All photographers use light and shade, or highlight and shadow, to capture the form of the objects they are photographing. In the hands of a skilled aerial photographer highlight and shadow can reveal the presence of sites which, recorded in poor lighting or from the wrong viewpoint, would be virtually invisible (Figs 2.4 and 2.5). By and large, the lower the angle of the sun and the stronger the resulting highlights and shadows, the greater will be the photographer’s chance of recognising and recording
features degraded by ploughing or erosion. There are
limitations, of course – very low sun can confuse rather than reveal patterns and bright sunlight when photographing
buildings or substantial earthworks may hide important detail in the shadows. Subdued directional lighting beneath a slightly overcast sky can be very effective in these situations.
For this Cambridge University view of fields and open pasture in the Black Mountains of South Wales a particularly high viewpoint was adopted on a day of perfect visibility. Note the contrast between the enclosed land in the valleys and
Fig 2. 3 Flying high for landscape recording
Flights in late evening, early morning or in low winter sunshine can reveal earthworks that have previously escaped detection. Here, in the hills of north-east Wales, the dark stone rampart of a known Iron Age hillfort occupies the peak of the hill. However, both the rectangular enclosure slightly below and to its right and the double-ditched enclosure in the foreground were
unknown until this photograph was taken. More correctly, they were unknown until their
existence was declared in readily available written (and now computerised) records based on the air photo evidence.
Fig 2. 4 Discovery through the use of light and shade
Some of the key ways of using highlight and shadow are illustrated in Figs 10.2 to 10.4.
There are also comments on photographic techniques in many of the other captions throughout the book. The prime lesson is that the fledgling aerial photographer should cherish the chance to fly early in the morning or late in the evening, or in the winter
months when the sun is low in the sky throughout the day (the obscuring effects of trees, shrubs and other ground cover are also at their lowest in wintertime). Needless to say, both highlights and shadow will be more clearly defined when the visibility is good and the air free of water vapour, dust or industrial pollution. For this and other reasons a basic understanding of weather patterns, along with the regular use of internet and other weather forecasts, become essential tools for the archaeological air photographer.
Contrasting views of two earthwork enclosures in eastern Wales. In the left-hand photo only the well-preserved embanked enclosure at upper right can be seen with any clarity. In the right-hand view more oblique lighting and a different viewpoint reveal a heavily degraded second enclosure crossed by a hedge line at centre left.
Fig 2. 5 Using light and shade to reveal low earthworks
Soilmarks
It is a truism of aerial archaeology (and of archaeology in general) that once the soil has been disturbed, or overlain by another material, it can never be exactly restored to its original state. There will be greater or lesser changes in the texture, physical content, water retention or nutritional value of the deposits that have accumulated in ditches or pits; there will more stone or excavated subsoil on the line of degraded banks; and stone or clay walls will change the texture of the soil even if all upstanding trace has been removed by
ploughing or erosion. The impact of these below ground
differences, or the remnants of superimposed material, will in the right circumstances still be visible to the aerial
archaeologist.
This happens in a variety of ways, one of them being the appearance in recently ploughed or harrowed fields of
differences of colour, reflectance or dampness of the kind shown in Fig 2.6.
Known as ‘soilmarks’, or in some cases ‘dampmarks’, these can be fairly fleeting in their appearance, depending on the interval since ploughing or harrowing, or on changes in the dampness of the soil in response to sun, wind or temperature.
There is an added problem that in many countries ploughing now takes place at almost any time from late summer to early spring, so that in areas with a relatively low proportion of arable
cultivation the chance of arriving over fields at the moment when they show intelligible soilmarks is fairly low. Where the whole or virtually all of the landscape is under arable cultivation, however, soilmark survey can produce very worthwhile useful results.
This happens in a variety of ways, one of them being the appearance in recently ploughed or harrowed fields of differences of colour, reflectance or dampness of the kind shown in Fig 2.6. Known as ‘soilmarks’, or in some cases
‘dampmarks’, these can be fairly fleeting in their appearance, depending on the interval since ploughing or harrowing, or on changes in the dampness of the soil in response to sun, wind or temperature. There is an added problem that in many
countries ploughing now takes place at almost any time from late summer to early spring, so that in areas with a relatively low proportion of arable cultivation the chance of arriving over fields at the moment when they show intelligible soilmarks is fairly low. Where the whole or virtually all of the landscape is under arable cultivation, however, soilmark survey can produce very worthwhile useful results.
