Lateglacial and Holocene River Development in the Upland Areas Of

Late Quaternary River Development and

Archaeology of the Middle Tywi Valley

Draft final report for Dyfed Archaeological Trust for the Exploration Tywi! Project.

February 2011

Anna F. Jones, Paul A. Brewer, Mark G. Macklin, Catherine. H. Swain, Graham Bird, Hywel M. Griffiths and Lynda Yorke.

Centre for Catchment and Coastal Research, and River Basin Dynamics and Hydrology Research Group Institute of Geography and Earth Sciences Aberystwyth University Aberystwyth SY23 3DB

CONTENTS

Page List of Figures ii List of Tables iv 1. Introduction and literature review 1 2. Study site 5 3. Methods 5 4. Results 10 4.1 Historical river channel change 10 4.2 River terrace sequences 14 4.3 Abermarlais 23 4.4 Sawdde alluvial fan 24 4.5 College Farm 30 4.6 Cwrt Henri 30 5. Valley floor development in the middle Tywi valley 44 5.1 The Afon Tywi 44 5.2 The Afon Dulas 47 6. Discussion 48 6.1 Comparison of the Late Quaternary river record in the middle Tywi valley with other upland UK rivers 48 6.2 Valley floor development and the preservation of the archaeological resource 52 7. Conclusions 58

Acknowledgements 59 References 60

Appendix 1: Geomorphological amps and LiDAR relief 64 Appendix 2: Raw and interpreted GPR profiles 70 Appendix 3: River channel change maps 87 Appendix 4: River channel change overlay maps 92

i LIST OF FIGURES

Page Figure 1 The Tywi catchment 6 Figure 2 Geology of the Tywi catchment 7 Figure 3 Location of the sites chosen for detailed field studies 9 within the Tywi Valley Figure 4 Individual channel position maps showing lateral river 11 channel changes at Abermarlais between 1761 and 2006. Figure 5 Overlay of lateral river channel changes at Abermarlais 12 between 1761 and 2006. Figure 6 Meander bend evolution upstream of Llandeilo, and air 13 photo evidence of multiple palaeochannels meander cut- offs. Figure 7 Geomorphology and river terrace units in the Tywi study 16-20 reach Figure 8 Long-profiles of river terrace units in the Tywi study reach. 21 Figure 9 Geomorphology and river terrace units in the Dulas study 22 reach Figure 10 Location of sediment cores and GPR surveys at the 25 Abermarlais site Figure 11 Sediment sequence recovered from Abermarlais Core 1 26 Figure 12 Sediment sequence recovered from Abermarlais Core 3 27 Figure 13 Sediment sequence recovered from Abermarlais Core 2 28 Figure 14 Location of sediment cores and GPR surveys at the 31 Sawdde alluvial fan site Figure 15 Sediment sequence recovered from Sawdde alluvial fan 32 Core 1 Figure 16 Sediment sequence recovered from Sawdde alluvial fan 33 Core 2

ii Figure 17 Sediment sequence recovered from Sawdde alluvial fan 33 Core 3 Figure 18 Sediment sequence recovered from Sawdde alluvial fan 34 Core 4 Figure 19 Sediment sequence recovered from Sawdde alluvial fan 34 Core 5 Figure 20 Sediment sequence recovered from Sawdde alluvial fan 35 Core 6 Figure 21 Sediment sequence recovered from Sawdde alluvial fan 35 Core 7 Figure 22 Location of sediment cores and GPR surveys at the 36 College Farm site Figure 23 Sediment sequence recovered from College Farm Core 1 37 Figure 24 Sediment sequence recovered from College Farm Core 2 38 Figure 25 Sediment sequence recovered from College Farm Core 4 39 Figure 26 Sediment sequence recovered from College Farm Core 5 40 Figure 27 Location of sediment cores and GPR surveys at the Cwrt 41 Henri site Figure 28 Sediment sequence recovered from Cwrt Henri Core 1 42 Figure 29 Sediment sequence recovered from Cwrt Henri Core 2 42 Figure 30 Sediment sequence recovered from Cwrt Henri Core 3 43 Figure 31 Sediment sequence recovered from Cwrt Henri Core 4 43 Figure 32 Sediment sequence recovered from Cwrt Henri Core 5 44 Figure 33 Fluvial discontinuities in selected Welsh river catchments 51 Figure 34 Distribution of archaeological finds and sites on alluvial 53-55 units of different ages Figure 35 Distribution of archaeological sites on alluvium of different 56 ages in the Dulas valley floor Figure 36 Distribution of archaeological sites/finds on Late 57 Quaternary river terraces in the middle Tywi valley

iii LIST OF TABLES

Page Table 1 River terrace sequence of the middle Tywi valley. 15 Table 2 River terrace sequence of the Afon Dulas. 23 Table 3 Radiocarbon-dated samples recovered from cored 29 sediment sequences

iv 1. Introduction and literature review Lateglacial and Holocene river development in the upland areas of northern and western Britain has been characterised by incision into Devensian glacial and glaciofluvial valley floor deposits (Harvey and Renwick, 1987, Hooke et al., 1990, Howard et al., 1999, Macklin and Lewin, 1986, Passmore and Macklin, 2000, Taylor and Lewin, 1997, Taylor et al., 2000, Tipping, 1995). This incision has been episodic, rather than continuous, and, coupled with extensive lateral channel migration, has resulted in the production of sequences of river terraces, of which the oldest are at the greatest elevation above the modern channel and the youngest form the modern floodplain (Macklin and Lewin, 1986, Passmore and Macklin, 2000). Initial incision into the pre-existing glacial and glaciofluvial sediments occurred as a result of the reduction in sediment supply caused by the establishment of vegetation on valley floors and slopes as climate ameliorated (Lewin and Macklin, 2003). Subsequent episodes of incision have also been ascribed to perturbations in climate, as well as to sea level change and isostatic adjustment (Foster et al., 2009). The preservation of the evidence of human occupation and use of valley floors in these environments is dependent on the number and timing of incision episodes and the extent of lateral channel migration between incision episodes (Howard and Macklin, 1999, Macklin et al., 1992a). A theoretical model of the preservation potential of valley floor alluvial units in upland glaciated catchments (Lewin and Macklin, 2003) suggests that the most recently formed terraces have a medium to high potential for preservation while those formed during the early Holocene have a low preservation potential, due to destruction of these units during subsequent episodes of lateral channel migration. In contrast, substantial incision during the Windermere Interstadial and at the start of the Holocene tended to isolate Late Devensian and Younger Dryas age terraces close to valley edges and units of this age are more likely to be preserved than those formed during the early and mid Holocene (Lewin and Macklin, 2003).

