Editing OR/17/042 Conceptual geological model

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The consensus within the literature is that these buried valleys were produced by glacial over‐deepening (subglacial erosion) and/or subglacial meltwater incision (Gresswell, 1964<ref name="Gresswell 1964"></ref>; Howell, 1973<ref name="Howell 1973"></ref>). Buried valleys produced by subglacial meltwater incision are commonly called tunnel valleys (or tunnel channels in North America) and occur widely around former glacier margins (Ó Cofaigh, 1996<ref name="Ó Cofaigh 1996">Ó COFAIGH, C. 1996. Tunnel valley genesis. ''Progress in Physical Geography'', Vol.&nbsp;20, 1–19.    </ref>; Piotrowski, 1997<ref name="Piotrowski 1997">KRISTENSEN, T B, PIOTROWSKI, J A, HUUSE, M, CLAUSEN, O R, and HAMBERG, L. 2008. Time‐transgressive tunnel valley formation indicated by infill sediment structure, North Sea&nbsp;—&nbsp;The role of glaciohydraulic supercooling. ''Earth Surface Processes and Landforms'', Vol.&nbsp;33, 546–559.    </ref>; Dürst Stucki ''et al''., 2010<ref name="Dürst 2010">DÜRST STUCKI, M, REBER, R, and SCHLUNEGGER, F. 2010. Subglacial tunnel valleys in the Alpine foreland: an example from Bern, Switzerland. ''Swiss Journal of Geosciences'', Vol.&nbsp;103, 363–374.    </ref>; Kehew ''et al''., 2012<ref name="Kehew 2012">KEHEW, A E, PIOTROWSKI, J A, and JØRGENSEN, F. 2012. Tunnel valleys: Concepts and controversies&nbsp;—&nbsp;A review. ''Earth‐Science Reviews'', Vol.&nbsp;113, 33–58.</ref>). Incision of tunnel valleys occurs under immense hydraulic gradients with flow regimes constrained by channel morphology and the thickness of overlying ice. A common characteristic of tunnel valleys is that their bases (referred to as the thalweg) are often undulating with significant normal and reverse changes in gradient developed along their long‐profile. Infills to buried valleys tend to be highly‐chaotic encompassing intercalated beds of till, glaciolacustrine (silt and clay) and glaciofluvial (sand and gravel) sediment that typically give‐rise to chaotic and unpredictable hydrogeological behaviour.
 
The consensus within the literature is that these buried valleys were produced by glacial over‐deepening (subglacial erosion) and/or subglacial meltwater incision (Gresswell, 1964<ref name="Gresswell 1964"></ref>; Howell, 1973<ref name="Howell 1973"></ref>). Buried valleys produced by subglacial meltwater incision are commonly called tunnel valleys (or tunnel channels in North America) and occur widely around former glacier margins (Ó Cofaigh, 1996<ref name="Ó Cofaigh 1996">Ó COFAIGH, C. 1996. Tunnel valley genesis. ''Progress in Physical Geography'', Vol.&nbsp;20, 1–19.    </ref>; Piotrowski, 1997<ref name="Piotrowski 1997">KRISTENSEN, T B, PIOTROWSKI, J A, HUUSE, M, CLAUSEN, O R, and HAMBERG, L. 2008. Time‐transgressive tunnel valley formation indicated by infill sediment structure, North Sea&nbsp;—&nbsp;The role of glaciohydraulic supercooling. ''Earth Surface Processes and Landforms'', Vol.&nbsp;33, 546–559.    </ref>; Dürst Stucki ''et al''., 2010<ref name="Dürst 2010">DÜRST STUCKI, M, REBER, R, and SCHLUNEGGER, F. 2010. Subglacial tunnel valleys in the Alpine foreland: an example from Bern, Switzerland. ''Swiss Journal of Geosciences'', Vol.&nbsp;103, 363–374.    </ref>; Kehew ''et al''., 2012<ref name="Kehew 2012">KEHEW, A E, PIOTROWSKI, J A, and JØRGENSEN, F. 2012. Tunnel valleys: Concepts and controversies&nbsp;—&nbsp;A review. ''Earth‐Science Reviews'', Vol.&nbsp;113, 33–58.</ref>). Incision of tunnel valleys occurs under immense hydraulic gradients with flow regimes constrained by channel morphology and the thickness of overlying ice. A common characteristic of tunnel valleys is that their bases (referred to as the thalweg) are often undulating with significant normal and reverse changes in gradient developed along their long‐profile. Infills to buried valleys tend to be highly‐chaotic encompassing intercalated beds of till, glaciolacustrine (silt and clay) and glaciofluvial (sand and gravel) sediment that typically give‐rise to chaotic and unpredictable hydrogeological behaviour.
  