Left. The traces of relatively recent ploughing strips, perhaps of two different dates to judge by the overlap in the upper part of the photograph. Top right.
An ancient road in the form of a lighter mark cutting diagonally across the modern ploughing strips. Bottom right. A small enclosure in southern Italy.
Note how the plough has ‘dragged’ the soil in opposing directions on successive transits across the darker mark of the buried ditch.
Fig 2. 6 Typical Italian soilmarks
Cropmarks
In their most typical form cropmarks are represented by variations in the colour, height or density of grain and other arable crops, especially in the weeks immediately before
harvesting. They are one of the most productive sources of new discoveries. The principles underlying their formation are
illustrated in Fig 2.7 but there are many subtleties, only the most obvious of which can be mentioned here. Comments on other aspects of cropmark formation can be found in captions
throughout the book.
Cropmarks fall into two main categories: ‘positive’ and
‘negative’ (Figs 2.9, 2.10). Positive marks, which typically appear in grain crops as green marks against a yellow background,
usually denote greater nutritional or moisture content in buried ditches, pits, foundation trenches or areas of deeper natural soil.
Negative marks, appearing as yellow marks and stunted growth in still-green crops, usually result from reduced nutritional value, thinner soil or impeded drainage above buried walls, roads or other impermeable surfaces. Both positive and negative
cropmarks can sometimes survive as yellow-on-yellow marks in the ripened crop, as illustrated at bottom right in Fig 2.9. In
certain conditions cropmarks can be ‘reversed’, ditches or water courses appearing along part or the whole of their course as negative rather than positive marks. The mechanisms involved here are not entirely understood but for most purposes this is of
little importance – the meaning of the cropmarks is usually quite clear in terms of the buried ditches or other features that they represent. (It may, on the other hand, be important to try to work out the precise mechanisms when analysing an individual site in detail, perhaps as a preliminary to excavation.)
While cropmarks appear most typically in grain crops just before and after ripening, they can also be seen at other times of year.
For example, variations in the temperature, dampness or
nutritional content of the soil in early spring may accelerate (or delay) growth above buried archaeological features, producing so-called ‘germination marks’. These differences may persist throughout the following weeks, producing stronger or weaker areas of growth (but no change of colour) long before the classical time of cropmark appearance, which may vary from mid-to-late May around the Mediterranean rim to early early August in northern Europe. These dates are liable to quite wide variations, however, since cropmarks are a response to the weather patterns and planting regimes of individual years (as well as to the local soils and geology – clay in Britain, for instance, for the most part only shows cropmarks in very dry seasons). For these reasons the timing or appearance of cropmarks may be significantly different from year to year or place to place within any country, region or locality. In this sense there is no ‘average’ year, no ‘normal’ pattern of cropmark
development and no fixed or ‘ideal’ time for cropmark survey.
In addition to grain crops (wheat, barley, oats and rye) other deep-rooting crops such as peas, beans, carrots, clover, lucerne and alfalfa (erba medica, Fig 2.10), along with a variety of root crops (notably beets of various kinds), can produce cropmarks at various stage in their growth, some of them startling in their
clarity (Fig 2.9, top). Shallower-rooting crops (such as mustard, kales and cabbages) rarely produce cropmarks since they lack the capacity to seek out water or nutrients at deeper levels when they come under stress; but the flowers of oil-seed rape and linseed (and very rarely potatoes) can also produce useful
cropmark evidence on occasions. In general the coarser the spacing of the plants, and the greater their individual leaf area, the more ‘blurred’ will be the cropmarks. Interpretable marks are therefore rarely seen in maize (apart from the more recent and shorter growing varieties) and cropmarks will only rarely occur in vineyards or orchards, except as weedmarks in the ground
beneath the trees or vines. Poppies in abandoned or temporarily dormant arable fields can sometimes produce cropmarks that are startlingly beautiful as well as archaeologically useful.
Fig 2. 7 The formation of cropmarks
Crops grow taller and ripen later over the deeper, more nutritious and damper soil of a buried ditch or pit. Growth is stunted and the ripening of the crop earlier in the shallower soil above buried walls or other impervious deposits. Ditches and pits create green marks in the yellowing crop (‘positive’ cropmarks). Walls and similar features give yellow marks in the green crop (‘negative’ cropmarks). Both can persists as ‘yellow-on-yellow’ marks in the ripened crop (Fig 2.9, bottom right).
Three views of a complex of ritual and funerary monuments first discovered through aerial survey in eastern Wales.
