Non-Destructive Archaeology

One drawback of archaeological digs is that they are destructive. Once dug, it is not possible to come back with more advanced techniques to re-dig the site and possibly find more than was discovered on the first dig.

There are methods of investigating what lies below the surface without damaging it.  To finally prove what is present will require a dig or at least sample digs of the site, but non-destructive archaeology can determine where digging is likely to provide results.  These methods include Lidar, Aerial Photography, Magnetometry, Resistivity and Ground Penetrating Radar.  Examples of what these have shown us around Tadcaster appear below.

Lidar (Light detection and ranging)

An aircraft moves systematically across a site shining a laser downwards.  It measures how long it takes for the light to return and from this calculates the distance from plane to the ground.  It can measure very small changes in the height of the ground.

Taking a local example of Newton Kyme, the picture above shows a number of things.  The most obvious item is the rectangular Roman fort.  To the right of this can be seen medieval “Rig and Furrow” or “Ridge and Furrow” strips of farmland.  Below the rig and furrow is a circular object.  This is a neolithic “Henge” consisting of concentric ditch and bank.

Careful inspection of Lidar plots can reveal things which can be easily missed when viewing a site.  Lidar can “see through tree foliage” and can reveal “lumps and bumps” even in woodland.

Aerial photography

Aerial photography has been recognised as a major aid to archaeology since the 1920s.

An aerial view of Toulston showing a Roman road’s crossroads

In this aerial picture a junction of two Roman roads can be seen as light marks in the soil.  The road running approximately north is Rudgate, and the east-west road is the Tadcaster to Manchester and to Skipton and Ribchester road.  In other aerial pictures features can be seen in crops where, because of material below the surface, some parts of the crop are early or late in ripening.

Magnetometry

A magnetometer can detect varying strengths of magnetism.  If a ditch is exposed for some time, small magnetic particles can accumulate.  Because they are in standing water, they are free to align with the earth’s magnetic field.  If the ditch is later filled in, these particle remain aligned.  These can be detected.

In the pictures below we see one of the fields that appeared in the previous view.

Magnetometry survey results

Roman roads shown in red, quarrying in blue and Iron Age field boundaries in green

The two Roman roads can be seen, marked by the ditches that once existed on both sides of the road.  The ditches show up as a series of lighter coloured dots or smudges.  However, this survey also reveals a third road (slightly narrower) joining the two at the same junction.

There are also some other marks visible.  These cross the roads in various places.  They are almost certainly Iron Age ditches that were there before the Roman roads were built.

There is also some later quarrying visible close to the road at the top of the picture and alongside the road near the bottom of the picture.

Resistivity

Another means of non-destructive archaeology is measuring the electrical resistance of the ground.  If there is some stonework below the surface, the electrical resistance will be higher than if the there is deep soil.

The picture shows a resistivity survey on the site of the 2015 dig on the riverside just north of the church.  The darker areas show higher resistance representing buildings just below the surface, and the lighter areas show lower resistance where there is deeper soil.

Ground Penetrating Radar (GPR)

GPR equipment sends high frequency radio waves into the ground and measures the time taken for reflection of these waves from different features beneath the surface. From this, it can determine how deep an object is and how good a reflector it is.  Detection is possible up to 3 metres below ground.