Insane?

The sun was shining, we have a GPR and a Park to play in, so why not? Mike, Peter and I headed out to Verulamium Park to do a little more GPR, although with a difference. We used a 25cm spacing between lines instead of 50cm. We managed a 40m x 60m block.  Without taking into account turning around, or getting to the spot and back again, that is 9.6km of walking, or just under six miles in old money. The results were great, however.  Later, I will do a posting comparing the 0.5m spacing with 0.25m to see if the extra 4.8km was worth it.

First things first.  Where in the Park were we?  I am especially interested in Insula XXIV simply because the Wheeler’s did not dig anything there and we have very good mag results.

Location of the mag grids in the northern half of Insula XXIV.

Location of the mag grids in the northern half of Insula XXIV.

Here is a more detailed view of the mag results.

Detail of the magnetometry results from the northern half of Insula XXIV.

Detail of the magnetometry results from the northern half of Insula XXIV.

The stark black-and-white feature in the NE corner is a practice cricket wicket made of concrete and green carpeting. It overlies the Roman road.  The diagonal feature in the NW corner is the other Roman road, and at least two buildings show clearly as white lines against a dark background.  The western 40m square was surveyed by Ralph Potter at 0.5m intervals.  The middle square and the western 20m of the eastern square were surveyed yesterday.

I used Larry Conyer’s program to slice the data we collected.  The slices start at 8ns and go down in 4ns thick pieces.  Ground surface in the Mala radargrams is usually at about 7ns and so the first slice is very much the top surface of the field.

Slice 1: 8 to 12ns.

Slice 1: 8 to 12ns.

The cricket wicket can be very clearly seen in the top-right corner.  The change across the plot at about 27m is where we went for lunch.  But what on Earth is the big circle?  Here is a clue…

GPR in the Park.

GPR in the Park.

Yup, it is the centre circle of the soccer pitch.  The second slice reveals the striping we have often seen in the Park.

Slice 2: 12 to 16ns.

Slice 2: 12 to 16ns.

I am guessing these are cultivation marks before the land was turned into the Park in the 1930s.  How old they are I am unsure, but could they be the residue of cultivation ‘strips’ as seen at Ashwell?

In the third slice, we begin to see the Roman archaeology.

Slice 3: 16 to 20ns.

Slice 3: 16 to 20ns.

Notice how there is a white line running through the lower building cutting the black lines representing internal walls.  This is a wall which has been partly robbed.  We also appear to have some surviving floors as shown by the larger black areas in the building.

Slice 4: 20 to 24ns.

Slice 4: 20 to 24ns.

In the fourth slice down we can now see the road running across the top-right hand corner in a similar, but not identical position to the cricket wicket.  I wonder if we might be seeing evidence of a hypocaust in the middle of the lower building?

Slice 5: 24 to 28ns.

Slice 5: 24 to 28ns.

In slice five we can see the outer walls of the building are much fainter suggesting shallower foundations, but the main wall which was only a light line previous is now showing more clearly.  This is presumably what is left at the bottom of the robbed wall.  There is also a building along the road which was not visible in the higher time slices.

Slice 6: 28 to 32ns.

Slice 6: 28 to 32ns.

Slice six is very black-and-white.  This is because the radar signal is now very weak and the slices are either showing something or nothing.

All in all, a very nice set of results.  As always, these are my rather rough-and-ready time slices.  With some experimentation and practice they could be better.

Here are Ralph’s grid square (processed by Mike Langton of Mala) and our new one roughly plotted together in Google Earth.

Ralph's and our blocks of GPR data plotted together using Google Earth.

Ralph’s and our blocks of GPR data plotted together using Google Earth.

I think it would be worth surveying the whole of Insula XXIV if we can!

 

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