The relationship between spring discharge, drainage, and periglacial geomorphology of the Frome valley, central Cotswolds, UK

Proceedings of the Geologists’ Association 125 (2014) 182–194

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The relationship between spring discharge, drainage, and periglacial geomorphology of the Frome valley, central Cotswolds, UK Jonathan D. Paul * Bullard Laboratories, Department of Earth Sciences, University of Cambridge, CB3 0EZ, UK

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 October 2013 Received in revised form 2 December 2013 Accepted 5 December 2013 Available online 26 February 2014

The Frome near Stroud is an unusual example of a Cotswold stream that flows west, against regional topographic and geological dips. As a result, a deeply incised and irregularly indented valley has been carved through a succession of Jurassic strata of varying competencies and permeabilities. The landscape has been modified by intense, locally variable periglacial erosion during Devensian times, resulting in a number of characteristic landforms including landslip, valley bulging, and limestone cambering. This study assesses the importance of spring and river discharge upon the sculpting of such a unique landscape. An extensive discharge survey of 67 hillside springs has revealed two well defined springlines that form at stratigraphical interfaces. Groundwater issues in greater abundance from the lower, Inferior Oolite, aquifer; discharge here is more regular throughout the year. Groundwater flow is a function of the regional SE strata dip, and of the heavily fissured character of the limestone, which provides rapid preferential flow pathways. Discharge of the River Frome was measured at four localities and cannot be explained by a simple model using upstream drainage area, as the channel can run completely dry over limestone in summer. The position of the springs has influenced the development of a line of settlements along the valley sides, as well as the proliferation of industry in the valley floor, with mills sited at points of high stream power. Geology affects valley shape, width, and orientation; the structure of the jointed limestone aquifer guides spring discharge and the orientation of many dry valleys. ß 2014 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved.

Keywords: Cotswolds Jurassic Limestone Periglacial geomorphology Springs Fluvial processes

1. Introduction 1.1. Solid geology Exploration of the Jurassic rocks of the Cotswold Hills, Gloucestershire (Fig. 1) led to some of the most significant discoveries in the early days of geology, from vertebrate fauna of international importance to widely accepted uniformitarian models (e.g. Wright, 1856; Witchell, 1882; Buckman, 1901). These rocks form part of the thin, lenticular sheet of Jurassic strata running from Dorset to the North Sea off the Yorkshire coast. Deposition took place along the eastern margin of the deep (1.5–2 km), asymmetrical Permo-Triassic Worcester Basin (Hancock, 1969; Goudie and Parker, 1996; Barron et al., 1997). Subsidence of this basin probably continued through Jurassic times allowing thick piles of shallowwater sediment to accumulate, reaching a maximum thickness of 2 km in the axial part of the basin SE of Cheltenham. A stratigraphic framework for the Cotswolds has been established for some time, with many studies devoted to minor revisions

* Tel.: þ44 (0)7784772818. E-mail address: [email protected]

(e.g. Buckman, 1901; Arkell, 1933; Mudge, 1978; Barron et al., 1997; Cox et al., 1999). The mid-Cotswold Jurassic succession is almost entirely marine, characterised by a complex distribution of shallowshelf ooidal limestones and deeper water clays and mudstones that were deposited in quieter conditions. Sedimentation was affected by a repeated series of transgressions and regressions, causing often subtle lateral and vertical facies changes and non-sequences, which reflect a variety of different depositional environments. Uncertainty in detailed correlation across the rhythmic alteration of these deposits persists (Mudge, 1978; Barron et al., 1997). The oldest strata encountered in the mid-Cotswolds are the finegrained clays and mudstones of the Lias Group, a name originally adopted by William Smith from quarry workers in the 1790s (Arkell, 1933). These rocks crop out in the Severn Vale and extend at depth (to 100 m thickness) beneath higher strata to the east. Of particular note is the Marlstone Rock Formation, a striking red, shelly ferrugineous limestone deposited following an increase in terrigenous sediment supply and the formation of extensive, shallowwater continental shelves after a rapid regression at the beginning of Toarcian times (Self and Boycott, 2004). After this event, a further deepening and subsequent shallowing of sea level led to muddy deposition (Whitby Mudstone) followed by coarser material. These sediments constitute the Bridport (formerly Cotteswold or

0016-7878/$ – see front matter ß 2014 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pgeola.2013.12.007

J.D. Paul / Proceedings of the Geologists’ Association 125 (2014) 182–194

Cotswold) Sands, the uppermost Lias strata in the mid-Cotswolds, becoming more clayey with distance north (Besien et al., 2006). A long period of shallow, tropical seas followed, producing widespread carbonate deposits. In 1799, William Smith identified a ‘freestone’ above strata now assigned to the Lias, which he later augmened to ‘under oolite’, now known as the Inferior Oolite Group (Wright, 1856; Witchell, 1882). Buckman (1901) originally established the tripartite structure to Inferior Oolite limestones (i.e. Salperton, Aston, and Birdlip Formations). High energy, open water depositionalenvironmentsare indicatedby thedominanceof currentinfluenced oolite shoals and disarticulated shelly material, though marly lenses and mudstone horizons suggest the periodic establishment of more protected lagoonal conditions. The Fuller’s Earth formation is one such horizon, and separates the porous limestones of the Inferior Oolite from the succeeding Great Oolite. The impermeability of this horizon, and of the Lias clays, has profound implications upon regional hydrology and groundwater flow (Section 1.3; Goudie and Parker, 1996; Neumann et al., 2003; Besien et al., 2006). Structurally, the oolite limestones are a heavily fractured rock mass. Hancock (1969) identified six joint sets, of which four are oriented normal to bedding. This bedding is inclined SE at angles <58 (Section 1.2). The effects of E-W-striking normal faults are most apparent in the northern Cotswolds; the area of interest to this study is shown in Fig. 1.

