chitin, were the reproductive bodies (statoblasts) of Bryozoa, a group of fresh-water, filter-feeding organisms, along with the softer egg pouch (ephippium) of the water flea (Daphnia sp.), which is actually a small crustacean.

A further class of material were the hard, siliceous skeletons (frustules) of diatoms, a group of algae found in fresh or brackish water, in puddles or even in damp soils.

How were environmental remains recovered?Most environmental remains were very small and could not simply be retrieved by hand. Rather, they had to be recovered from soil samples taken from sediments and from archaeological deposits.

A large proportion of the environmental samples were taken from long sequences of water-lain sediments that had accumulated in abandoned river channels, or across the flood plain of the Lea Valley.

A selection of freshwater and terrestrial (Cepaea nemoralis) mollusc shells from the Park site © Wessex Archaeology

Gravel terrace, sand-bars and channel sequences © MoLAS/PCA

A range of remains was found including plants, molluscs, pollen, and insects.

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These deposits, found at depths of up to nine metres below the modern ground surface, spanned hundreds and sometimes thousands of years. They were sampled both in 5–10 litre bulk samples taken at regular intervals from columns through the sediment sequence, or by monoliths – long, thin, continuous blocks of sediment. The monoliths were used to sub-sample sediments for microscopic remains, such as pollen, diatoms and ostracods.

The ditches of ancient fields, and the pits, hearths, gullies and post-holes of prehistoric settlements, all potentially contain environmental material. Because the Park settlements were usually located on drier parts of the gravel terraces, with features seldom penetrating deep into the water-table, they produced few waterlogged remains. Charred plant remains and wood charcoal, however, were relatively common, providing information about those settlements’ economies.

How were environmental remains processed?The ways in which environmental remains were recovered depended largely on their size. Microscopic remains not readily visible to the naked eye, such as pollen and diatoms, had to be processed under strict laboratory conditions, usually involving combinations of fine sieves, chemicals and centrifuges to separate them from the sediments in which they

Pollen slides from the Park site © Wessex Archaeology

Examination of flots and residues for molluscan remains © Wessex Archaeology

were preserved. They were then mounted on slides and examined under high-powered microscopes.

Larger, macroscopic remains, such as seeds, molluscs, insects and small animal bones were extracted from the bulk samples, and then sorted and identified using low-powered microscopes.

Waterlogged plant remains, molluscs and ostracods were recovered by passing the samples through a stack of sieves with meshes of decreasing size (from 4mm down to 0.25mm, or 0.063mm for ostracods). The molluscs and ostracod remains were dried before being sorted, extracted, identified and quantified, but the waterlogged plants were processed while still wet.

Charred remains were separated using a technique known as flotation, which involves the separation of charred and organic material using water, in which the charred material floats to the surface with the aid of fine jets and is then caught using a sieve or a nest of sieves.

Insect remains were recovered by a similar process, although here paraffin is mixed into the sample; this coats the impervious chitinous exoskeletons so that when cold water is slowly added the oil rises to the surface bringing the insect remains with it.

Deposits up to nine metres below the modern ground surface were sampled.

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Plant remainsWaterlogged plant remainsMany preserved seeds, fruit stones, nuts, leaves, stems, thorns and twigs will fall close to where the plants grew, although they can be carried downstream by water. As a result, they usually provide a very localised picture of the vegetation in the area; this can complement the wider evidence provided, for example, by pollen.

Unsurprisingly, waterlogged remains, identified by reference to modern plant material, are dominated by aquatic species that grew in the shallows of the river, and species growing on the damp soils along the river bank and on the flood plain. These included alder cones, catkins and seeds, hazelnuts, and a great many seeds of wet grassland and marshland species, such as buttercups, rushes and sedges.

The recovery of remains from many different locations across the Park has provided a detailed picture of a variable and changing river environment. It has revealed the gradual transition from the Late Glacial marshland to the open scrub woodland that spread through the valley bottom during the Early Mesolithic.

There then followed a long period of alder carr (wet woodland dominated by water-tolerant alder trees), which gave way eventually to marsh and wet-grasslands. These grasslands

Waterlogged plant macrofossil remains recovered, demonstrate a range of environments including woodland, riverine, marsh and wet grassland © Wessex Archaeology

Alder carr in July, showing young ripening female catkins ‘cones’ © Karen Nichols

emerged around the time of the first evidence for permanent settlement in the Park, in the Bronze Age, and they persisted right up to the widespread construction of factories across the valley in the 19th century.

