13 Geology and Soils
13.0.1 The Stage 1 and 2 assessments demonstrated that there are no geologically designated sites, areas of contaminated land or past mining activities associated with the local environment, which takes in the Proposed Scheme development footprint. The assessments concluded that potential impacts for online schemes within the corridor would be negligible. As the detailed assessment progressed the above conclusions held in relation to the Proposed Scheme despite being located offline. As such, a detailed assessment of geology and soils was scoped-out.
13.0.2 It was however recognised that baseline data related to geology and soils would be of value to the understanding of the baseline environment when considering other topics of environmental interest, such as landscape, ecology and water quality, which remains the purpose of the inclusions within this Chapter.
13.0.3 The Stage 1 and 2 assessment did however recognise the potential impact on soil quality where top-soils would be stripped and replaced to allow earthworks to be modified along with the associated loss of productive agricultural land to the new road and areas severed between the existing trunk road and proposed new section of trunk road. Accordingly, the loss of agricultural land is addressed in Chapter 9, Land Use and the effect of stripping and topsoil-removal are discussed in Chapter 15, Disruption due to Construction.
13.0.4 The following data sources were consulted to establish the baseline environment related to geology and soils.
- British Geological Survey (BGS), Scotland Sheet 10W, Lochmaben (Drift Edition), 1:50,000, (1983) and associated memoir.
- BGS, Scotland Sheet 6, Annan (Drift Edition) 1:50,000, (1983).
- BGS Sheet NY17SW Dumfries (Solid Edition), 1:10,000 (6’’: 1 mile), (2000).
- BGS, Sheet NY16NW Dumfries (Solid and Drift), 1:10,000, (1994).
- BGS, Sheet 62 Dumfries (Solid and Drift), 1:10,000 (2000).
- The Macaulay Land Use Research Institute, Soil Survey of Scotland, Sheet 85, Carlisle and Solway Firth, 1:50,000 (1986).
- Mouchel Parkman, A75 Hardgrove to Kinmount Improvements Geotechnical Desk Study (June 2006).
13.0.5 The following bodies were consulted with specific reference to contaminated land:
- Dumfries and Galloway Council; and
- The Scottish Environment Protection Agency (SEPA).
13.0.6 Their associated responses are contained within Appendix B.
13.1 Baseline Conditions
13.1.1 The Proposed Scheme corridor is underlain by sedimentary deposits from the Carboniferous period. The sequence of deposits comprises:
- Tyne Limestone;
- Fell Sandstone;
- Annandale Sandstone; and
- The Ross Formation.
13.1.2 Tyne Limestone of the Liddesdale Group is found at the top of the sequence. The deposit is characterised by mainly fine-to-medium grained sandstone, seatearth35, siltstone and thin limestone. This solid deposit extends across the site from the current A75/U82a junction to the A75/Upper Mains junction.
13.1.3 The Fell Sandstone Formation lies below the Tyne Limestone and is present as medium-to-thinly-bedded, mainly coarse-grained and cross-bedded sandstones with seatearth, mudstone and siltstone. The deposit extends from the A75/U82a junction to the junction with Cuddie Lane.
13.1.4 The Annandale Sandstone beds lie below the Fell Sandstone and comprise red, fine to coarse- grained sandstone and locally derived conglomerate. This deposit is present between Cuddie Lane crossroads and the position of Kiln Knowe. It is truncated at Kiln Knowe by the Gillhall Fault that runs northeast/southwest across the line of the A75, with the downthrow to the southeast (of unknown depth). Running northwest and intersecting the Gillhall Fault 30 m north of Kiln Knowe is the Dalton Fault with a downthrow to the east.
13.1.5 Fell Sandstone and Annandale Sandstone beds; both belonging to the Border Group.
13.1.6 The Ross formation, a Silurian deposit of the Hawick Group, is found at the bottom of the sequence. The formation is present as grey and red wacke siltstone. The oldest of the solid deposits, the formation is represented to the west of the Gillhall and Dalton Fault intersection at Kiln Knowe, and extends beneath Carrutherstown.
13.1.7 There is a difference in the drift deposits east and west of the existing Nether Stenries junction. To the east, the drift largely comprises glacial till, though there is the likelihood that isolated mounds of sands and gravels may be found along the Proposed Scheme construction corridor.
13.1.8 To the west, fluvioglacial sands and gravels overlie glacial till. The glacial till is characterised by grey to red-brown deposits containing cobbles of greywacke36, sandstone and local granite.
13.1.9 Soil types and their extent are shown in Figure 13.1.
13.1.10 Most of the local area has a cover of brown forest soils and brown forest soils with gleying of the Canonbie Association. At the eastern end of the study area (Kelhead Moss Plantation area) peat is found, also of the Canonbie Association. Approximately 25% of the soils associated with the corridor are classified as being of prime agricultural capability (class 3.1), 60% are of class 3.2 and only 15% are of relatively low agricultural quality. This is discussed further in Chapter 9.
13.1.11 The Geotechnical Desk Study Report (June 2006) indicates that localised deposits of peat have been identified between Carrutherstown and A75/U81a Junction to the north of the Proposed Scheme footprint. The report goes on to state that made ground is also present at several locations adjacent to the existing A75, though there was nothing to indicate the potential presence of contaminants or visual signs of contamination.
13.1.12 The solid carboniferous deposits of a sedimentary nature comprise aquifers where flow is predominantly through fissures and discontinuities. They tend to be highly productive but usually only of local importance.
13.1.13 These calciferous sandstone measures consist of medium grained sandstone with subordinate mudstones, siltstones and limestone. Borehole yields are generally moderate and not greater than 10 l/s. Groundwater chemistry is dominated by calcium and bicarbonate with magnesium, and occasional sulphate. Iron may be present in deleterious amounts where reducing conditions prevail.
13.1.14 To comply with the Water Framework Directive, SEPA has produced a map denoting ‘Vulnerability of Groundwater in the Uppermost Aquifer’37. Aquifer vulnerability has been categorised on a scale of 1-5 (least-to-most). The aquifer vulnerability beneath the site rates as 4b (whereby the uppermost aquifer is vulnerable to those pollutants not readily absorbed or transformed). The vulnerability classification has been derived from a risk assessment of pollutants entering the aquifer vertically from ground surface level.
13.1.15 The average annual rainfall for the area is within the range 800-1200 mm/year. Average estimated infiltration ranges between 100-300 mm/year.
Mining and Mineral Extraction
13.1.16 There is no evidence of underground workings in the area, and based on knowledge of local geology, underground workings are unlikely locally. Some surface sand and gravel extraction is likely to have taken place along the existing alignment; however no formal gravel or sand quarries were shown on the historical maps within the footprint of the Proposed Scheme. It is also likely that waste deposits comprising a mixture of spoil, arising from previous road construction works, may be located along the verges of the current footprint.