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1.2. Geomorphology The geology of the Stroud area was first documented in detail by Witchell (1882), followed by Gardiner (1938), who established the strong influence of bedrock geology on the landscape in this area. The preacher and essayist Sydney Smith wrote of the limestone plateau above Stroud in 1845: ‘‘. . .this region of stone and sorrow . . .one of the most unfortunate, desolate countries under heaven, divided by stone walls, and abandoned to screaming kites and larcenous crows.’’ The oldest geomorphological analysis of the Cotswold Hills includes references to ‘sundry valley forms’, a ‘high, featureless plateau’, and a ‘steep escarpment facing the Severn Vale’ (Lycett, 1857). The hills stretch for 100 km from Bath to Warwickshire. Topography essentially reflects the underlying geology: the gentle SE dip of the Jurassic limestone strata is expressed as a broad, asymmetrical, intensely dissected plateau tilted SSE-SE at typical angles of 0.5–1.58 (Goudie and Parker, 1996; Neumann et al., 2003). The plateau ends in the NW at the heavily indented Cotswold Escarpment (Fig. 1), which forms a surface water divide between the Rivers Severn and Thames at maximum elevations of >300 m near Cheltenham (Besien et al., 2006). The gentle Great Oolite dip slope to the SE is progressively covered by younger deposits.

Fig. 1. Location of Frome valley within Gloucestershire. Note significant embayment in the Cotswold Scarp in the Stroud area as indicated by the 15 km deflection of the main Thames/Severn (Atlantic) drainage divide to the east, and the paucity of rivers that flow through the scarp edge. Key to principal rivers of Frome catchment: A: Avening Brook; F: River Frome; H: Holy Brook; N: Nailsworth Stream; P: Painswick Stream; S: Slad Brook; T: Toadsmoor Stream.

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What has driven the development of this dip-scarp morphology? Watts et al. (2005) used 2D flexural unloading models to argue that the Cotswolds were uplifted as an isostatic response to the removal of large volumes of Upper Triassic and Lower Jurassic mudrocks in the English Midlands. The discovery of allochthonous quartzite clastics in Quaternary-aged river gravels of the Upper Thames suggests a drainage switch towards the SW and Bristol Channel, possibly in response to recent uplift (Rose, 1994; Watts et al., 2005). Others have inferred a phase of regional uplift, caused by magmatic activity associated with the opening of the Atlantic Ocean 59 Ma, from the change in strike of Jurassic limestones in Gloucestershire to NE-SW from N-S elsewhere (Cope, 1994; White and Lovell, 1997). Whatever the origin, it is likely that rock uplift of the Cotswolds is relatively young and that eastward scarp retreat has taken place over the last 1 Myr, since there is no evidence of topographic barriers to impede NW-SE river flow prior to Early Quaternary times (Watts et al., 2000, 2005). The formation and occurrence of geomorphological phenomena in the Stroud area such as landslip, dry valleys, valley bulging, and limestone cambering, can be attributed to the interaction between bedrock geology and periglacial conditions during the last glacial period (Devensian: 75–12 ka). Large and Sparks (1961) reported on the discovery of mammoth remains and elephant teeth from the Devensian-aged Cainscross Terrace river gravels of the River Frome near Stroud. These fossils are suggestive of a cold, open grassland or tundra environment; coarse, poorly sorted and unbedded gravel deposits imply coeval frost weathering of uplands and mass movement to the valley floor. Furthermore, since the gradient of this terrace was found to be steeper than present valley gradient, the loadto-discharge ratio of the palaeo-Frome was greater, which is expected under cold conditions (Large and Sparks, 1961). However, it is unlikely that any ice entered the central or southern Cotswolds as no evidence for glacial erosion or deposition has been documented (Castleden, 1977; Rose, 1994; Goudie and Parker, 1996; Cope et al., 1999). Dry periglacial conditions are geomorphologically benign. During periods of moisture availability, dynamic surface processes including solifluction, intense scouring, and freeze-thaw weathering, result in a characteristic set of landscape morphologies, all of which are genetically linked (Kellaway et al., 1971; Castleden, 1977; Paterson, 1977). South of Stroud, the River Frome is underfit relative to its valley width (smaller by a ratio of 10:1 than its valley width and meander wavelength: Goudie and Hart, 1975). After Devensian times, gradual thawing supplied the drainage system with great volumes of water that were subsequently reduced. River regimes in periglacial conditions have been characterised as ‘nival’, involving greater discharge and rates of incision (Brown and Courteney, 1969; Paterson, 1977). Dry valleys (or ‘coombes’) are a relic topographic feature of such niveo-fluvial processes in the Cotswolds and elsewhere, involving discharge reduction and increased limestone porosity (e.g. joint enlargement through solution) since endPleistocene times. Many smaller coombes contain the sites of former springs at their head; spring discharge only occurred in the absence of permafrost. Seasonal streams or winterbournes may flow episodically along the coombes (Goudie and Hart, 1975; Neumann et al., 2003; Connelly, 2005). Based on analyses of the ‘staircase’ pattern of nearby Thames river terraces, such an episodicity could reflect multiple climate cycles and denudational isostatic events (Maddy, 1997). Dry valleys are often oriented at right-angles to chalk scarps, reflecting the importance of steep scarp-face slopes on valley development under periglacial conditions (Van der Hammen, 1951; Brown and Courteney, 1969; Paterson, 1977). Landslipping is responsible for the shapes of these dry valleys: ancient landslides obscure most of the solid geology outcrops in the Frome valley (Ackermann and Cave, 1967). The Cotswolds are one of the most concentrated areas of landsliding in UK (Goudie and Hart, 1975; Connelly, 2005). Causes include differential

