The Great Escape

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

The mag team have pulled-off the Great Escape.  They waved a tearful goodbye to the field they have been working-in all of this season.  Today saw the completion of the available area of Prae Wood Field.  In fact, we did more than we agreed to because I did not know they had grubbed out a hedgerow since the images on Google Earth were taken last summer. Figure 1 shows the entire mag survey from Abbey Orchard (we must finish that!) to Prae Wood.

Figure 1: the entire Verulamium magnetometry survey as of 17/8/2019.

The complete survey now consists of over 93ha of mag data, which is about 18,700,000 readings.  No wonder my computer is feeling the strain!  The Prae Wood field survey is 10.7ha in extent.  Figure 2 shows the whole field.

Figure 2: the Prae Wood Field magnetometry survey.

Compared to the riches of surveying inside the town, the field provided slim pickings.  We did find an enclosure and a large ditch at the eastern end of the field, and some enclosures towards the west.  Much of the field, however, is a remarkably blank canvas, magnetically speaking.  As always, absence of evidence is not evidence of absence, but in this case there is not much to encourage further work here.  Figure 3 shows the western area completed today.  Well done Ruth, Jim, Dave and Pauline, along with Rhian and Ellen on previous days.  Tomorrow sees the great move to Church Meadow.

Figure 3: the western end of Prae Wood Field.

The one new and obvious feature in the field is the big black blob.  It is quite large, about 10m across, and moderately magnetic (-3.5nT to about 8nT).  My best guess this is a ploughed in pit.  Hertfordshire is full of pits, for chalk, gravel or clay (and sometimes all three in the same field).  This one would be quite modest compared to many.

The Earth Resistance team consisted of Ellen and Anne, assisted in the morning by myself until I went to help with the GPR after lunch.  We completed another six grid squares as we work our way north. Figure 4 shows today’s data.

Figure 4: the Earth Resistance survey. Area completed today outlined in red.

Towards the north of today’s area is a nice little apsidal building orientated NW-SE.  Although it reminds me a little of an early church, apses were not uncommon in the Roman world, and we may not be seeing all the building.  It can be seen a little more clearly if we apply a high pass filter (Figure 5).

Figure 5: the 2019 area after the application of a high-pass filter to remove the background trends.

We have seen this little building before in the GPR data, but not quite so clearly.  Once thing that I am finding very intriguing is why some buildings show in the mag data and the res/GPR data, and some buildings do not show at all in the mag data.  Figure 6 shows the mag data from this area.

Figure 6: the mag data from the same area as Figs 4 and 5.

As you can see, there is no sign at all of the apsidal building in the mag data.  Compare this to the buildings further to the south which line the road and show very clearly in both mag and Earth Resistance data.  There is a mystery to investigate!

The GPR team were on some of the steepest slopes at Gorhambury (Figure 7).

Figure 7: John Ridge (SAHAAS) gallantly pushes the GPR up the steep incline.

Not often do I hear survey teams hoping not to find something, but afraid I might make them resurvey the area at 0.5m transect spacing, the team were keeping their fingers crossed that nothing exciting showed!  The radargrams on screen seemed to be fulfilling their hopes (Figure 8).

Figure 8: example radargram from today.

The data were duly processed in GPR Slice.  I present just one slice, No. 4 (Figure 9).

Figure 9: Time slice 4 from today’s survey.

Although there are areas of high reflectance (in red) and low (in blue) I suspect this is mainly to do with geology on the side of this steep dry valley.  The underlying mag data shows a few features (Figure 10).

Figure 10: the mag data in the same area as the today’s GPR survey.

Although there are a couple of pit-like features, including one quite strong one near the northern edge of the block, there isn’t that much to suggest there is much going on here.  Not enitrely surprising given the geology.

Today’s weather was highly variable.  We had some rain just before lunch, but sunny skies in the afternoon (Figure 11).  Fingers crossed for tomorrow.  The afternoon’s weather is currently forecast to be “unsettled”.

Many thanks to everyone for an excellent day’s progress.

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