The rockhead surface model provides a valuable insight into the nature of the rockhead surface beneath the study area. However, it only provides a generalisation of the rockhead surface with local  variation  also  influenced  by  relative  borehole  density.    The  model  shows  a  radial arrangement of buried valleys fanning outwards from the Liverpool‐Skelmersdale area southwards and eastwards beneath the Cheshire/north Shropshire lowlands (Figure 4.1). The radial pattern conforms to the geometry of the hydraulic gradient that would generate perpendicular to the margins of a piedmont‐style glacier lobe that fanned outwards across the Cheshire lowlands towards the west, south and east. This style of glacier geometry has previously been inferred for the Late Devensian ice lobe based upon the mapped distribution morainic landforms around the region (Boulton and Worsley, 1965<ref name="Boulton 1965">BOULTON, G, and WORSLEY, P. 1965. Late Weichselian glaciation in the Cheshire‐Shropshire basin. ''Nature'', Vol.&nbsp;207, 704–706.</ref>; Yates, 1967<ref name="Yates 1967">YATES, E. 1967. A contribution to the glacial geomorphology of the Cheshire Plain. ''Transactions of the Institute of British Geographers'', 107–125.</ref>; Thomas, 1989<ref name="Thomas 1989"></ref>).
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The rockhead surface model provides a valuable insight into the nature of the rockhead surface beneath the study area. However, it only provides a generalisation of the rockhead surface with local  variation  also  influenced  by  relative  borehole  density.    The  model  shows  a  radial arrangement of buried valleys fanning outwards from the Liverpool‐Skelmersdale area southwards and eastwards beneath the Cheshire/north Shropshire lowlands (Figure 4.1). The radial pattern conforms to the geometry of the hydraulic gradient that would generate perpendicular to the margins of a piedmont‐style glacier lobe that fanned outwards across the Cheshire lowlands towards the west, south and east. This style of glacier geometry has previously been inferred for the Late Devensian ice lobe based upon the mapped distribution morainic landforms around the region (Boulton and Worsley, 1965<ref name="Boulton 1965">BOULTON, G, and WORSLEY, P. 1965. Late Weichselian glaciation in the Cheshire‐Shropshire basin. ''Nature'', Vol.&nbsp;207, 704–706.</ref>; Yates, 1967<ref name="Yates 1967">YATES, E. 1967. A contribution to the glacial geomorphology of the Cheshire Plain. ''Transactions of the Institute of British Geographers'', 107–125.</ref>; Thomas, 1989<ref name="Thomas 1989">THOMAS, G S P. 1989. The Late Devensian glaciation along the western margin of the Cheshire‐ Shroshire Iowland. ''Journal of Quaternary Science'', Vol.&nbsp;4, 167–181.</ref>).
  
 
Whilst a glacial origin for several of the larger buried channels is logical, some channels may have existed in the landscape prior to the Late Devensian glaciation and originally be of fluvial origin. For example, Worsley ''et al''. (1983)<ref name="Worsley 1983"></ref> describes a buried channel that contains preglacial organic sediments overlain by glacial till and meltwater sediments. Of particular relevance to the study area is the existence of a major buried channel beneath the modern River Mersey (Figure 4.2). Small, broadly north‐south trending offshoots of this buried valley occur to the west and east of Thornton‐le‐Moors. However,  the  resolution  of  the  rockhead  model  mean  that  the  true geometry of these buried valleys remains poorly constrained. Therefore, the presence of a buried valley beneath the Cheshire Energy Research Field Site is ‘about as likely as not’ ([[OR/17/042 Methodology#Table 2.1|Table 2.1]]). Local perturbations in the rockhead surface up to 47&nbsp;m below OD, some likely associated with buried channels, have been identified to the east of the village of Elton, beneath Ince Marshes and are described by Burke ''et al''. (2016)<ref name="Burke 2016"></ref>.
 
Whilst a glacial origin for several of the larger buried channels is logical, some channels may have existed in the landscape prior to the Late Devensian glaciation and originally be of fluvial origin. For example, Worsley ''et al''. (1983)<ref name="Worsley 1983"></ref> describes a buried channel that contains preglacial organic sediments overlain by glacial till and meltwater sediments. Of particular relevance to the study area is the existence of a major buried channel beneath the modern River Mersey (Figure 4.2). Small, broadly north‐south trending offshoots of this buried valley occur to the west and east of Thornton‐le‐Moors. However,  the  resolution  of  the  rockhead  model  mean  that  the  true geometry of these buried valleys remains poorly constrained. Therefore, the presence of a buried valley beneath the Cheshire Energy Research Field Site is ‘about as likely as not’ ([[OR/17/042 Methodology#Table 2.1|Table 2.1]]). Local perturbations in the rockhead surface up to 47&nbsp;m below OD, some likely associated with buried channels, have been identified to the east of the village of Elton, beneath Ince Marshes and are described by Burke ''et al''. (2016)<ref name="Burke 2016"></ref>.

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