erosion of weaker underlying Lias clays, leading to slope oversteepening; the nature of coherent limestone-plastic clay stratigraphy; and the lowering of bulk rock strength due to water interaction, including thawing from periglacial conditions, destablising steep hillslopes. Composite rotational slips leave concave hillslope scars and are most pronounced in the presence of Fuller’s Earth (Ackermann and Cave, 1967; Parks, 1991). According to an eminent erstwhile Stroud geologist (Witchell, 1868, p. 223): ‘‘. . .it is scarcely possible to find a coombe . . .in which there are not one or more slips. Sometimes they are stationary, but frequently in winter, after heavy rains and severe frosts, they move a little forward, until, in the course of time, the masses reach the bottom of the valleys, where they are eroded by the streams.’’ Other periglacial-related phenomena in the Stroud area include valley bulging, where previously frozen plugs of clay and silt become plastic and are pushed upwards, mobilised by the weight of overlying strata, through more competent rock (normally Inferior Oolite) flooring the Frome valley (Ackermann and Cave, 1967; Parks, 1991). The result is a broad, anticlinal deformation to lower valley slopes, which may disrupt river flow. A related effect is pervasive cambering and foundering of competent, rigid cap rock (e.g. Inferior Oolite) in response to subsidence of underlying less coherent rocks (Parks, 1991; Self and Boycott, 2004). Cambering results in a rounding of hillcrests and is best developed where there is a rigid de´collement bed within the plastic rocks, for instance a stratum of Marlstone Rock within the Lias. Fig. 2 shows an idealised cross-section, corresponding stratigraphy, and a perspective view of the Frome valley. The presence of Marlstone Rock results in a conspicuous flat ledge in the lower levels of the hillside (Self and Boycott, 2004). The middle course of the River Frome was chosen as the study area since it is one of the few water courses to run through the escarpment as opposed to down the dip slope (Fig. 1). Lane et al. (2008) divided Cotswold rivers into three broad families: dip slope streams; short, steep scarp streams; and break-through streams, the fewest in number, and the group to which the River Frome belongs. High rates of river incision have revealed a proliferation of springs deep into the Jurassic succession. The Frome may have once run SE towards the London Basin, but was captured following uplift of the Cotswold Plateau and beheaded by a river running towards the Bristol Channel (e.g. Buckman, 1901; Goudie and Parker, 1996; Connelly, 2005). Lane et al. (2008) analysed 66 river profiles in the central Cotswolds to demonstrate that this uplift is relatively young and probably reflects a flexural control on the landscape. A potential wind gap, through which a proto-Frome may have flowed SE, exists at Sapperton, 5 km from the present-day Thames Head. 1.3. Hydrogeology Limestones of the Great and Inferior Oolite groups are important aquifers in the Cotswolds, the third most important source of groundwater in the UK, considered essentially separate when divided by Fuller’s Earth (Neumann et al., 2003). Mean transmissivity of the latter, considered to operate in hydraulic continuity with the Bridport Sands, was measured as 139 m2 d1; for the former trasmissivities of up to 1500 m2 d1 have been recorded (Allen et al., 1997; Jones et al., 2000; Besien et al., 2006). Geophysical logging of several boreholes in the confined Inferior Oolite elucidated the importance of structural features, rather than stratigraphy alone, in influencing patterns of groundwater flow (Morgan-Jones and Eggboro, 1981). Groundwater hydraulics operate along preferential flowpaths such as faults, high transmissivity fissures and fracture zones. Locally, flow is controlled by topography, with deeply incised valleys exerting a strong influence on hydraulic gradients (Beckinsale and Beckinsale, 1976). Valley cambering has also

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Fig. 2. Cartoon showing idealised geology and geomorphology of hillside of the Frome valley. Perspective view direction facing north. Also shown is the stratigraphy of Jurassic rocks in the mid-Cotswold area, and possible patterns of groundwater hydraulics and spring discharge from the oolite (Ackermann and Cave, 1967; Hart, 1976; Besien et al., 2006). Note that recharge can be focused and forced up along a fault (in this instance, where the Fuller’s Earth is downfaulted).

resulted in open tension cracks (‘gulls’). As a result, aquifers respond quickly to rainfall events, leading to ‘flashy’ hydrographs and rapid changes in groundwater levels, especially in valley areas due to increased aperture sizes (Hancock, 1969; Beckinsale and Beckinsale, 1976; Neumann et al., 2003; Besien et al., 2006). This process is augmented by the high limestone surface area exposed for recharge (Fig. 2). In addition, the Inferior Oolite aquifer is recharged by seepage from the overlying Fuller’s Earth Formation (which is not laterally continuous), and by downward flow along faults. Total recharge for both limestone aquifers has been measured as 370 mm yr1, compared to average annual rainfall of 650 mm yr1 (Morgan-Jones and Eggboro, 1981). Groundwater is mainly extracted for public use. Discharge from pumped boreholes, however, accounts for only a small proportion of total aquifer discharge (Neumann et al., 2003). The vast majority occurs in the form of springs and seepages, which occur at two distinct stratigraphical (and topographical, due to the very gentle dip of the Jurassic limestones) levels. These are an upper level, at the boundary of porous Great Oolite limestones and impermeable clays of the Fuller’s Earth, and a lower, generally more productive, level, where spring water issues from the Inferior Oolite limestones and Bridport Sands where they meet the Lias (Fig. 2; Richardson, 1930; Tann, 1965; Besien et al., 2006). Discharge of these springs is seasonally highly variable and a function of aquifer piezometric level; other springs associated with intra-aquifer hardbands are structurally controlled (Neumann et al., 2003). Settlement patterns along the Frome valley directly reflect the elevated positon of the spring line (and therefore geological boundaries), as a reliable source of good-quality water has been sought for generations. On the valley floor, the linear spread of cloth and fulling mills along the River Frome has been directed by the steepness of the Frome’s longitudinal profile compared to rivers of similar length flowing SE down the dip slope (Gardiner, 1938; Tann, 1965; Goudie and Parker, 1996). However, the flow rate of the predominately spring-fed Frome is intimately linked to the condition and water level of the limestone aquifers that it drains (Besien et al., 2006). Although Cotswold limestones are not well known for development of classic karstic terrain (compared to e.g. Carboniferous limestones of the Mendips), small caves have

