Spotting ridge and furrow in historic aerial photographs

Ridge and furrow is the easiest medieval earthwork to miss on a modern image and the easiest to miss on the wrong historic one too. The corrugated pattern left by generations of ox-plough teams working the same reversed-S strip survives today as a few inches of relief across a field. On a flat, overhead summer photo taken at midday, a few inches of relief is nothing. The sun is too high to throw a shadow worth reading.

Why the wartime sorties show what later ones don't

The RAF and OS verticals flown from the 1940s through the early 1960s catch two things modern satellite passes generally can't. First, a lot of this pasture had not yet been drained, deep-ploughed, or bulldozed flat for postwar arable expansion, so the ridges were still there to photograph. Second, those older sorties were often flown in winter or early spring, when a low sun angle rakes across the ground and throws a shadow off every ridge and furrow crest. That combination, earthwork still present plus light low enough to model it, is why a scratchy 1946 print can show a field system that a crisp 2024 pass over the same parcel shows as nothing but a flat green rectangle.

If you're assessing a conservation area and the modern imagery reads blank, that blankness is itself a finding. It tells you the earthwork was levelled sometime between the archive date and now, which matters for a report as much as the earthwork's original extent does.

Reading the shadow, not the shape

Ridge and furrow doesn't photograph as a shape in the way a building platform or a moat does. It photographs as a stripe pattern of light and dark, repeating at a fairly regular interval, usually curving gently in a reversed S along the length of the furlong. A few things to check before you call it:

• Light direction and time of print. Look at the photo's annotated flight details if they survive, or infer from shadow direction relative to field boundaries. Shadows should fall consistently across every ridge in the same direction. Random or inconsistent shading is more likely a scan artefact or plough damage from recent cultivation.
• Spacing and curvature. Strips in an open-field system typically run somewhere between 5 and 12 metres apart, curving slightly, almost never dead straight for their full length. Dead-straight parallel lines at a wider interval are more often drainage grips or a much later field drain pattern.
• Headlands. Where the ridges meet a field boundary at right angles, look for a slightly raised cross-strip, the headland where the plough team turned. Its presence is a reasonably strong confirmation you're looking at ridge and furrow rather than modern tramlines.
• Parchmarks as a second check. In a dry summer, the same strips can show up as pale and dark banding in the crop or grass, drought stress sitting differently over the buried furrow fill than over the ridge core. A photo from a known drought year, cross-referenced against the shadow reading from a winter sortie, is about as good a confirmation as you get from aerial evidence alone.

None of this replaces walking the field or checking the National Mapping Programme record where one exists for the parish. What it does is let you work out, from a desk, which of the dozen fields in a proposed conservation area are worth a site visit at all, and which ones were already ploughed flat before the earliest usable photograph was taken.

What to put in the report

A planning officer's record of ridge and furrow needs the extent, the condition at the date of each available photograph, and ideally a note on survival between then and now. That last part is the one desk study alone struggles with, because it means holding a decades-old print and a current high-res pass side by side at the same scale and orientation, something scanned archive sorties were never georeferenced to do on their own. Historic Landscape Change orthorectifies that older imagery against a recent pass so the two sit in the same frame, letting you trace a furlong boundary from the 1940s print straight onto the current field and see exactly what's left of it.

If you've got a conservation area where the archive photos might hold more ridge and furrow than the current site visit found, it's worth lining them up properly rather than eyeballing two separate prints.