River system responses to environmental changes may vary both within and between catchments (Macklin et al., in prep., Taylor and Lewin, 1997). These responses are dependent on the magnitude of the environmental change, the

1 characteristics of the catchment and the sensitivity of each catchment to change. The substantial changes in climate during the Lateglacial and at the start of the Holocene produced similar responses from rivers across Britain. The nature of the fluvial response to fluctuations in climate at this time was largely governed by whether the catchment was located within the predominantly upland areas which were glaciated or within the lowlands beyond the ice margin (Lewin and Macklin, 2003). Subsequent variations in climate during the Holocene, being of lesser magnitude, have not produced such a consistent response across such a wide area: recent analysis of the extent and timing of Holocene incision in upland catchments indicates that the number and timing of incision episodes varies considerably between upland catchments in Britain (Macklin et al., in prep.). In addition, fluvial system response, such as channel-bed incision to form terraces, to the same environmental change may vary between the individual sub-catchments within a single catchment. This was first noted in a comparison of the sequence of river development in the Tanat and Vyrnwy catchments, in the upper Severn basin (Taylor and Lewin, 1997), and is also apparent within the Tyne catchment (cf. Macklin et al., 1992a, Macklin et al., 1992b, Macklin et al., 1994, Moores et al., 1999, Passmore et al., 1993, Passmore and Macklin, 2000).

Studies of British upland postglacial terrace sequences have revealed diversity between catchments in terms of both the numbers of terrace units formed (and subsequently preserved) and their ages. The majority of dated terrace units have formed during the late Holocene but the rate of occurrence of incision episodes during this period varies widely between catchments. In the Swale, for instance, six episodes of incision have occurred at Catterick and four at Muker during the past seven centuries (Foulds, 2008, Taylor and Macklin, 1997). By contrast, a single episode of incision has occurred at Burholme Bridge on the Hodder during the past five millennia (Harvey and Renwick, 1987). In the Till catchment (a tributary of the Tweed) there is significant variation in the occurrence of terrace-forming incision episodes between different sites: three incision episodes have occurred during the past three centuries in the Calroust Burn (Tipping, 1994) while only one episode of

2 incision occurred during the past four millennia in the Harthope Burn (Tipping, 1992). Relatively few early and mid Holocene terrace units have been found in British upland catchments, which corresponds to the lower preservation potential of units of this age demonstrated by Lewin and Macklin (2003). The River Ribble is unusual in this respect as units dating to the early and mid Holocene are found in various part of the catchment (Chiverrell et al., 2008, Foster et al., 2009, Harvey and Renwick, 1987). Lateglacial terraces in British upland catchments have usually been identified on the basis of the depth of incision which formed the unit and their sedimentology rather than by absolute dating. Terraces of this age are entirely absent from many sites studied. However, single terraces of this age are reported from sites in catchments including the Kirtle Water (Tipping, 1995), Tweed (Foster et al., 2008), Sulby (Chiverrell et al., 2001), South Tyne (Passmore et al., 1993, Passmore and Macklin, 2000) and Irthing (Cotton et al., 1999). Sites which preserve two Lateglacial terrace units include the Feshie (Bain et al., 1993, Robertson- Rintoul, 1986), North Tyne (Moores et al., 1999) and Wharfe (Howard et al., 1999) catchments.

In catchments draining the uplands of Wales terrace sequences typically consist of one or two terraces which on the basis of sedimentology and elevation are thought to be Pleistocene in age, and three or four terrace surfaces (including the modern floodplain) which are late Holocene in age. The most comprehensively studied terrace sequence in Wales, that of the Afon Dyfi typifies this type of sequence. Two pre-Holocene terraces, one probably from the Late Devensian and the other probably formed during the Younger Dryas, occur predominantly in the upper part of the Dyfi catchment and the four Holocene terraces date from c. 3700, c. 1950, c. 650 and c. 450 cal. BP (Johnstone, 2004). Similar sequences, with terraces of slightly different ages, are found in the Rheidol (Macklin and Lewin, 1986), upper Severn (Jones et al., 2010, Maas et al., 2001), and Tanat catchments (Taylor and Lewin, 1997). The terrace sequence in the upper Teifi, near Lampeter, however, consists of a single probable Lateglacial terrace and two main Holocene units, the older of which covers most of the valley floor and may have been the floodplain for an extended period until c. 1450 cal. BP with

3 incision to form the younger unit occurring by c. 1230 cal. BP (Jones et al., 2010). This difference may in part be due to the presence of a number of bedrock gorges in the lower part of the Teifi catchment, which have effectively isolated the upper catchment from changes in sea level. The history of valley floor development in the Lower Dee, around Erbistock and Bangor-on-Dee, differs considerably from most other studied sequences in Wales. Here, the majority of the incision occurred before the late Holocene, with two pre- Holocene and five Holocene units of which the latest two have been dated to c. 4760 cal. BP and 1170 cal. BP. This is, at least in part, a consequence of the blocking of the former course of the river in this area by glacial deposits, which resulted in the development of a new channel, which is considerably shorter and steeper than the original course (Wills, 1912). In the Arrow catchment, a tributary of the Lugg, terraces of mid and early Holocene age have also been preserved: the earliest Holocene terrace is undated but the second (of four) has been dated to c. 6060 cal. BP, with the third dating to c. 3230 cal. BP (Macklin et al., 2003).

This paper reports the results of a study of Late Quaternary river development in the middle Tywi valley, south-west Wales. The work described here was undertaken as part of Exploration Tywi!, a project investigating the development of the landscape of the Tywi Valley since the last glacial, with a focus on archaeological investigations. The history of river and valley floor development, therefore, provides a broad context within which the human occupation and use of the valley has occurred and has also determined which parts of the valley floor archaeological record still survive. This study aims to (1) elucidate the history of valley floor development in the, previously unstudied, Tywi catchment; (2) compare the nature and timing of changes in the river system with other British rivers; and (3) assess the impact of environmental changes on the preservation of the archaeological resource in the valley.