been noted (Richardson, 1930; Goudie and Parker, 1996). Furthermore, localised (metre-wide) subsidence depressions have been recognised in some valley floors of the central Cotswolds, known locally as ‘whorley pits’ or ‘swilly holes’ (Guise, 1877). Dip streams undergo discharge diminution as they pass over heavily fissured Inferior Oolite limestones. Hart (1976) reported a significant decrease in discharge of the River Churn as the channel bedrock changed from Lias clays (320 ft3 min1/ 0.15 m3 s1, 9 km from source) to limestone (10 ft3 min1/ 0.005 m3 s1, 23 km from source). The Frome valley is an excellent place to study the interaction between geomorphology, hydrology, and solid geology in the Cotswolds since it is the only substantial embayment in the 100 km Cotswold escarpment, and a rare example of a ‘scarp stream’. Intense river incision has revealed a line of springs and resulted in steep-sided valleys that were modified under periglacial conditions. The purpose of this work is threefold. First, a comprehensive survey of spring discharge has been assembled that will assess the role of seasonal variability, aquifer productivity, and strata dip upon groundwater hydraulics. Secondly, an analysis of the Frome’s longitudinal profile will determine the effects of solid geology upon profile gradient, discharge, and valley shape. Thirdly, the geomorphological phenomena described in Section 1.2 will be analysed quantitatively using new digital elevation data. Specific questions addressed include: (i) How and why do spring and scarp stream discharge vary; do they ever run dry? (ii) How do periglacial geomorphological phenomena affect drainage? (iii) What are the main controls on groundwater flow patterns close to the escarpment edge? (iv) Do dry valleys exhibit a preferred orientation in limestone?

2. Methods In the field, research for this study involved an intensive discharge sampling campaign of 67 springs issuing into the Frome valley, Stroud. Also, river discharge was measured at four points along the Frome. These field studies are complemented by a morphometric analysis of topography, slope aspect and gradient,

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Table 1 Channel metrics and velocity measurements for River Frome discharge calculations.

Location

Xa

Ya

Elevation, m

Channel width, m

Average channel depth, m

Mean velocity, m s1

Mean discharge, m3 s1

Bowbridge Brimscombe Chalford Trellis Bri.

2.20933 2.18300 2.15767 2.09942

51.73616 51.71777 51.71994 51.72758

48 66 88 123

4.05 3.65 3.10 2.45

0.60 0.55 0.40 0.25

0.37 0.32 0.04 –

0.90 0.65 0.05 Dry

a

Longitude and latitude measured in decimal degrees.

drainage, river profile shape, discharge and stream power, using high-resolution digital elevation data as a starting point. The Frome longitudinal profile and upstream drainage area data were extracted from a 30 m  30 m digital topographic dataset generated from the Shuttle Radar Topographic Mission (SRTM; Farr et al., 2007). Standard flow-routing algorithms from the ArcGIS software package were used to extract hillslope aspect data, valley cross-sections, and a drainage network (Tarboton, 1997). Along with elevation data, the gradient of the Frome bedrock river channel, S, was extracted and averaged using a 150 m moving window, to eliminate artefacts (spikes and sinks) in the SRTM dataset.

River discharge, Q (measured in cumecs or m3 s1), can be estimated as a function of downstream distance by assuming a direct relationship with upstream drainage area A: Q ¼ vAm :

(1)

In non-urbanised river channels, Q generally scales linearly with A (i.e. the fractional exponent m 1 (e.g. Dunne and Leopold, 1978; Galster et al., 2006). v is a measure of river base flow speed; v = 0.5 m s1 is an average value for rivers of the Frome’s length (30–50 km) in their middle course (Dunne and Leopold, 1978).

Fig. 3. Solid geology of Frome valley. Circles represent springs; size is scaled according to (a) winter and (b) summer discharge. Dark/light blue colour = lower/upper springline, respectively (see Fig. 2). Numbered boxes = position of later figures. Appendix A contains further details of the spring inventory. Red lines = normal faults; tick on downthrown side. Thin black line = 100 m topographic contour.

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Fig. 4. Manifestations of water egress. See Fig. 3 for figure locations. (a) Spout discharging from the Bridport Sand Formation at Lower Butterrow (Ref. 1 in Appendix A). (b) Village spout draining Inferior Oolite limestone at Upper Butterrow (Ref. 2). (c) Water seepage along bedding planes (indicated by arrows), Aston Limestone Formation, Rodborough Common. (d) Typical Upper Springline spring, discharging from the Great Oolite Formation in a field at Snakeshole (Ref. 63). Photos taken October 2013.

