Thirlwall CaStle (Figure 1) is one of the finest surviving examples of a mid-14 th c~ntury\ud Borders Hall House. It stands on a high bank on the north bank of the Tipalt Burn\ud approximately 2.5 km east of Gilsland, about 0.75 km north of Greenhead and approximately\ud 120 m north of the course of Hadrian's Wall. Since coming into the care of Northumberland\ud National Park the castle has been the subject of a variety of studies, including its history and\ud archaeology (Ryder, 199311997) and the lichen flora found on the walls (Simkins, 2000a;\ud 2000b, 2000c).\ud At the time of this investigation scaffolding was in place to the full height of the\ud building. All parts of the external and intern~l walls, accessible at the time of the\ud investigation were examined. Some areas, not safely accessible, were not examined at close\ud range though general features of the stonework were observed from appropriate vantage\ud points. Lithological characteristics discernible on site, e.g. rock type, comparative grain size,\ud colour, reaction to 10% hydrochloric acid, state of weathering, etc. were noted. In addition,\ud samples of a number of characteristic rock types were obtained, under the supervision of\ud Northumberland National Park staff and masonry contractors, for more detailed\ud petrographical examination. The study concentrated on the geological materials present in the\ud original fabric of the building. This report offers comments on stone types and the associated\ud mortar. Care was taken to distinguish any materials incorporated into the fabric during the\ud restoration work on the building.\ud The descriptions and discussion presented in this report are based entirely upon\ud available published sources, unpublished BGS and other records, the observations made\ud during site visits during November 2000 and the results of petrographical examination of\ud rock samples obtained from rock samples collected during the investigation. No attempt has\ud been made at any appraisal of the structUral condition of the building or of the ground\ud conditions beneath them: the report is not in any sense the report of a structural survey. None\ud of the data contained in this report should be regarded as a substitute for full and appropriate\ud structural and geotechnical investigations if any remedial or other works are contemplated
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