4 2. Study site The Tywi catchment is located in south-west Wales (Figure 1). The river drains west-south-westwards from its headwaters in the Tywi Forest and is the sixth longest in the British Isles (Frost and Jones, 1989). The 1373 km2 catchment is underlain by mudstones and greywackes of Ordovician and Silurian age and, in the south, by sandstones (the Old Red Sandstone) and limestones of Devonian and Carboniferous age (Figure 2). The highest point in the catchment is the 802 m high Fan Brycheiniog above the north-facing Old Red Sandstone escarpment of The Black Mountain on which the Sawdde tributary rises. The reach of the middle Tywi Valley selected for study extends from Llanwrda (SN 718310) to Llanarthne (SN 540205). This reach was chosen for its broad valley floor, its representativeness of the valley as a whole and its extensive alluvial archaeological resource. Within this reach a number of tributaries flow into the Tywi including the Marlais, Bran (Llangadog), Sawdde, Cennen and Dulas. Of these, the Bran, Sawdde and Cennen drain catchments partly underlain by the Old Red Sandstone and supply distinctively pink or reddish sediment to the Tywi floodplain, which is absent from north-bank tributaries.

3. Methods A range of geomorphological and sedimentological methods have been used to reconstruct valley floor development within the study reach. The historical development of the river channel was first reconstructed using published large-scale maps and aerial photographic records of former and current river channel positions. Maps and aerial photographs were georeferenced using ArcGIS™ and channel edges, gravel bars and islands were digitised from sources dating between 1761 and 2006.

Figure 1: The Tywi catchment, showing the main drainage network and the study area in the middle reach of the Tywi Valley (inside the dashed line) and, inset, the location of the Tywi catchment in south-west Wales.

Figure 2: Geology of the Tywi catchment, showing the distribution of rocks of Ordovician and Silurian age, which are predominantly mudstones and greywackes, and those of Devonian (Old Red Sandstone) and Carboniferous age (limestone).

Geomorphological mapping was undertaken from LiDAR data using the method described by Jones et al. (2007). The locations of river terrace fronts, palaeochannels and alluvial fans were digitised directly from the LiDAR data in ArcGIS along the Tywi and its main tributaries (see Appendix 1). The geomorphological maps were subsequently field-checked at the sites selected for detailed study (see below). Following identification of terrace fronts, the surface elevation of each terrace fragment was extracted from the LiDAR data

7 and these were plotted against distance downstream, along with the channel long-profile. The process was repeated for a 2.5 km reach of the Afon Dulas, a north bank tributary of the Tywi, around Cwrt Henri. Individual terrace fragments were correlated on the basis of their height above the channel and their surface gradient in order to identify discrete alluvial units which extended throughout each study reach.

Based on the geomorphological map and terrace correlations, four sites were selected for detailed study (Figure 3). These were at (1) Abermarlais in the middle Tywi Valley upstream of the main input of sediment derived from the Old Red Sandstone (ORS); (2) The Sawdde alluvial fan at the mouth of the largest tributary draining the part of the catchment underlain by the Old Red Sandstone; (3) College Farm, in the middle Tywi Valley downstream of the major inputs of ORS material; and (4) Cwrt Henri on the Afon Dulas, a north bank tributary of the Tywi.

Ground penetrating radar (GPR) surveys (14 in total, covering 1404 m) were carried out using a pulseEKKO 100 system (Sensors and Software, Inc.) with 100 MHz antennae to establish sub-surface alluvial sedimentary structures and sequences. An antenna separation of 1.0 m and a step-size of 1.0 m were used with a 200 ns time window. A velocity of 0.07 m ns-1 was assumed in order to calculate approximate depths for the GPR profiles. This is a typical velocity for silty substrates (Sensors and Software Inc., 1996). Differential GPS was used to provide the high-resolution survey data necessary to correct the GPR traces for the effects of surface topography. The results of GPR surveys were used to aid the precise location of coring sites). The radargrams were also used to interpret the sub-surface stratigraphy (Appendix 2) and, in combination with LiDAR and ground survey data, to determine widths and depths of palaeochannels on the younger alluvial units.

Figure 3: Location of the sites chosen for detailed field studies within the middle Tywi Valley.

9 Alluvial sediment coring was undertaken on the main alluvial units at each site; a total of 19 sites were drilled and 26 m of core recovered. Sedimentary sequences were logged (grain size, fabric and colour) in the field and organic samples suitable for 14C dating were collected where available. Twelve samples from three of the sites were dated by Beta Analytic Inc. (Table 3). The results were calibrated using OxCal version 4.1 (Bronk Ramsey, 2009) and the IntCal09 calibration curve (Reimer et al., 2009) and the 2  ranges of the results are reported. A single sample of sand from the College Farm site was sent to the Research Laboratory for Archaeology and Art History at Oxford University for dating using optically stimulated luminescence

4. Results

4.1 Historical river channel change Figures 4 and 5 show the pattern of river channel change at Abermarlais between 1761 and 2006 (equivalent channel change maps and channel change overlays for all study reaches are provided in Appendix 1 and Appendix 2, respectively). The channel change maps show that in many parts of the Tywi study reach the river has been a laterally dynamic river over the last few centuries. For example, at Abermarlais (Figures 5 and 6), and upstream of Llandeilo (Appendices 3 and 4), large meander loops have developed and then been cut-off or abandoned leaving remnant palaeochannels on floodplain and terrace surfaces. These are clearly visible in the field as depressions on the ground surface and are also evidence on aerial photography (Figure 6). Palaeochannels are present on most terrace surfaces in the Tywi study reach and were the subject of detail coring and GPR investigations. The results of these surveys underpin much of the interpretation of the Late Quaternary valley floor development of the mid Tywi Valley presented in the remainder of this report.

Figure 4: Individual channel position maps showing lateral river channel changes at Abermarlais between 1761 and 2006.

Figure 5: Overlay of lateral river channel changes at Abermarlais between 1761 and 2006.

Figure 6: (top) Meander bend evolution upstream of Llandeilo. (bottom) Evidence of multiple palaeochannels (former meander loops that have been cut-off) are evident on the colour aerial photograph (red box on upper figure shows field of view. Aerial photograph courtesy of Google Earth.

13 Rates of river channel migration have been high when compared to other UK rivers. For example, at Abermarlais some meander loops were migrating in excess of 6.5 m per year in the late nineteenth century. However, in the latter half of the twentieth century these rates have typically reduced to less than 2.0 m per year. In contrast, upstream of Llandeilo at Bethlehem (Figure 6), meander migration rates have remained high until the present day: between 1946 and 1992 some meanders migrated up to 5.0 m per year, and between 1992 and 2006 rates were as high as 6.8 m per year.

