Brookfield, Michael

McMillan, Andrew

Merritt, Jon

English

NERC Open Research Archive

The Powfoot Boulder Pavement 
 
Michael Brookfield, Jon Merritt and Andrew McMillan 
 
A striated boulder pavement is intermittently exposed at low tide on Priestside Bank 
following the removal of intertidal sands and clays during changes in course of the 
estuarine channel of the River Eden within the Solway Firth.  The main locality is 
located on a boulder scar [NY 125 645], 500 m south of the shoreline at Nethertown, 
just north of a large, prominent boulder on the foreshore (Merritt and Phillips, this 
volume, fig.1). Striated boulders also may be found on Powfoot Scar [NY 145 650] 
and Howgarth Scar [NY 136 643], to the east 
 
The pavement, where undisturbed, consists of a single layer of interlocking bullet- 
and flatiron-shaped boulders up to 50 cm in length (Figure 1). The clasts are 
composed predominantly of wacke sandstone (95%) with some red Triassic sandstone 
(5%) and sparse granodiorite derived from Criffell, some 15 km to the west. The 
flatirons have smoothed and striated upper surfaces and both the long axes of the 
boulders and the striations show a consistent WNW-ESE alignment, but with no 
unambiguous directional indicators. They are, however, parallel to the general ESE 
ice flow deduced from drumlins to the east of the River Annan on the Scottish side of 
the Solway Firth (BGS, 2005, 2006; Livingstone et al., 2008, flow set LT4). The 
pavement is only locally developed and passes laterally into patches of pebbles both 
overlying, and interbedded with coarse, well-sorted, reddish brown sand; elsewhere, it 
is replaced by scattered pebbles lying on till.  
 
The pavement rests on compact diamicton and is overlain locally by a unit of similar 
till that is exposed along the high tide mark, north of the main pavement location. The 
diamicton underlying the pavement is exposed where boulders have been removed or 
dislodged by recent marine erosion. It is a pale reddish brown (10R 5/4 - Munsell 
Rock Color Chart on wet material), pebbly silty clay diamicton with well dispersed 
clasts (<20%) dominated by Lower Palaeozoic wackes. A pebble count at the main 
locality gave: wackes (95%), red Triassic sandstones (4%), with less than 1% of 
Carboniferous conglomerate, and, significantly, with no granodiorite. The clasts are 
dominantly subangular to subrounded and many are bullet- or flatiron-shaped, 
polished and scratched. There are a few large wacke erratics up to 1 metre diameter 
on the Powfoot foreshore.  
 
Discussion   
 
Striated boulder pavements are flat, tightly packed mosaics of ice-smoothed and 
striated boulders, one clast thick, with faceted upper surfaces commonly exhibiting 
orientated striations, lying within or between tills (Hansom, 1983, Eyles, 1988). They 
are a class of armoured surface produced when coarser particles are concentrated by 
the removal of intervening finer material (Benn and Evans, 1998). Striated boulder 
pavements are considered to form either at the base of grounded ice sheets or in 
intertidal areas in cold climates; they have been recorded from both Quaternary 
(Eyles, 1988; McCabe and Haynes, 1996) and older glacial deposits (Visser and Hall, 
1985; Lucio and Rocha-Campos, 2000).  
 
Subglacial boulder pavements may be created by: 
a) preferential lodgement of boulders against bed obstructions beneath a wet-based 
glacier (Boulton, 1978); the clasts form a ‘traffic jam’ with their long axes parallel to 
flow and may show an up-glacier imbrication. Such subglacial boulder pavements 
have limited areal extent and should be dominated by bullet shaped striated clasts 
typical of transport at the glacier base.  
 
b) a deforming bed at the base of a glacier in which case clasts may either be similar 
in lithology to the overlying till, having settled through the deforming bed (Hicock, 
1991; Clark, 1991), or  similar in lithology to the underlying sediment following 
‘excavational deformation’ (Boulton, 1996). Such pavements commonly pass laterally 
into lines of widely spaced clasts with flattened and striated upper surfaces, then into 
either pebble-rich bands without striated surfaces or sharp unconformities separating 
distinct units of till. 
 
c) basal glacier meltwater erosion and concentration of clasts from underlying till or 
other sediments (Shaw, 1988), in which case a wide variety of clast morphologies 
may be present. 
 