Charred plant remains and charcoalCharred plant remains, such as seeds, grains and chaff (the waste from cereal processing), are usually identifiable from their size, shape, texture, and the presence of appendages – in comparison to their modern uncharred counterparts.

Charcoal, in contrast, is identified by its often unique cellular structure – consisting of rays and pores – which is visible when the charcoal is split along three planes and examined using a high-powered microscope.

By its nature, most of the charred material was recovered from those parts of the Park where human settlement, or craft and industrial activity, was once present, as indicated by hearths, pits, house gullies, post-holes and ditches.

Much of it was either the remains of food plants, mainly cereal crops, that were discarded into fires during the preparation of food, or the remains of wood collected from the local woodland and flood plain scrub as fuel for cooking, heating and craft activities, or even, in a few cases, for funeral pyres with some of the charcoal ending up in the graves with the cremated bone.

Plant remains, charcoal and pollen were used to build up a picture of the local environment.

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Unlike the preparation of many foodstuffs, the processing of cereals produces a lot of combustible waste, particularly the more traditional hulled wheats – emmer and spelt – that were used in prehistoric and Roman times.

While the grain and chaff of modern cereals are separated when the crop is threshed, emmer and spelt tended to be stored as ‘spikelets’ – the unprocessed heads of the plants. The chaff was only removed as and when grain was needed to make into flour, with the result that the waste was often tossed into the nearest hearth.

However, also charred during this process were the seeds of weeds that grew among the cereals, such as vetch, wild pea, oats, brome grass, bedstraw, pale persicaria and cleavers. Because some weeds prefer particular soils, they provide clues as to what types of soils, and therefore which areas of the landscape, were being cultivated.

For example, some samples contained weed species that are rarely found in arable fields today, such as sedge and common spike-rush; these grow around the wetter edges of the river channels, possibly indicating the seasonal flooding of fields.

Schematic diagram showing the sources of charred material seen at within Middle Bronze Age to Middle Iron Age samples from the Park sites © Wessex Archaeology

PollenOnce the pollen sample is prepared from a monolith sub-sample, the pollen grains are identified to species, genus or broader taxonomic groups, and quantified. This allows the production of a ‘pollen diagram’ that shows the number of grains of each species as a percentage of the total pollen count within each sediment sub-sample down through the monolith column. The changing frequencies of these species in the diagram reflect changes in vegetation back through time.

The pollen of different species varies in the way it is dispersed. Pine, for example, produces a large amount of pollen that is dispersed on the wind (anemophily) and so is widely distributed. Aquatic species, such as pondweeds (Potamogeton sp.), disperse their pollen by water (hydrophily), so potentially may be over-represented in waterlain sediments. Other pollen (such as lime) is dispersed by animals, in particular by insects (entomophily).

Some plants are self-fertilising, either on the same plant (geitonogamous), or even within the same flower (autogamous) such as wheat; because such pollen is produced in much smaller quantities and is less widely dispersed, it is often very poorly represented in sedimentary deposits.

Some samples contained weed species rarely found in arable fields today, possibly indicating seasonal flooding.

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As a result, some plant species were better represented in the Park’s peats and sediments than others, and all these factors had to be taken into account when trying to reconstruct the past vegetation of the Lea Valley landscape.

Nonetheless, the pollen has provided evidence of the most significant changes in that landscape, recording the major transitions from the treeless tundra steppe that dominated the valleys at the end of the last Ice Age 12,000 years ago, through pine and hazel forests, to oak and elm with alder carr on the flood plain, cumulating in the open grassland and marshes that dominated the landscape before the arrival of the Victorian factories.

InsectsWhile insects will not travel as far as some of the pollen, they often move much further than the plant remains recovered from waterlogged samples, and so provide a picture of the ecological habitats within the general vicinity of the deposits from which they were recovered.

The most common insects were beetles (Coleoptera), although heads and cases of the caddis fly (Trichoptera) were also frequently recovered. Some species of insect recovered indicate general habitat types, such as the leaf beetle (Donacia sp.) whose larvae feed in the submerged leaves and stems of aquatic plants.