River bank erosion and channel migration clearly pose a threat to floodplain infrastructure and can cause the temporary loss of agricultural land. However, with the exception of the meanders at Bethlehem, it appears that the Tywi is less laterally active now than it was in the eighteenth and nineteenth centuries. We also know from other research work on the Tywi that there has been a dramatic reduction (56%) in the area of exposed gravel bars in the latter half of the twentieth century. The causes for this apparent reduction in channel activity are complex, but are related to changes in flood magnitude and frequency, flow regulation (Llyn Brianne), and to the history of metal mining and gravel extraction in the catchment.

4.2 River terrace sequences Eleven alluvial units were identified in the Tywi valley, up to 15 m above the present channel (Figures 7 and 8 and Table 1). The lowest of these units represents vegetating bar surfaces, which lie just above low flow levels (unit 1). The most extensive terrace is alluvial unit 3, covering more than 65 % of the valley floor. Numerous palaeochannels on the surface of this unit provide evidence for extensive lateral channel migration. Unit 2 is inset approximately half a metre below unit 3, adjacent to the present river channel, in parts of the reach in which lateral channel changes occurred between successive historical maps and aerial photographs during the past century or more. Unit 4 lies around half a metre above unit 3 and has a surface area characterised by the presence of palaeochannels, but the density of palaeochannels on this surface is less than that on alluvial unit 3. Fewer palaeochannels can be

14 identified on the surfaces of units 5 and 6, which constitute 2.68 % and 8.55 % of the valley floor, respectively. Height differences between alluvial units increase above unit 6 and the surface areas of units 7 to 11 are significantly smaller than those of the younger alluvial units. Units 8 and 9 are present throughout the majority of the study reach, as are units 2 to 6, but fragments of units 7 and 10 are only found upstream of Llandeilo / Ffairfach and unit 11 is restricted to a small area around Llandeilo and Dinefwr . Palaeochannels were not found on the surfaces of units 7 and 11. This may be partly due to the small remaining areas of these surfaces but because of colluvial activity infilling the channels, as these units are found adjacent to valley side slopes.

The river terrace sequence in the valley of the Afon Dulas, a small right-bank tributary of the Afon Tywi, consists of eight alluvial units including vegetating bar surfaces (unit 1) (Figure 9 and Table 2). Alluvial units 4 and 6 are the most extensive of these surfaces. The highest and oldest alluvial unit (unit 8) is located 9 m above the level of the modern river channel. Alluvial units 7 and 8 are only present in the northern part of the reach, upstream of the confluence with the Nant Lash. The majority of the remaining units are present throughout the reach. Correlation on the basis of height of the terrace sequence of the Dulas with that of the Tywi was not possible because of the lack of elevation data for the area surrounding the confluence.

Table 1: River terrace sequence of the middle Tywi valley.

Alluvial unit Height above channel % of valley floor area (m) 1 Vegetating bars 0.03 2 1.5 4.37 3 2.0 65.05 4 2.5 14.47 5 3.4 2.68 6 3.9 8.55 7 5.4 0.90 8 7 2.34 9 10.5 0.94 10 13 0.19 11 15 0.46

Figure 7a: Geomorphology and river terrace units in the middle Tywi study reach at Abermarlais.

Figure 7b: Geomorphology and river terrace units in the middle Tywi study reach at Sawdde confluence.

Figure 7c: Geomorphology and river terrace units in the middle Tywi study reach at Llandeilo.

Figure 7d: Geomorphology and river terrace units in the middle Tywi study reach at College Farm.

Figure 7e: Geomorphology and river terrace units in the Tywi study reach at Dryslwyn.

20 8 3a 20000 7 2a 6 10a 9a 15000 5 11a 8a 4 Channel 7a 10000 3 11 6a 5000 2 10 5a 1 9 4a 0

60 55 50 45 40 35 30 25 20 15 10 Figure 8: Long-profiles of river terrace units in the middle Tywi study reach. Solid lines link continuous sections of terrace. Dashed lines link separate terrace fragments belonging to the same alluvial unit.

Figure 9: Geomorphology and river terrace units in the Dulas study reach.

22 Table 2: River terrace sequence of the middle Tywi valley.

Alluvial unit Height above channel % of valley floor area (m) 1 Vegetating bars 0.21 2 0.9 13.67 3 1.6 15.10 4 1.9 30.51 5 2.7 5.84 6 3.9 25.75 7 4.8 2.38 8 9 6.52

4.3 Abermarlais Three cores were recovered from palaeochannels on alluvial units 3 and 4 to the north of the Afon Tywi at Abermarlais (Figure 10). As this site is upstream of the major input of sediment derived from the Old Red Sandstone to the Tywi, sediments comprise material mainly derived from Lower Palaeozoic mudstones and greywackes. All three cores contained abundant organic matter suitable for radiocarbon dating. Core 1 was recovered from a palaeochannel c. 450 m from the present river channel and revealed a sequence consisting principally of silts and clays, in which the amount of organic matter increased with depth (Figure 11). Three organic samples recovered from this sequence were radiocarbon-dated (Table 3): (i) charred material from the base of a clayey silt unit was dated to 890±40 BP (916 – 730 cal BP), (ii) plant material recovered from the underlying clay unit returned a date of 900±40 BP (916 – 736 cal BP), and (iii) a piece of wood recovered from close to the base of the recovered sediment sequence, which did not reach channel gravels, was dated to 1730±40 BP (1727 – 1538 cal BP).

A second core (Core 3; Figure 12) was recovered from a palaeochannel on alluvial unit 3 c. 550 m from the present river channel, and beyond the course of a former Roman road across the floodplain, and revealed a sequence which fined upwards from gravels to clayey silts. Two samples, of wood and charred material, from gravelly sand units beneath the clayey silts were

23 radiocarbon-dated and returned ages of 3510±40 BP (3893 – 3690 cal BP) and 3390±40 BP (3818 – 3486 cal BP), respectively.

The third core at this site was recovered from a palaeochannel on alluvial unit 4 (Figure 13). Above the basal gravels and sands there was a 1.62 m thick sequence of clayey silts and silty clays. Three samples of charred material from this sequence were radiocarbon-dated: a sample from 0.83 m depth in a clayey silt unit returned an age of 1940±40 BP (1994 – 1818 cal. BP), and two samples from 1.18 m and 1.49 m depth within a silty clay unit were dated to 3460±40 BP (3840 – 3640 cal BP) and 4110±40 BP (4822 – 4544 cal BP), respectively.