Having obtained Q it is possible to derive a prediction for stream power, V (Bagnold, 1966):

V ¼ rgSQ ;

(2)

where rg is the unit weight of water. Stream power (measured in W m1) is the rate of energy dissipation to the channel bed and banks per unit downstream length (e.g. Bagnold, 1966; Dunne and Leopold, 1978; Flores et al., 2006). V governs the ability of a river to incise and is a common component of landscape evolution models, since river incision sets the pace for other erosive processes (Bagnold, 1966; Whipple and Tucker, 1999). In order to validate predicted discharge, Q was measured at four points along the middle course of the River Frome. These measurements were carried out on 17–19 July 2010, following a three-week dry period. Groundwater conditions were low. In each instance, a roughly rectangular channel reach was chosen, access permitting, and the average current calculated from 6 evenly spaced current meter measurements across the channel. Table 1 contains further information of discharge calculations at each locality. At the farthest locality upstream, the river bed was completely dry (Inferior Oolite bedrock).

The locations of productive springs were of importance for the historic fulling, dyeing, and cloth industries of the Stroud area, and have therefore been well documented (Richardson, 1930). Guided by a qualitative assessment of the most productive springs, in December 2009 discharge flow rate was measured at 67 springs along the Frome valley hillside. Potential springs were selected for inclusion if (i) water egress took the form of a single flow from solid rock; (ii) at least 200 mL of water could be collected and measured in one minute; and rejected otherwise. Further information pertaining to the inventory of springs is given in Appendix A. Measurements were repeated at the same locations in summer 2010, when 22 of the 67 sites, all along the upper springline (previously observed throwing out water from the Great Oolite limestones) were discovered to be dry. 3. Results 3.1. Spring discharge Fig. 3 summarises measurements of spring discharge at 67 sites along the Frome valley in summer and winter. The existence of two clearly defined spring lines is immediately apparent.

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Fig. 5. Topography and drainage of Frome valley. Blue lines represent water courses of Strahler order 3 (Strahler, 1952). Numbered circles = location of major textile and woollen mills along valley floor (see key). Four solid black lines that cut across valley are topographic transects shown in Fig. 8. White asterisks = location of river discharge measurements (Fig. 6). Dotted white line = top of Fuller’s Earth (i.e. upper springline), above which many streams become seasonal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

Groundwater of the lower level springs issues from the Bridport Sands in the west of the study area, rising to the lower Inferior Oolite in the east, at elevations of 100 m. The upper level is demarcated where the Fuller’s Earth throws out groundwater

from the Great Oolite limestone aquifer around 150–170 m. These stratigraphical positions and elevation ranges are remarkably consistent along the entirety of the Frome valley (Appendix A).

Fig. 6. (a) Longitudinal profile of portion of River Frome shown in Fig. 5 (Black line) and channel gradient calculated within a 150 m moving window (grey dashed line). Coloured bar = bedrock geology (see Fig. 3 for key). (b) Predicted river discharge, Q. Arrowed numbers = measured discharge. See Fig. 5 for locations. (c) Stream power, V. Arrowed numbers = position of major textile and woollen mills; see Fig. 5 for numerical key.

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the Frome to flow into the Thames valley is also shown (Section 1.2). The longitudinal profile of this middle portion of the Frome is steep, with channel slopes of up to 48 over Inferior Oolite limestones (Fig. 6). Gradients decrease markedly to 0.28 once the valley floor incises down the succession to Marlstone Rock. Predicted discharge varies between 0.6 and 1.2 m3 s1; measured discharge was of the same order of magnitude as these predictions in three locations, whereas the channel was dry in one upstream locality, 15 km from the source, where the bedrock was limestone (Fig. 7). Based on measured channel slope, and the discharge predicted according to Eq. (1), Fig. 6c shows how the ability of the river to incise (i.e. river power) reaches a maximum at a geological boundary between Inferior Oolite limestones and the Bridport Sands. River power is reflected in the distribution of industry along the valley floor: mills cluster together along reaches of high stream power, for instance at confluences with major tributaries. 3.3. Geomorphological analysis

Fig. 7. Dry channel of River Frome at Trellis Bridge (see Table 1 and Fig. 5 for location), 15 km from its source (October 2013). The channel is floored by Inferior Oolite limestones at this locality. Note large boulders indicating periods of much higher discharge.

Springs of the lower line are fewer in number but generally yielded greater discharge compared to the upper line (Fig. 4a and b). Some named springs have been documented for their use in the local brewing industries and domestic water supply, such as the Black Gutter at Chalford (Richardson, 1930). Some outcrops of Inferior Oolite showed evidence of water seepage, especially along bedding planes (Fig. 4c), giving rise to the local phenomenon known as ‘hot rock’. By contrast, upper springs of the Great Oolite were more numerous and diffuse, often taking the form of slow upward leakages via fissures in the limestone (Fig. 4d). Average discharges were lower and somewhat harder to measure, since fewer of the sources were channelled for drinking via spring-line village spouts, unlike the lower springs (Fig. 4). Summer discharges were lower for all springs compared to winter measurements. 22 of the upper springs were completely dry at this time, while flow rates of the lower springs showed a lower proportional decrease. In general, a greater density of springs and other groundwater seepages was observed on the gentler northern and western (dip) valley slopes. Furthermore, the discharge of these springs was normally greater than their counterparts across the valley. 3.2. Frome longitudinal profile Fig. 5 shows the gently SE-tilted Cotswold topography and the drainage network of the Frome valley. The low-topography wind gap channel that potentially acted as a conduit for headwaters of