Pollen found, from a range of environments spanning thousands of years including grasslands, alder carr and deciduous and coniferous woodland © Wessex Archaeology

Other insects are very closely associated with particular types of plant, such as, for example, three types of weevil: Notaris sp. commonly associated with sweet-grasses; Sitona sp. which favours clover; and Tanysphyrus lemnae found on duckweed. These plant species are not always represented by their pollen or plant macrofossils, so the insects can provide valuable insights into the vegetation that cannot be gleaned from elsewhere.

Some insect species indicate the presence of animals. For example, dung-beetles (Aphodius sp.) were useful in showing areas where large domesticated mammals were grazing, especially in deposits that were dated to the Roman and Saxon periods for which there was little direct archaeological evidence.

Their appearance in Early Mesolithic deposits at a location at the north of the Park is also of some interest as it suggests that wild animals, such as aurochs (wild cow), wild horse, or boar, were using this location as a regular place to drink from the river.

MolluscsShells of various aquatic molluscs survived in many of the deposits from the Park, as well as those of species characteristic of grasslands, marshes and wooded carr. Taken together they provided a detailed picture of the changing environment.

Insects provided evidence for plant species not otherwise represented.

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Fresh water molluscs and land snailsDifferent species of mollusc will favour different sorts of habitat. Terrestrial species, for example, can be separated according to whether they are shade-loving, open-country or intermediate species.

However, given the prominent role of the River Lea in the formation of many of the sampled sediments it is unsurprising that most of the mollusc shells recovered were from fresh-water species. These could be grouped into amphibious species, marsh-loving species, species associated with moving-water, ditch-loving species, or intermediate fresh-water species inhabiting environments between slow and fast flowing water.

Counts of the shells of each species within a sample can be displayed as percentages in a bar-chart, and when the species from columns of samples are grouped by habitat this can show changes to the environment over time.

Terrestrial snails, which often range only tens of metres at most, provide a localised picture of the environment at any given time. Aquatic shells, however, can be moved by the water, although an indication of how far they might have moved is given by calculating the ratio of the number of shells to ‘opercula’ (the hard ‘trap-doors’ that cover the shell’s mouth) of the aquatic snail Bithynia.

The operculum is lighter than the shell, and more easily transported by water, so where the ratio is close to 1:1 we might suppose that the material has been not moved far and that the water was flowing relatively slowly.

The most significant aspect of the mollusc analysis was in their indication of the fluctuating rates of water-flow within the channels and tributaries of the River Lea. In the earliest periods the molluscs indicate relatively fast- flowing waters, but by later periods they show that, apart from intermittent periods of increased flow, many channels had become stagnant, with some silting up completely and developing into damp flood plain grassland.

Ostracods and diatomsOstracods are minute aquatic crustaceans (related to crabs, crayfish and lobsters). While some species live in the sea or in brackish water, only those associated with freshwater habitats were recovered from the Park.

Diatoms, which are the hardened silica bodies of photosynthesizing algae, are found in many different watery environments from rivers, ditches, estuaries and the sea, to just a puddle or wet mud. Because they are highly sensitive to water conditions they can provide information not only on its salinity and temperature, but also on its chemical environment, and whether it is stagnant, slow-moving or fast-flowing.

As such, diatoms are the main source of evidence for the increased tidal reach which occurred during the Roman period and in the 13th–15th centuries AD. Although no other environmental indicators for tidal influence were found along the channels of the River Lea within the Park, diatoms may have been transported slightly further upstream at times when the tidal reach was at its maximum, such as on tidal surges during off-shore storms.

Molluscs provided an indication of water-flow in the channels and tributaries of the River Lea.

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References1 Available from: http://learninglegacy.london2012.com/documents/pdfs/

archaeology/ll102geoarchaeology-aw.pdf (accessed 5 December 2011).

AcknowledgementsThis report was written by Chris J Stevens with contributions by Nigel Cameron, Catherine Barnett, Dana Challinor, Michael J. Grant, David Norcott, John Russell, David Smith and Sarah Wyles.

© 2011 Olympic Delivery Authority. The official Emblems of the London 2012 Games are © London Organising Committee of the Olympic Games and Paralympic Games Limited (LOCOG) 2007. All rights reserved.

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For more information visit: london2012.com/learninglegacy Published October 2011 Ref: ODA 2011/031