4.4 Sawdde alluvial fan Seven cores were recovered from the Sawdde alluvial fan (Figures 14 to 21); this tributary provides the main input of sediment derived from the Old Red Sandstone to the Tywi. Sediments at this site were coarser than those at Abermarlais, containing a larger proportion of sand-sized material, and were also generally red or pink in colour, in contrast to the grey or brown sediments recovered at Abermarlais. Little organic matter was preserved in the sediment sequences recovered from the Sawdde alluvial fan, but two fragments of charred material from Core 7 (Figure 21), from a channel on a unit inset beneath the oldest fan surface, were radiocarbon-dated. A sample from 0.59 m depth, close to the base of a clayey silt unit, returned a date of 2160±40 BP (2313 – 2011 cal. BP) and the sample from the gravel unit at the base of the recovered sequence was dated to 3300±40 BP (3636 – 3446 cal. BP).

Figure 10: Location of sediment cores and GPR surveys at the Abermarlais site.

Figure 11: Sediment sequence recovered from Abermarlais Core 1.

Figure 12: Sediment sequence recovered from Abermarlais Core 3.

Figure 13: Sediment sequence recovered from Abermarlais Core 2.

28 Table 3: Radiocarbon-dated samples recovered from cored sediment

sequences.

412 870

1949

1220 1214 1782 1894

– –

92BC

61 BC

–

– – – –

AD AD 136

–

1684BC 2801BC 2786 BC 2572 BC 2504 BC 1740BC 1847BC 1605 BC 1536 BC 1496BC

–

– – – – – – – – – –

363 363

1667

AD 223 AD 668 AD

Or 68

AD 1034 AD 1034 AD BC 44 1890 2872 1943 1868 1686 AD

) in calendar )calendar in years

Or1905 AD

Or 2792 Or2792 Or2780 Or2512 Or1774 Or1582 Or1796 AD



Calibrated14C age (2 BC/AD

1 1

–

730 736

168

–

4454 3486 2011

1538 1814 3634 4751 4736 4522 3690 3797 3555 2042 3446 1080

– –

–

C C age

– – –

– – – – – – – – – – – –

Or 45

916 916 916

283 283

Or 154

1727 1994 3840 4822 3893 3818 3724 2313 3636 1282

Or 4462 Or4462 Or3532 Or2018

Or 4742 Or4742 Or4730 Or

) in calendar )calendar in



Calibrated (2 yearsBP

C C age

890±40 900±40 140±40

1730±40 1960±40 3460±40 4110±40 3510±40 3390±40 2160±40 3300±40 1260±40

al al

Convention

------

Laborator y identifier Beta 282515 Beta 282516 Beta 282517 Beta 282518 Beta 282519 Beta 282520 Beta 282522 Beta 282523 Beta 282528 Beta 282529 Beta 282524 Beta 282526

Sampl e materi Charre al d materia Plant l materia l Wood Charre d materia Charre l d materia Charre l d materia l Wood Charre d materia l Charre d materia Charre l d materia Wood l Wood

e e

depth

Sampl

1.38 1.38 m 1.75 m 2.84 m 0.83 m 1.18 m 1.49 m 1.65 m 2.09 m 0.59 m 1.30 m 0.86 m 1.10 m

702303

Grid

reference

SN 705 SN 303 705 SN 303 705 SN 303 706 SN 309 706 SN 309 706 SN 309 702 SN 303 SN 702 SN 272 702 SN 272 554 SN 222 554 SN 221

1 1 1 2 2 2 3 3 7 7 2 3

Core

Site Abermarlais Abermarlais Abermarlais Abermarlais Abermarlais Abermarlais Abermarlais Abermarlais Sawdde alluvialfan Sawdde alluvialfan Dulas at CwrtHenri Dulas at CwrtHenri

4.5 College Farm Four cores were recovered from palaeochannels on three different alluvial units at the College Farm site (Figure 22). The sediments at this site were frequently reddish in colour and coarser than those at the Abermarlais site (Figures 23 to 26). Although no organic matter suitable for radiocarbon dating was recovered at this site, sands from 0.75 m to 1.0 m depth within Core 4, which was recovered from alluvial unit 6, were dated using optically stimulated luminescence to 4610±410 before 2010 (Figure 26). The relatively large 2  range of this age (5370 – 3730 cal. BP) is a result of uncertainty about the normal moisture content of the sediments, as the sample was collected after prolonged dry spell.

4.6 Cwrt Henri Five cores were recovered from three alluvial units at this site on the Afon Dulas (Figures 27 to 32). The recovered sequences consisted of sediment derived from the Lower Palaeozoic rocks which underlie most of the catchment and without any material derived from the Old Red Sandstone. Cores from two alluvial units contained organic material suitable for radiocarbon dating. Wood recovered from a palaeochannel on alluvial unit 2 returned a date of 140±40 BP (283 – 1 cal. BP) (Figure 26) and wood recovered from a depth of 1.10 m within a palaeochannel on alluvial unit 4 was dated to 1260±40 BP (1282 – 1080 cal. BP) (Figure 27).

Figure 14: Location of sediment cores and GPR surveys at the Sawdde alluvial fan site.

Figure 15: Sediment sequence recovered from Sawdde alluvial fan Core 1.

Figure 16: Sediment sequence recovered from Sawdde alluvial fan Core 2.

Figure 17: Sediment sequence recovered from Sawdde alluvial fan Core 3.

Figure 18: Sediment sequence recovered from Sawdde alluvial fan Core 4.

Figure 19: Sediment sequence recovered from Sawdde alluvial fan Core 5.

Figure 20: Sediment sequence recovered from Sawdde alluvial fan Core 6.

Figure 21: Sediment sequence recovered from Sawdde alluvial fan Core 7.

Figure 22: Location of sediment cores and GPR surveys at the College Farm site.

Figure 23: Sediment sequence recovered from College Farm Core 1.

Figure 24: Sediment sequence recovered from College Farm Core 2.

Figure 25: Sediment sequence recovered from College Farm Core 4.

Figure 26: Sediment sequence recovered from College Farm Core 5.

Figure 27: Location of sediment cores and GPR surveys at the Cwrt Henri site.

Figure 28: Sediment sequence recovered from Cwrt Henri Core 1.

Figure 29: Sediment sequence recovered from Cwrt Henri Core 2.

Figure 30: Sediment sequence recovered from Cwrt Henri Core 3.

Figure 31: Sediment sequence recovered from Cwrt Henri Core 4.

Figure 32: Sediment sequence recovered from Cwrt Henri Core 5.