The manifest role of geology in dictating the shape of the landscape is shown in Fig. 8. A valley bulge, farther downstream than any hitherto reported (e.g. Ackermann and Cave, 1967; Goudie and Parker, 1996) is well developed in the Marlstone Rock that floors the Frome valley close to Stroud. This bulge has forced the river to flow flush against the western break in slope of the valley floor. Immediately upstream from Chalford, a valley bulge is coincident with a Bridport Sand and Lias protrusion through the Inferior Oolite-floored valley base, disrupting the normal stratigraphic sequence (Fig. 3). Valley slopes are steepest, and landslips most common, where the bedrock is Inferior Oolite limestone. The surface expression of Great Oolite limestone is hummocky topography and a gently undulating broad plateau surface. Where exposed, Fuller’s Earth forms a laterally continuous bench in the hillside, often gently back-tilted against the main valley side. Landslips obscure solid outcrop for the most part, generating gently convex hillside profiles along the valley. Fig. 9 illustrates some of the observed geomorphological features in the study area. While the azimuth of the Frome varies greatly across the study area (but generally cuts against the prevailing SE dip of the Jurassic limestones), many of its smaller tributaries display a striking E-W trend (Fig. 10). This trend is also true of topography between the larger tributary valleys, implying the development of ubiquitous dry valleys (some of which are drained by seasonal streams) of variable dimensions. These valleys are generally asymmetric, with steeper slopes on their northern or northeastern (i.e. dip slope-facing) slope. Recent and historical landslips are observed in close proximity to existing springs and drainage (Fig. 9c and d). This reach of the Frome valley becomes substantially wider such that the river is misfit once it turns towards the NE and Stroud (Fig. 9a). The widening is coincident with a change in valley floor geology from Inferior Oolite limestones to muds and marls of the Lias Group. 4. Discussion The variation in discharge between springs of the Great and Inferior Oolite aquifers is striking. In spite of the higher transmissivity of the Great Oolite aquifer, spring discharge was typically less than that issuing from the Inferior Oolite. Very high flow rates (>2 L s1), for instance at the Black Gutter near Chalford, were restricted to springs with direct limestone rock egress (instead of those issuing from the Bridport Sands). Groundwater of these high-flow springs was generally thrown out by solutionenhanced bedding-parallel fissures, confirming the importance of

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Fig. 8. Topographic swaths across Frome valley (see Fig. 5 for locations). Solid geology across each swath is also shown (note cartoon, not cross-sectional, representation). (A) Lower course, where marlstone and clays of the Lias Group floor the valley. Note wide valley floor. (B) Rising through the stratigraphy, the Inferior Oolite limestones underlie the valley sides, causing them to steepen. (C) Swath revealing well developed Fuller’s Earth bench in the hillside. (D) Upper course.

rock structure (rather than stratigraphy) in dictating groundwater hydraulics in oolitic limestone (cf. Morgan-Jones and Eggboro, 1981). However, this study is probably too localised to capture the full range of flow conditions reported for the limestone aquifers. In addition to the general increase in transmissivities with increasing westward aquifer thickness, strong local effects can focus groundwater flow and aquifer recharge. For instance, of 12 pumping tests carried out at Bibury, 11 yielded a discharge of <10 m3 d1, while the borehole that intersected a ‘good fracture system’ yielded 5000 m3 d1 (Al-Dabbagh, 1975). Focused zones of low transmissivity have also been recognised where aquifer thickness is decreased along predominately E-W trending normal faults. However, in other locations, downfaulting of impermeable strata, such as Fuller’s Earth, can force water upwards along the fault (Al-Dabbagh, 1975; Allen et al., 1997; Fig. 2). Fig. 3 shows that few of the analysed springs are located close to mapped normal faults. Although the unconfined Great Oolite is less fractured than the Inferior Oolite (possibly owing to a lower clay content), its transmissivity is roughly double (Al-Dabbagh, 1975). The ‘flashiness’ and greater seasonal variability of the Great Oolite aquifer has been explained by modelling it as a fixed-volume receptacle. Water level fluctuation is small since the head never exceeds a certain level (Al-Dabbagh, 1975). The spring discharge values presented here cannot account for the ‘automatic’, focused draining of the aquifer via low storage but highly transmissive fractures.

The proximity of the lower springline to the deeply incised valley floor, where transmissivity is typically higher owing to elevated rates of solution weathering, could explain the disparity between the discharge of the two springlines. More extensive cambering in the Inferior Oolite limestones has led to a greater density and larger apertures of tension cracks, which act as preferential flow pathways. Recharge from the overlying Great Oolite aquifer could also occur where the Fuller’s Earth is absent farther north. The greater number of springs, associated discharge, and spring-line settlements on the gentler dip slopes (which, like the Cotswold Hills, face SE) implies that regional groundwater flow and hydraulic gradients mirror the dip of the landscape and Jurassic limestone strata. However, exceptions such as the powerful Black Gutter spring on the north-facing valley slope could reflect an unmapped fault or other local structural control providing a high transmissivity flow pathway. Compared to the winter observations, values of spring discharge in summer are lower. However, while some of the upper springs dry up completely, discharge of many of the lower springs does not noticeably change, suggesting a buffer against seasonal effects. The upper springline is more prone to rapid recharge, while residence times in the Inferior Oolite aquifer may be greater, resulting in steadier and more sustained yearround discharge. The correlation of the springline settlement pattern with lower level springs could result from this more reliable water supply.

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191

Fig. 9. Geomorphological phenomena along the valley of the River Frome. See Fig. 3 for locations. (a) General view north towards Stroud. Note springline settlement at two levels and industry in valley floor. Orientation of dry valleys is E–W. The Inferior Oolite limestone underlying the prominent hillside in the foreground has been rounded and obscured by successive ancient landslides. (b) Typical upper reach valley bottom, with evidence of soil creep on rounded, convex lower slopes. (c) Landslide of Inferior Oolite limestone leaving concave scour and limestone detritus on underlying Bridport Sand Formation. (d) Landslip and solifluction creep in regolith overlying Inferior Oolite limestone.

