Untitled - Council for British Archaeology

6
seems to line up with the existing road from Evesham to
Winchcombe. At the end of July, cropmarks in the Northampton
area were poor and only a few new sites were recorded
there. The Trent Valley around Alrewas and between
Nottingham and Cromwell showed cropmarks of sites for
several weeks, but the nearby Witham valley was virtually
blank. The northern edge of the Cotswolds had some unusual
and interesting sites showing but the northern edge of the
Lambourn Downs overlooking the Vale of White Horse was
devoid of indications of sites. The Yorkshire moors and
north of these to Newcastle on Tyne was uninteresting this
year.
'Three more weeks of intensive flying were carried out
from the beginning of August.
1 Bicester (Cherwell Valley)
2 Cottesmore (Nene Valley & East Leicestershire)
3 Leconfield (Yorkshire S. Wolds)
This programme was devised to carry out a number of
experiments and areas selected for intensive flying and the
time when the flying was carried out over them was carefully
calculated. Combined with the longer ranging surveys, the
programme made it possible to check the state of cropmarks
in a variety of geologies and in similar geologies both in the
same week and after an interval.
The programme and its timing was based on earlier reconnaissances
in May and June. Rainfall and cool weather at the
beginning ofJuly lengthened the time of growth before crop
maturity in some areas and the final week in Leconfield
proved to be the best recording week. This was four weeks
after the normally best recording week in the Thames Valley.
Areas in which cropmarks develop in any year have a direct
relation to the spring and summer rainfall in the area, but
the relation is not a simple one. Rainfall statistics provide
some guidance, but it is easier to predict areas where cropmarks
are unlikely to develop than areas where cropmarks
will develop with a good contrast.
Even a 'conventional' cropmark of green growth persisting
over soil of higher moisture content is more complicated
than was previously thought. The three main constituents
of a cereal plant are 1) the base foliage, 2) the straw stalks
and 3) the ears. A colour, difference can develop between
any two of these and a colour difference can be created not
only from soil moisture differences but from rainfall and dew.
At an early stage of growth in June, when the cereal plant
was mainly base foliage, rainfall produced growth variations
in a few hours. Far from 'washing out' variations in growth,
rain accelerated the growth plants, and after a dry spell, the
acceleration of growth of plants over moister soils was greater
than that of plants whose growth had been retarded by dry
soil.
Whether the growth differences are reflected in foliage,
straw or ear colour has some bearing on whether the cropmarks
are clearest from an oblique angle or from above. On
the South Wolds of Yorkshire, many fields of cereals produced
what it has been conventional to describe as 'reversal'
cropmarks. These occur at full plant maturity and after the
ears have bent over and turned downwards. These marks are
usually almost invisible from above and are best visible from
a limited oblique angle and limited sector of an orbit. An
analysis of some photographs shows that the best angle for
photographing is not constant throughout the day, nor is it
at a constant angle to the sun. Possibly wind direction may
swing round the inverted plant head.
Both these marks, and marks produced by rainfall in a few
hours, are not easy to pick up during normal observations.
Many of them would be totally missed on vertical cover,
even if this was done on the days of their maximum contrast.
It became obvious during the weekly periods of intensive
flying in limited areas that more sites were noticed after a
few days tha.n had been seen on the first day and this experience
was repeated at each of the areas. The term 'reversal'
to describe some of these marks can be misunderstood as
many of them develop towards full crop maturity without
there having been an earlier stage when the site produced a
darker cropmark. Also, in some areas, early cropmarks did
not persist through to 'reversals' at crop maturity and the
marks either partially or totally disappeared or became
'negative' marks (intaglio in the crop) or maintained a height
differential instead of a colour differential.
The large number of factors that determine whether cropmarks
will develop and what type of cropmark will develop
makes close prediction of their occurrence more difficult
than has perhaps been realised. At the moment, it is less
urgent to try and predict than to realise that knowledge of
cropmarks at best is superficial and in general is based on
misconceptions. Interest in the information they provide
has preceded an understanding of the processes that produce
the evidence.
Some 2000 colour transparencies and 500 black and white
photographs were taken in the course of the above reconnaissances.
Some areas that normally produce cropmarks were
totally devoid of any evidence of previous land use. Some
cropmarks seem to develop over some sites every year that
cereals are planted regardless of the weather. In one area I
have flown over for nearly 20 years some sites have only been
visible once, for a few days only, and these sites have not all
been visible in the same year. Every time a cropmark develops,
there will be details in it that are different from its other
appearances.
Whilst the interest of archaeologists in the results of aerial
archaeology tends to be towards sites in their areas that are
suitable for excavation, there is increasing evidence of a type
that cannot be obtained from excavations. As an example,
several areas under annual surveillance are producing evidence
of large areas of graticular field systems, laid out mainly without
regard to contours. These contrast with field systems
determined by terrain. The relationship between these different
field systems and the domestic sites associated with
them requires investigation. Some field systems in East
Leicestershire seem to be orientated on and to include elements
of earlier features of, in some cases, linear earthworks,