5. Late Quaternary valley floor development in the middle Tywi valley

5.1 The Afon Tywi The results of the geomorphological and sedimentological investigations indicate that there have been at least seven phases of river channel and floodplain sedimentation, and subsequent channel-bed incision during the development of this reach of the Afon Tywi. The oldest units identified (units 10 and 11) are found only on the south-eastern side of the middle Tywi Valley and their current extent is limited to a number of small fragments in two or three locations. The preserved fragments of units 7 to 9 are more extensive and are found at both margins of the valley floor and throughout a large portion of the reach. These three terraces were formed some time between the Late Devensian, after the retreat of ice from this area at around 18,000 cal B.P., and the mid Holocene, prior to c. 4550 cal. BP, by which time aggradation of unit 6 was in progress. The relatively large magnitude of the incision between units 9 and 8 (minimum 3.5 m), in comparison with subsequent episodes of incision, suggests that this phase of incision probably occurred during the last glacial-Holocene transition at a time of significant change in climate and, consequently, in vegetation cover in the catchment, which reduced the sediment supply to the river. The lack of sedimentological data and datable material from terrace units 7 to 9 precludes more precise dating of the episodes of channel and floodplain sedimentation and incision which formed them.

The palaeochannel on terrace unit 6 at College Farm had been abandoned by the river and was being infilled with well-sorted sands at some time between c. 5370 and c. 3730

44 cal. BP. The number of palaeochannel fragments preserved on the surface of this unit is too small to determine whether the river channel at this time was single-thread (as it is through the majority of the reach today) or multi-thread in planform. The thick fining- upwards sequence of well-sorted sands found in this palaeochannel was unusual within the reach: similar sediment units were not found in palaeochannels on the younger alluvial units.

The period following the deposition of the sand sequence in the palaeochannel on terrace 6 was one of considerable activity in the middle Tywi Valley. Between 5370- 3730 and c. 3650 cal. BP three episodes of channel bed incision occurred along with extensive lateral channel migration. The first episode of channel bed incision was relatively minor (c. 0.5 m) and the preserved extent of the alluvial unit created following this episode of incision (unit 5) is relatively small. Few palaeochannel fragments are preserved on the surface of this terrace and it is not possible to identify whether it was formed by single-thread or multi-thread channel. The subsequent phase of channel bed incision was more significant (minimum 0.9 m) and the terrace unit formed as a result of this episode of incision (unit 4) provides evidence for considerable lateral channel migration at this period. Organic material extracted from close to the base of a palaeochannel on this unit dated to c. 4640 cal. BP. This age falls close to the midpoint of the 2  age range of the OSL-dated sediment recovered from the palaeochannel on unit 6. However, whereas optically stimulated luminescence dating of sediment provides an age for the deposition of the sediment dated, radiocarbon dating of organic matter extracted from a sediment layer provides a maximum age for the incorporation of the organic matter into the sediment body (normally at deposition), since there may have been a time lag between the death of the organic matter dated and its transport by the river and incorporation into the sediment sequence. Indeed it is possible that fragments of wood or other organic matter may be deposited in one sediment sequence, subsequently eroded and re-deposited elsewhere. Nevertheless, these two dated samples do suggest that the two episodes of incision which formed units 4 and 5 may have been completed within a period of 1300 years or less.

A much larger number of palaeochannels are preserved on the extensive remains of terrace unit 4 than on the surfaces of the older alluvial units. Throughout the reach palaeochannel patterns indicative of both a single-thread meandering river channel and a multi-thread anabranching (wandering) or braided river channel are preserved.

45 Examples of the latter on the surface of unit 4 are found to the south of Cwmifor (Figure 7c), close to College Farm (Figure 7d) and also close to Pentre-Davis (Figure 7e). However, at the College Farm site there is also an excellent example of a cut-off palaeochannel, abandoned by the river when progressive erosion upstream, and possibly also downstream, of it resulted in the river breaking through the neck of the meander and thereby shortening its channel. This cut-off palaeochannel is almost circular in planform and was almost certainly part of a single-thread meandering channel. This suggests that during the period of formation of terrace unit 4 the river may have alternated, both in time or space, between a single-thread meandering planform and a multi-thread anabranching or braiding one. This variability in channel pattern is likely to be due to variations in discharge and / or sediment supply, probably as a consequence of periodic large floods.

Incision into unit 4 probably occurred between c. 4640 cal. BP and c. 3650 cal. BP, by which time the older of the two dated palaeochannels on unit 3 was in the process of being abandoned and beginning to infill. The dated samples from unit 6, 4 and 3, suggests that the three episodes of channel-bed incision, resulting in total in a minimum of 1.9 m of incision, occurred during 1900 years, or possibly, considerably less. The vertical channel activity evident within this period, between c. 5370 cal. BP and c. 3480 cal. BP, contrasts with the subsequent behaviour of the river which, between c. 3480 cal. BP and c. 200 cal. BP has been characterised by little change in river bed elevation. In this period, however, in which terrace unit 3 was the active floodplain, extensive lateral channel migration occurred and numerous palaeochannels are preserved on the surface of the unit. The majority of these channels provide evidence for migration, cut- offs and avulsions, which occur in a single-thread meandering channel. However, particularly near Abermarlais (Figure 7a), palaeochannels provide evidence for a multi- thread channel pattern. Particularly good examples of progressive meander migration are found in the Tywi immediately upstream of the Sawdde alluvial fan (Figure 7a), just upstream of Llandeilo (Figure 7c) and near Ro-Fawr (Figure 7e). Examples of locations in the study reach where palaeochannels indicate that an avulsion, a switching of the channel location either into an entirely new channel or into a palaeochannel, has occurred on unit 3 include College Farm and Dyslwyn at the downstream end of the study reach (Figure 7e).

46 Incision into terrace unit 3 has occurred during the period for which historical maps and aerial photographs are available (1761 to present). Comparison of the distribution of alluvial unit 2 throughout the reach with the river channels derived from historical sources shows that incision did not occur in all parts of the reach simultaneously. The surface of this alluvial unit, which is inset only c. 0.5 m below unit 3, is covered with palaeochannels, the majority of which are indicative of a river with a single-thread channel planform. The small areas of vegetating bar surface classified as unit 1 will eventually, as a result of fine-grained overbank deposition, aggrade to the level of unit 2. The small magnitude of the incision between units 4 and 3 and units 3 and 2 (c. 0.5 m in each case) has permitted fine-grained sediment to be deposited on surfaces up to that of unit 4 during high-magnitude flood events, as shown by the presence of organic material dating to c. 1900 cal. BP at a depth of 0.83 m within the fine-grained palaeochannel fill on unit 4 at Abermarlais. At the same site there has been as much as 1.75 m of fine-grained sediment deposited in a palaeochannel on unit 3 since c. 830 cal. BP, suggesting a possible increase in sedimentation rate around or subsequent to this time, possibly as a result of accelerated soil erosion associated with agricultural activities in the Tywi catchment.