Discharge of the River Frome is inextricably linked to the water table, especially spring discharge. This finding agrees with Richardson (1930)’s observation that ‘most of the water carried off by the Frome is derived from springs issuing from the Cotteswold Sands’. A simple model of discharge variation with downstream distance cannot account for the influence of heavily fissured oolitic limestone, rendering the channel dry some 10 km from its source. Although the discharge of dip rivers has been observed to vary with bedrock geology, and highly permeable solution features exist in isolation, it is striking that a scarp river should be completely dry (Richardson, 1930; Hart, 1976; Goudie and Parker, 1996). Together with the seasonal variation in spring discharge, the piezometric level in the oolite aquifers must therefore be subject to regular and rapid fluctuation. Water loss and unpredictable discharge were also experienced during the construction of the nearby Thames and Severn Canal in the 1780s, ultimately causing its abandonment: company surveyor Robert Whitworth blamed water escape on ‘bad, rocky ground’ (Household, 1969). The development of heavy industry correlates closely with stream power. Most textile and woollen mills are located on the Lias, whose presence leads to a widening of the valley floor and guarantees reliable river flow. While channel gradients are greater as the river flows over Inferior Oolite limestones, the steep, narrow valley sides and irregular discharge preclude large-scale industrial

development. Also, the lower springline is best developed where the contact between permeable oolite or Bridport Sands and impermeable Lias clay is exposed in the hillside in the west of the study area. Although asymmetrical uplift of the Cotswold Plateau probably governed the distribution of present-day drainage, explaining why so few major Cotswold rivers flow west into the River Severn, the role of periglaciation is also critical in modifying the form of the Frome valley. In many instances there is a complicated series of interactions between periglaciation, geomorphology, bedrock geology, and drainage. For instance, the Frome originally carved out a much wider valley when its discharge was greater during Devensian times. Subsequent mobilisation of hitherto frozen clays resulted in broad, convex-upward bulging of the wide valley floor, which diverts the Frome to the very edge of the valley between Stroud and Chalford. Many of these bulges were observed at confluences of the Frome with tributaries; at these points, a greater depth of stratigraphy is exposed (cf. Fig. 3). This finding suggests that the majority of valley bulges form at points of minimal valley bottom load. Therefore, the distribution of springs (giving rise to winterbournes and tributary streams) directly influences mass movement along the valley floor. The diversion of the river channel by valley bulges to the edge of the valley floor results in intense lateral erosion of Lias clays and

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the water table was higher. Alternatively, this topography could be a manifestation of pre-existing lines of structural weakness, since tributary valleys in the headwaters of the Frome are shaped by E-W trending normal faults. The preservation of ridges, troughs and dry valleys in lieu of being integrated into simpler drainage forms suggests rapid and recent formation. It is likely that niveo-fluvial processes in operation during Devensian times acted in concert with spring discharge, in the absence of permafrost, generating well defined loci of intense weathering that resulted in many of the landforms observed in the Frome valley today. 5. Conclusions

Fig. 10. Hillslope aspect and drainage map of Frome valley, south of Stroud. Dashed blue lines = seasonal streams (winterbournes). Blue circles = position of springs (Fig. 3). Arrows = prominent landslides. Stippling = areas where hillside gradient >158. Note scarp-parallel ENE-WSW pattern of Frome tributaries and secondary valley (slip-trough) topography.

mudstones, destablising the valley side, leading to landslips and cambering of limestone strata. Fluvial erosion may act as a trigger; however, the cyclical superposition of incompetent clays and rigid, competent limestones along the Frome valley has created natural conditions that favour mass movement. Lubrication of slip planes due to spring discharge or other groundwater leakage can provide other potential triggers. The greater number of springs and intensity of groundwater discharge along slopes with an easterly or southeasterly aspect could explain the asymmetry of many smaller dry valleys. Most dry valleys follow a remarkable E-W orientation, oblique to the general trend of the Cotswold escarpment. This orientation could reflect structural inheritance of a pre-Pleistocene palaeotopographic high. Paterson (1977) noted the largely perpendicular orientation of chalk coombes to the principal scarp of the North Downs. It is therefore likely that the numerous cross-cutting coombes of the Frome valley developed most rapidly during Devensian times, when solifluction and running water followed the steepest gradient of the hillslope, increasing their erosive power. The undulatory hillslope topography, where E-W ridges and troughs form perpendicular to the Frome valley, has only previously been observed along the valley of the River Windrush in the northern Cotswolds (Briggs and Courteney, 1972; Goudie and Hart, 1975). The regular etching of these small troughs into the valley side could imply the former issue of groundwater from regularly spaced limestone fissures during periglacial times, when

 The permeability of the oolite limestones, their interaction with bounding incompetent clay strata, and periglacial erosive processes, have all modified the deeply incised Frome valley near Stroud.  Spring discharge occurs at two well defined stratigraphic interfaces and, like river discharge, is dependent on aquifer levels and is seasonally highly variable.  Groundwater issuing from the Inferior Oolite takes the form of a smaller number of intense point sources.  Groundwater flow is affected by the regional SE dip of the limestone strata and by the presence of pervasive fissuring in the limestone.  The valley has undergone a complicated geomorphological evolution: intense episodes of physical weathering took place during cold, dry periglacial conditions, and subsequent meltwaters made a major contribution to valley incision. Springs probably acted as loci for the most intense freeze-thaw action, leading to the development of many smaller E-W striking valleys that are now dry.  Fissuring and jointing of the limestone aquifer provides a framework explaining the orientation of these valleys and subsequent patterns of groundwater flow and mass movement.  While spring discharge directs mass movement along the hillside, the effects of this movement, including landslip and valley bulging, have modified present patterns of drainage.  Springline settlement has developed along the base of the Inferior Oolite, but not along the upper Great Oolite springline, as many springs here run dry in summer.  The position of these springs has affected industrial development on the valley floor: mills are concentrated on clay-floored channel reaches where stream power is high, reflecting steep gradients and the high density of adjoining tributaries providing additional discharge.