Intertidal, periglacial boulder pavements originate as gently undulating boulder lag 
surfaces produced by wave and current winnowing of till in relatively shallow seas, 
followed by abrasion and transport by a grounded ice shelf (Eyles, 1994). Intertidal 
ice has been observed freezing downwards to encase boulders on subarctic flats as the 
ice cover is raised and grounded repeatedly (Rosen, 1979; McCann et al., 1981). 
However, the boulders typically show very divergent scratch alignments and clasts 
have neither the preferred alignment nor the characteristic bullet shape of clasts 
transported at a glacier base (Eyles, 1988). Repeated abrasion by debris within ice 
blocks grounded across the intertidal zone at low tide produces flattened upper 
surfaces and short, randomly orientated striations on the pavement clasts (Eyles, 
1994), dissimilar to the unidirectional striations produced beneath a glacier.  
 
Conclusions 
 
The Powfoot boulder pavement has characteristics of both subglacial and intertidal 
boulder pavements. On the one hand, the aligned clasts (dominated by flatiron and 
bullet shapes) and consistent striation directions strongly suggest a subglacial origin. 
On the other hand the local close packing and rapid passage into dispersed pebbles 
both underlying and overlying well-sorted sands may indicate an intertidal origin. It is 
possible that estuarine wave action first concentrated boulders from the underlying 
till, accounting for the variability of the pavement and its apparent passage into 
pebbles overlying beach deposits. A wet-based glacier flowing from the WNW 
subsequently moulded the boulders into the present polished and unidirectionally 
striated mosaic, accounting for the numerous bullet and flatiron clasts together with 
the strong pebble and striae alignment.  
 
It is important to test the hypothesis outlined above, because either a simple intertidal 
origin or an intertidal followed by subglacial origin for the pavement raises questions 
regarding the established sea level history of the region (Wells, 1999). For example, 
fragmentary Late-glacial raised beaches occur up to about 11m above OD within the 
area to the north-west of the pavement that would have been affected by such a 
readvance of ice (Haggart, 1999; BGS, 2005; McMillan et al., 2010b), necessitating a 
significant sea level rise following the readvance. The simplest explanation is that the 
pavement formed subglacially beneath wet-based ice that readvanced ESE across 
previously deposited till and glaciofluvial deposits during the ‘Powfoot Oscillation’ 
(Merritt, 2010, this volume). The presence of granodiorite boulders from Criffell in 
the till overlying the pavement, but not below it, is compatible with such a late-stage 
readvance of ice splaying out into the Solway Firth from Nithsdale and Annandale 
(Figure 2). 
 
References 
 
Benn, D.I, & Evans, J.A. 1996. The interpretation and classification of subglacially-
deformed materials. Quaternary Science Reviews, 15, 23-52. 
  
Boulton, G.S., 1978. Boulder shapes and grain size distribution of debris as  
indicators of transport paths through a glacier and till genesis. Sedimentology, 25, 
773-799.  
 
Boulton, G.S. 1996. Theory of glacial erosion, transport and deposition as a 
consequence of subglacial sediment deformation. Journal of Glaciology, 42, 43-62. 
 
British Geological Survey. 2005.  Solway West. Scotland Special Sheet. Superficial 
Deposits and Simplified bedrock. 1:50 000 Geology Series. (Keyworth, Nottingham: 
British Geological Survey.) 
British Geological Survey. 2006.  Solway East. Scotland Special Sheet. Superficial 
Deposits and Simplified bedrock. 1:50 000 Geology Series. (Keyworth, Nottingham: 
British Geological Survey.)  
Clark, P.U., 1991. Striated clast pavements: products of deforming subglacial  
sediments? Geology, 19, 530-533.  
 
Eyles, C.H., 1988. A model for striated boulder pavement formation on glaciated  
shallow-marine shelves: an example from the Yakatanga Formation, Alaska. Journal  
of Sedimentary Research, 58, 62-71.  
 
Eyles, C.H., 1994. Intertidal boulder pavements in the northeastern Gulf of Alaska  
and their geological significance. Sedimentary Geology, 88, 161-173.  
 
Hansom, J.D., 1983. Ice-formed intertidal boulder pavements in the sub-Antarctic.  
Journal of Sedimentary Petrology, 53, 135-145.  
 
Hicock, S.R., 1991. On subglacial stone pavements in till. Journal of Geology, 99,  
607-619.  
 