5.2 The Afon Dulas At least six episodes of channel-bed incision have occurred in the Afon Dulas study reach during the Late Quaternary. The oldest terrace unit (unit 8), which probably formed the valley floor in this reach at some time during the Late Devensian period, lies significantly above all the remaining units. The magnitude of the incision into this unit, a minimum depth of 4.2 m, suggests that it probably occurred as a result of the significant changes in climate that occurred either during the Lateglacial or at the start of the Holocene. Three episodes of channel bed incision, totalling 2.9 m of incision, occurred subsequent to this initial episode prior to c. 1180 cal. BP. Only a small area of unit 7 has been preserved but unit 6 is the second most extensive within the reach. The sedimentary sequence on this unit consists predominantly of sand or gravelly sand overlying gravels at depths of less than 1 m, and in one core at only 0.35 m depth. A palaeochannel on the surface of unit 4, the most extensive within the study reach, was occupied by the river at c. 1180 cal. BP, but was shortly afterwards abandoned and infilled with predominantly silty sediment. The small number of palaeochannels preserved on the surface of this unit appear to include both single-thread and multi-

47 thread planforms. Two phases of channel bed incision (minimum depths 0.3 m and 0.7 m respectively) occurred in the Dulas between c. 1180 cal. BP and c. 150 cal. BP, resulting in the formation of units 3 and 2 close to the present river channel. The majority of the palaeochannels preserved on the surfaces of these units suggest that the channel pattern was similar to that of the actively migrating, single-thread meandering channel of the Dulas today.

6. Discussion

6.1 Comparison of the Late Quaternary river record in the middle Tywi valley with other upland UK rivers. The occurrence of episodes of channel-bed incision, forming river terraces, during the Late Pleistocene and Holocene periods is typical for British upland catchments. However, in terms of the number and timing of incision episodes which have occurred, the sequence in the Tywi catchment is noteworthy in a number of respects. First, the preserved extent of probable Late Devensian and Late glacial age terraces is relatively small. Second, the number and magnitude of incision episodes in the period between c. 5700 cal. BP and c. 3400 cal. BP is unusual. Third, following this period of activity, the occurrence of the long period of vertical stability between c. 3400 cal. BP and c. 200 cal. BP is remarkable. The alluvial unit preservation model of Lewin and Macklin (2003) suggests that terrace units of Late Devensian and Younger Dryas age have, as a result of the large magnitude of the incision which occurred during the Windermere Interstadial and at the start of the Holocene, a relatively high preservation potential. In other Welsh catchments, such as the Dyfi and Rheidol, this is the case: extensive areas of Late Devensian and Late glacial terraces are preserved at the valley sides and frequently at an elevation as much as 20 m above the current channel (Johnstone, 2004, Macklin and Lewin, 1986). In the Tywi, relatively small fragments of units older than c. 4550 cal. BP are preserved at the valley sides but, throughout the reach, the preserved extent of the younger units is many times greater than those of probable Late Devensian and Late glacial age and the preserved fragments of the older units do not present the almost continuous terrace surfaces seen in parts of other river systems. This difference may be due in part to the higher gradients of rivers such as the Dyfi and Rheidol in comparison with the Tywi and the presence of bedrock controlling lateral and / or vertical channel changes in parts of those catchments.

Repeated incision at the transition from the mid-Holocene to the late Holocene, followed by a long period of vertical stability in the river channel, is atypical for Welsh and, indeed, for British upland rivers. Two typical patterns of terrace development may be distinguished in the majority of upland British catchments which have been previously studied: an absence of terraces of mid-Holocene age followed by an increasing number of episodes of incision during the Late Holocene, typified by the Dyfi sequence in which terraces have been dated to c. 3700, c. 1900, c. 650 and c. 450 cal. BP (Johnstone, 2004), and a relatively uniform distribution of terrace units, and therefore episodes of incision, throughout the Holocene, typified by the Ribble in northern England (Chiverrell et al., 2008, Foster et al., 2009, Harvey and Renwick, 1987) and also by the Arrow, which drains the Welsh massif towards the west (Macklin et al., 2003). The dated mid- to late Holocene terrace sequence present in the Tywi does not fit into either of these two categories.

The lower River Dee has experienced a recent extended period of vertical stability accompanied by extensive lateral channel migration and preceded by a number of incision episodes (Jones et al., 2010). However, only the age of the last of the episodes of incision is constrained (to between c. 4760 cal. BP and c. 1170 cal. BP). The particular circumstances which caused incision in the lower Dee, the blocking of the former, considerably longer, course of the river with glacial sediments, are not replicated in the Tywi catchment and as a result it appears unlikely that the history of earlier incision episodes in the Dee would be similar to that of the Tywi. It is also worth noting that there is no evidence for a recent episode of channel bed incision in the lower Dee and that although lateral channel migration was formerly extensive in this reach the channel has in recent centuries been characterised by a high degree of lateral channel stability whereas rates of lateral channel migration have remained high throughout the historical period in the Tywi.

The variation in the underlying geology in the Tywi catchment results in a striking variability in the types of sediment sequence preserved. Of the four sites studied, two were almost or exclusively supplied with sediment derived from Ordovician and Silurian mudstones and greywackes and, as a result, had sediment sequences dominated by silt-sized, and sometimes finer, sediment, which was generally grey or light brown in colour and which frequently contained preserved organic matter including wood and

49 hazelnuts. The preservation of the organic matter was promoted under these circumstances by the fine particle sizes which are conducive to moisture retention. By contrast, the sediment sequences at the Sawdde alluvial fan and College Farm sites, both of which received significant inputs of material derived from the Old Red Sandstone, contained much greater proportions of sand-sized sediment, were generally slightly pink or red in colour, were well-drained and consequently contained little preserved organic matter with the exception of isolated fragments of charcoal. The presence of such a substantial difference in the type of floodplain sedimentary sequence is unusual in Welsh rivers and has significant implications for the preservation of the archaeological resource (see below).

