Using Modern Technology
Modern Technology in Operation Ground Penetrating Radar Investigation Potential Heritage Grave Sites on the Great North Road
The Old Northern Road was the section of the Great North Road south of Wiseman's Ferry. The section of the road which we will be looking at is about 3 km north of Glenorie on a bend in the road about 1.5 km north of Mount View Road (see map). It was an early part of the road constructed by chain gang convicts, before the winding road was built to get down to Wiseman's Ferry, possibly before 1825.
Legend has it that a number of convicts rebelled against their overseer for the treatment to which he was subjecting them. The story goes that, as a result, they were flogged and then hung for the trouble they caused. It went further to say that they were hung and buried on the spot. A number of local residents whose families have lived in the area for a long period have verbal history of the site passed down through the generations. An old map certainly had the spot, where convicts were buried, marked on it but no official documents have been found which would verify the event. While it is conceivable that convicts were buried in this location, the hanging after flogging does not seem to be consistent with the procedures which were adopted in those years. Only a magistrate could order hanging, though the assistant surveyor on the work site could order flogging but it would have to be reported officially.
Whether it is legend or fact that convicts were buried at this site, Councils responsible for the area and the Road & Traffic Authority (RTA) considered it wise to use modern technology to investigate if graves were really to be found. Each of these organisations is conscious of the need to study and maintain possible historical and heritage sites before any drastic changes to the area are made. The RTA was currently upgrading the Old Northern Road and the local historial societies and the Hornsby and Baulkam Hills Councils became concerned. A company CMP-GBG was commissioned by the RTA Consultancy Services (Heritage) to use non-destructive testing methods to determine if disturbed ground could be located which might be related to the presence of grave sites. The company had collected data from previous investigations at a cemetery in Parramatta which could be used as a basis for comparison. Ground penetrating radar (GPR) was selected as the most suitable technology.
The Potential Heritage Site
The rough location of the supposed burial area was sufficiently well established that it was narrowed down to a site 35m x 12m (420 square metres in area), about 1.5 km north of Mount View Road, using as the reference point a large boulder with an original land portion marker chiselled onto it. The 35m length of the site is parallel to the Old Northern Road, with its nearest longitudinal line about 1 metre from the edge of the existing road pavement.
A grid of survey pegs and paint marks was set out by the RTA survey crew, separated transversally and longitudinally on 1 metre centres in the 35m x 12m block and located relative to RTA fixed datum points.
The survey was carried out in November 1998 by a two man team, one operating the control and recording equipment, carried in the rear of a vehicle, and the second pulling the transducing elements across the ground. No special preparation other than burning off the low lying scrub and grass was carried out on the site. The transducer used (900 MHz antenna) was chosen because of its ability to resolve detail and have a reasonable depth of penetration. Transverse and longitudinal lines were profiled with a maximum scan depth of about 1600 mm. Because of the nature of the site, with local sandstone located at a reasonably shallow depth, it seemed reasonable to assume that any graves dug here would also be at a shallow depth. Digging a normal full depth grave would have been extremely hard work, probably not felt warranted in this situation.
Results of the Investigation
We cannot go to the detail of the methods of analysis used but rather will point to the way that data were interpreted. The interpretation and plotting of sub-surface anomalies was aided by comparison with the Parramatta cemetery study. Disturbances in the sub-surface material and anomalies found at boundary between weathered rock and the soil were plotted from the data. If these disturbances appeared on 2 or 3 adjacent profile lines, one metre apart (giving a length of 2 metres approximately) they were plotted as potential grave sites. All possible effort was made to exclude anomalous signals from tree roots and remains of trees but a small percentage of the plotted results will include error in signal interpretation, resulting in some roots being plotted.
The study located three probable grave sites. The largest sub-surface anomaly was found at the South end of the site, adjacent to the large rock with the old survey marker chiselled into it. The area was quite large and may consist of more than one grave. The other two sites were spread out along the site at about 8-10m and 26-28m. A fourth site was located at 30-31m but the shape, size and position of this anomaly suggests that it may be more likely to be associated with the root system from a nearby cluster of trees. In the profile A-A in plan view of the grave site above, high amplitude reflections near the large rock appeared to come from disturbed ground.The location is depicted in the Transverse Section A-A shown in the drawing below.
There appear to be three potential grave sites in the investigated area, the largest of which could possibly be a multiple grave at the South end of the site, adjacent to the large rock with the chiselled marker on it.
- The potential grave sites are located about 5-6m from the edge of the existing road pavement.
- The radar investigation was non-intrusive with no damage to the site and has provided good sub-surface resolution, pinpointing some significant anomalies.
- The potential grave site is to be provided with a marker stone explaining what has been determined may exist there and what the story of its origin could be.
We are most grateful to the RTA Technical Services for the provision of a copy of the CMP-GBG Final Report of November 1998. We compliment CMP-GBG on the work which was done at the Glenorie site and the analysis which they performed.