Acknowledgements The large number of farmers and landowners who allowed access to the springs in their fields are thanked. The helpful and constructive comments of two anonymous reviewers greatly strengthened the manuscript. Figures were prepared using GMT4.2.0 (Wessel and Smith, 1991) and Inkscape. Appendix A. Spring discharge data See Table A.1.

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Table A.1 Position and discharge data of 67 springs used in this study. (a) U/L denote upper (Great Oolite) and lower (Inferior Oolite) springlines.

Ref.

Name

(a)

Xa

Ya

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67

Lower Butterrow Upper Butterrow Montserrat Bagpath Swellshill (1) Swellshill (2) Claycombe (1) Claycombe (2) Claycombe (3) Lower Besbury Upper Besbury (1) Upper Besbury (2) The Knapp Hyde Black Gutter Frampton Mansell (1) Frampton Mansell (2) Sapperton Pinbury Park E (1) Pinbury Park E (2) Pinbury Park W (1) Pinbury Park W (2) Daneway Tunley (1) Tunley (2) Tunley (3) Tunley (4) Tunley (5) Far Oakridge (1) Far Oakridge (2) Oakridge (1) Oakridge (2) Oakridge (3) France Lynch (1) France Lynch (2) France Lynch (3) France Lynch (4) France Lynch (5) France Lynch (6) France Lynch (7) Chalford (1) Chalford (2) Brownshill Bussage Toadsmoor ponds (1) Toadsmoor ponds (2) Toadsmoor E (1) Toadsmoor E (2) Toadsmoor E (3) Toadsmoor E (4) Toadsmoor E (5) Toadsmoor E (6) Toadsmoor E (7) Toadsmoor E (8) Quarhouse Upper Thrupp (1) Upper Thrupp (2) Upper Thrupp (3) Lower Thrupp (1) Lower Thrupp (2) The Heavens (1) The Heavens (2) Snakeshole (1) Snakeshole (2) Snakeshole (3) Field Rd Stroud Bowbridge

L L L L L L U U U L U U U U L L L U U U U U L L L L L L U U U U U U U U U U U U U U U L L L U U U U U U U U U U U U L L U U U U U L L

2.20783 2.21070 2.20789 2.20724 2.19852 2.19923 2.20349 2.20273 2.19833 2.18721 2.19140 2.18316 2.17282 2.16248 2.13693 2.12360 2.12216 2.08565 2.06986 2.06735 2.06170 2.07355 2.07308 2.09685 2.09953 2.09529 2.10614 2.09872 2.11188 2.10981 2.11943 2.12282 2.12870 2.14545 2.14204 2.14103 2.13766 2.13445 2.13337 2.13096 2.15267 2.15809 2.17187 2.17130 2.17687 2.17794 2.17846 2.17787 2.17876 2.18038 2.18080 2.17931 2.17799 2.18032 2.18663 2.19363 2.19524 2.19618 2.20142 2.20149 2.19468 2.19087 2.18565 2.18320 2.18563 2.21006 2.20890

51.73276 51.72946 51.72582 51.72246 51.71595 51.71428 51.71185 51.71062 51.71109 51.71290 51.71020 51.70941 51.71148 51.70873 51.71150 51.71254 51.71266 51.71521 51.72501 51.72687 51.73095 51.73420 51.72680 51.73293 51.73114 51.73181 51.72820 51.72940 51.72046 51.72118 51.71957 51.72308 51.72117 51.71976 51.72078 51.72416 51.72418 51.72682 51.72933 51.73050 51.71965 51.72213 51.71722 51.72854 51.73598 51.73630 51.73301 51.73118 51.73061 51.72922 51.72741 51.72685 51.72447 51.72298 51.72041 51.72478 51.72683 51.72949 51.73220 51.72958 51.73264 51.73419 51.73808 51.73897 51.74176 51.74078 51.73540

a

Longitude and latitude measured in decimal degrees.

Elevation, m 85 99 80 90 84 106 175 181 172 102 174 169 160 142 96 106 102 170 155 155 174 170 121 104 109 123 120 111 170 170 168 159 158 175 170 181 173 175 175 172 162 165 161 111 110 101 175 186 174 180 175 176 174 169 170 164 155 164 81 59 166 161 170 169 174 75 69

Winter discharge, L s1

Summer discharge, L s1

0.42 0.61 0.30 0.66 0.40 0.65 0.08 0.15 0.05 0.20 0.05 0.05 0.05 0.08 3.50 0.15 0.42 0.08 0.03 0.03 0.05 0.05 0.40 0.72 0.35 1.20 1.15 1.85 1.15 0.07 0.06 0.06 0.05 0.65 0.09 0.55 0.05 0.16 0.04 0.04 0.46 0.16 0.15 0.80 2.30 1.65 0.90 1.50 0.33 0.25 0.55 0.52 0.09 0.39 0.35 0.06 0.12 0.04 0.18 0.08 0.07 0.13 0.09 0.14 0.34 0.40 0.15

0.32 0.45 0.25 0.47 0.33 0.48 Dry Dry Dry 0.15 Dry Dry Dry Dry 1.80 0.11 0.37 Dry Dry Dry 0.05 0.04 0.33 0.18 0.15 0.45 0.65 0.32 0.18 0.02 Dry Dry Dry 0.52 0.02 Dry Dry 0.04 Dry Dry 0.17 0.12 0.05 0.57 2.20 0.95 0.18 0.60 0.11 0.12 0.15 0.15 0.02 0.16 0.13 0.02 Dry Dry 0.15 0.08 0.04 Dry Dry Dry 0.08 0.20 0.14

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