Comparison of the terrace sequences from the Tywi and the Dulas indicates that the history of incision in the trunk stream and its tributary has not been identical. The two valley floors of these two systems preserve different numbers of alluvial units and the available dating control suggests that in the recent period incision has not necessarily occurred at the same time in both sites. In the Dulas, for instance, there have been two episodes of incision between c. 1180 cal. BP and c. 140 cal. BP, whereas in the Tywi the long period of vertical stability from c. 3400 cal. BP to c. 200 cal. BP has been followed by a single episode of recent incision. This divergence between the sequence of valley floor development within adjacent catchments has previously been observed in other parts of Britain (e.g. Taylor and Lewin, 1997) and is a result of variable sensitivity to environmental changes which have the potential to alter discharge and sediment supply.

In Figure 33 the major fluvial discontinuities in the middle Tywi valley recorded at c. 730, 3890 and 4970 cal B.P. are compared with similar phases of accelerated change recorded in other Welsh catchments. It is evident that these coincide and would suggest that periods of river instability in the middle reaches of the Afon Twyi are similarly associated with major flooding episodes and climate change.

Figure 33: Fluvial discontinuities in selected Welsh river catchments (from Jones et al., 2010). Fluvial discontinuities on the Tywi at c. 730, 3890 and 4790 cal. B.P. are indicated by dashed vertical red lines.

6.2 Late Quaternary valley floor development and the preservation of the archaeological resource Two processes of valley floor development affect the preservation of the valley floor archaeological resource. These are the destruction and replacement of older valley floor as a result of lateral channel migration coupled with vertical incision and the burial of sites and artefacts on valley floors through the deposition of thick sequences, usually of fine-grained sediment. The latter may not destroy the archaeological resource, indeed burial within waterlogged sediments may actually be conducive to the preservation of organic materials, but sediment deposition may significantly reduce the likelihood of discovery of features and artefacts. Both of these processes have the potential to affect the available archaeological resource in the middle Tywi Valley.

The distribution of items listed in the Historic Environment Record (HER) within the valley floor has been superimposed upon the maps of the distribution of terrace units in the Tywi and Dulas (Figures 34 and 35). For this purpose items in the HER record have been divided into two categories: ‘sites’ which are immovable and therefore must be present in a primary context, such as a stretch of Roman road, and ‘finds’, portable artefacts, which may be in either a primary or secondary context. Items for which locational information was imprecise have been excluded. In the middle Tywi Valley repeated lateral channel migration during the formation of unit 3 has created a multi- period floodplain.

On this alluvial unit it could be expected that sites younger than c. 3500 cal. BP will be present, with sites dating to before 3500 cal. BP restricted to older alluvial units. Any archaeological finds dating from prior to c. 3500 cal. BP (c. 1500 cal BC) on alluvial unit 3 would certainly be in a secondary context, especially given the extensive lateral channel migration which has occurred during the development of this alluvial unit. Indeed, this does appear to have happened as a single Neolithic age find recovered from terrace 3 near Llandeilo (Figure 34b) clearly must be in a secondary context. But some or all of the Bronze Age, Roman and Medieval finds recovered from terrace 3 could be in primary context (Figure 36), which has implications for the river environment-people interactions in these periods.

Figure 34a: Distribution of archaeological finds and sites on alluvial units of different ages in the Abermarlais and Sawdde reaches.

Figure 34b: Distribution of archaeological finds and sites on alluvial units of different ages in the Llandeilo reach.

Figure 34c: Distribution of archaeological finds and sites on alluvial units of different ages in the College Farm lower reach.

Figure 35: Distribution of archaeological sites on alluvium of different ages in the Dulas Valley floor (Cwrt Henri).

56 a)

Neolithic Bronze Age Roman Medieval Unknown

b) 8

3 Number of finds / sites / finds of Number 2 1 0 1 2 3 4 5 6 7 8 9 10 Terrace c) Neolithic Bronze Age Roman Medieval Unknown

) within the study reach study the within ) 2

4 Area(km 2

0 1 2 3 4 5 6 7 8 9 10 11 Terrace

Figure 36: Distribution of archaeological sites/finds on Late Quaternary river terraces in the middle Tywi valley. a) distribution by period, b) distribution by terrace number, c) area of each terrace (km2).

57 The oldest archaeological sites which could be found on units 4 to 6 would date from c. 5700 to c. 3500 cal. BP, that is from the early Neolithic to the Bronze Age. Sites, and finds in a primary context, older than c. 5700 cal. BP may be preserved on terraces 7 to 11, and recovery of in situ Mesolithic and possibly earlier material should be focussed in these areas.

In the Dulas valley floor, few archaeological finds and sites have been discovered. Archaeological sites, and finds in a primary context, older than Medieval (c. 1180 cal. BP) will be restricted to river terraces 5 to 8. Artefacts which are thought to date to the period before c. 1180 cal. BP which are found on units 1 to 4 are most probably in a secondary context. Burial by fine- grained sedimentation is likely to be less of a hindrance to archaeological discovery in the Dulas since the maximum depth of fine-grained sediment recovered in the cores was a little over 1 m.

7. Conclusions This study represents the most detailed investigation of Late Pleistocene/Holocene river development ever carried out in Wales. It is the first time that long term river-archaeological relationships have been systematically evaluated in one of Wales’ largest rivers.

Since c. 3000 BC the River Tywi has down-cut by nearly 4 metres and has eroded, swept and sedimented almost its entire valley floor. Periods of rapid river down-cutting and channel straightening at shortly after AD 1220, 1940 BC and 3020 BC would appear to be associated with major floods and climate change. The historical period (c. 1750 to present) has been characterised by renewed channel bed incision and lateral channel migration leading to the development of multiple cut-offs.

The history of Late Quaternary valley floor development has significantly influenced the locations where both prehistoric and historical archaeological finds and sites have been preserved. The destruction and replacement of older valley floor sediments as a result of lateral channel migration has

58 undoubtedly led to the loss of archaeological resource in the middle Tywi valley. However, vertical incision and the burial of sites and artefacts on valley floors through the deposition of overbank sediment, although not destroying the archaeological resource has probably significantly reduced the likelihood of discovery of features and artefacts.

The value of the combined geomorphological-archaeological approach (with secure radiometric dating) used in this study has been effectively demonstrated in two respects. First, it provides the opportunity to identify archaeological finds that are in a secondary, rather than a primary, context. Second, it identifies terraces with the highest archaeological potential and thus enables more targeted and efficient archaeological surveys in these areas.

Acknowledgements We are grateful to Sarah Rassner, Kate Martin, Javier Gamarra, Marc Huband, Nia Williams and Menna for assistance with fieldwork and to the farmers at the Abermarlais, Sawdde alluvial fan, College Farm and Cwrt Henri sites for permission to access their land and for their interest in the research.

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