Top. Two Bronze Age burial mounds appear as slightly lighter patches of soil in the upper part of the field. In the lower left corner the external bank of a Neolithic ritual monument or ‘henge’ also appears as a lighter mark where the plough has cut into subsoil originally thrown up from the its now re-filled internal ditch.
Centre. When seen as cropmarks both the burial mounds and the henge appear as dark green ring-ditches, the latter with a narrow
entrance gap on the left-hand side. Note how the lighter soilmark in the top photo takes a wider circuit than the darker-coloured ring-ditch, showing that the marks in each photo represent different parts of the monument.
Bottom. The same cropmarks can be clearly seen at ground level, but without the coherence of overall pattern provided by the aerial view.
Fig 2. 8 Soilmarks and cropmarks in Wales
Top. Positive (dark green) cropmarks above the filled-in ditch of a Roman temporary camp in England, showing in sugar beet, a crop widely grown in some parts of Italy.
Lower left. The walls of a Roman building in Tuscany show as negative marks where the stunted crop has ripened more quickly above the buried stone foundations.
Lower right. Previously dark green marks above a complex of pits and ditched in Hungary now show as light yellow marks on the darker
background of the fully ripened crop. Low sunlight accentuates the extra height of the plants over the buried ditches and pits.
Fig 2. 9 Positive, negative and ‘yellow-on-yellow’
cropmarks
Some of the walls show as clear ‘negative’ marks, where prolonged dry weather has led to parching of the stunted crop above remaining stone foundations. Others walls show less clearly, though still as light green (therefore ‘negative’) marks, probably where there is a mixture of soil and stonework in the partially-robbed wall foundations.
Fig 2. 10 Roman villa showing in a crop of alfalfa in September 2012
Top. The ditches of a defended enclosure with a long ditch-defined entrance show as green marks in yellowing grass after a long dry spell on the Welsh borderland. The site, which has a Welsh place-name (Cloddiau = banks or ditches) which suggests the former existence of an enclosure, was watched for ten summers before this mark appeared in 1989. It has been seen very rarely since.
Bottom. Light green ‘negative’ cropmarks above the buried walls of a small church in southern Italy are here emphasised by the scarlet poppies that have taken over parts of the field. The eastern apse of
Fig 2. 11 Grass-marks and weed-marks
Cropmarks can sometimes be seen in grassland, as so-called
‘grassmarks’ or ‘parchmarks’. They usually occur, often quite suddenly, at the end of a prolonged summer drought. Grass above stonework or other hard surfaces parches out first, giving brown or yellow marks against a green background. Later, when almost all of the grass has lost its colour, the parts that lie above buried pits or ditches may stay green for a little longer, giving well defined cropmarks. On chalk, and for instance above the
‘crosta’ of the Tavoliere in southern Italy, the effect may be reversed, the shallow rooting grass turning brown first above ditches which have acted as drains rather than reservoirs.
Grassmarks may fade rapidly after rain but they are of particular value to the aerial archaeologist because they occur less
frequently than cropmarks in cultivated fields and thus give
access to parts of the landscape that are generally impervious to cropmark survey.
Grassmarks of even greater clarity can occur when the
harvesting of hay or silage is followed by a period of hot and dry weather. The cut grass quickly withers to a whitish colour, except above ditches or other reservoirs of underground moisture. In these conditions the grass and weeds (if not eaten by grazing animals) may put on rapid growth, producing remarkably clear cropmarks in fields that normally reveal nothing.
Cropmarks of a different kind, sometimes referred to as
‘weedmarks’ or ‘vegetation marks’, are caused by minor
differences in moisture content, nutritional value or microclimate which favour one plant over another, or give an advantage to plants in one position compared with their neighbours alongside.
Poidebard, on the Syrian steppe, for instance, noted how tiny variations in ground conditions resulted in differences in
vegetation which were difficult to see from the ground but clearly visible from the air. A similar thing can happen with weeds of cultivation, such as poppies in stands of corn (Figs 2.11 and 10.14) or the re-growth of weeds amid the stubble of harvested grain crops. Commenting on the prospects for cropmark survey in Mediterranean countries, and particularly in southern Italy, Bradford (1957, pp. 24ff) noted how wild plants and flowers can create cropmarks in sun-scorched pasture at almost any time from July to October. The grass itself may also show patterns when the first rains of autumn bring the summer drought to an end. Along with similar effects of flowering plants earlier in the year this could give the drier parts of Italy and other
Mediterranean countries a wider range of opportunities for
recording cropmarks than applies in the more temperate zones of Europe.