Haggart, B A. 1999. Pict’s Knowe: Holocene relative sea-level change. 62-74  in The 
Quaternary of Dumfries and Galloway: Field Guide. Tipping, R M. (editor). 
(London: Quaternary Research Association.) 
Lucio, M.P. and Rocha-Campos, A.C., 2000. Late Paleozoic glacial boulder  
pavements from Jumirim, Sp (Itarare Subgroup): new evidence on origin. Anais da  
Academia Brasiliera de Ciencias, 72, 600.  
 
Livingstone, S. J., O´ Cofaigh, C. and  Evans, D. J. A. 2008. Glacial geomorphology 
of the central sector of the last British-Irish Ice Sheet. Journal of Maps, 2008, 358-
377. 
 
McCabe, A.M. and Haynes, J.R., 1996. A Late Pleistocene intertidal boulder  
pavement from an isostatically emergent coast, Dundalk Bay, eastern Ireland. Earth  
Surface Processes and Landforms, 21, 555-572.  
 
McCann, S.B., Dale, J.E. and Hale, P.B. 1981. Subarctic tidal flats in areas of large 
tidal range, southern Baffin Island, eastern Canada. Géographie physique et 
Quaternaire, 35, 183-204. 
 
McMillan, A.A., Merritt, J.W., Auton, C.A. and Golledge, N.R. 2010b (in press).  The 
Quaternary Geology of the Solway. British Geological Survey Research Report, 
XX/00/00.  
 
Rosen, P. 1979. Boulder barricades in central Labrador. Journal of Sedimentary 
Petrology, 49, 1113-1124. 
 
Shaw, J., 1988. Subglacial erosion marks, Wilton Creek, Ontario. Canadian Journal  
of Earth Sciences, 25, 1256-1267.  
 
Visser, J.N.J. and Hall, K.J., 1985. Boulder beds in the glaciogenic Permo- 
Carboniferous Dwyka Formation in South Africa. Sedimentology, 32, 281-294.  
 
Wells, J M. 1999. Late-glacial and Holocene sea-level changes in the Solway Firth. 
27-32 in The Quaternary of Dumfries and Galloway: Field Guide. Tipping, R M. 
(editor). (London: Quaternary Research Association.) 
 
Figures  
 
1. View of the glaciated pavement. A: general view looking NW with main pavement  
outcrop arrowed; B: view of dominant flatiron- and bullet-shaped clasts. C: close-up 
of striated flatirons.  
 
2. Speculative reconstruction of the readvance of Scottish ice into the Solway 
Lowlands following the Last Glacial Maximum (after Stone et al., 2010), showing the 
approximate position of the ice front during the Scottish Readvance and subsequent 
Powfoot Oscillation. P, Plumpe Farm; PF, Powfoot. 
 
Acknowledgements 
 
This chapter is published with the permission of the Executive Director of the British 
Geological Survey, NERC 
 
Michael Brookfield, Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55 
Nankang, Taipei 11529, Taiwan 
 
 
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2010b (in press). The Quaternary Geology of the Solway. British Geological Survey Research Report,

A Late Pleistocene intertidal boulder pavement from an isostatically emergent coast,

A model for striated boulder pavement formation on glaciated shallow-marine shelves: an example from the Yakatanga Formation,

Boulder barricades in central Labrador.

Boulder beds in the glaciogenic PermoCarboniferous Dwyka Formation in South Africa.

Boulder shapes and grain size distribution of debris as indicators of transport paths through a glacier and till genesis.

Glacial geomorphology of the central sector of the last British-Irish Ice Sheet.

Ice-formed intertidal boulder pavements in the sub-Antarctic.

Intertidal boulder pavements in the northeastern Gulf of Alaska and their geological significance.

Late Paleozoic glacial boulder pavements from Jumirim, Sp (Itarare Subgroup): new evidence on origin. Anais da Academia Brasiliera de

On subglacial stone pavements in till.

Pict’s Knowe: Holocene relative sea-level change. 62-74

Solway West. Scotland Special Sheet. Superficial Deposits and Simplified bedrock. 1:50 000 Geology Series.

Subarctic tidal flats in areas of large tidal range, southern Baffin Island, eastern Canada. Géographie physique et

Subglacial erosion marks,

The interpretation and classification of subglaciallydeformed materials.

Theory of glacial erosion, transport and deposition as a consequence of subglacial sediment deformation.

The Powfoot Boulder Pavement

http://nora.nerc.ac.uk/11569/1/Powfoot_Boulder_Pavementtext.pdf

The Powfoot Boulder Pavement

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