The subglacial imprint of the last Newfoundland Ice Sheet, Canada.

Abstract

The former Newfoundland Ice Sheet was situated on the fringes of the northeast Atlantic Ocean during the Wisconsinan glaciation (∼80–10 ka BP). Its geographic position indicates that it was likely to have been influenced by a number of external and internal forcing mechanisms including configuration changes in the Laurentide Ice Sheet with which it converged during the last glacial maximum, ice streams, changes in oceanic circulation and fluctuating sea levels. This makes Newfoundland a key location for investigating the dynamic response of ice sheets to these types of internal and external drivers. An established methodology for investigating ice sheet dynamics is to use the landform record to reconstruct the dynamic behaviour and configuration of the ice sheet. This provides a relative chronology of former ice sheet events during glacial cycles. A fundamental requirement of this approach is a detailed glacial geomorphology map that records the spatial distribution of individual subglacial bedforms across the former ice sheet bed. This paper presents a new subglacial bedform map of the Island of Newfoundland. It was produced as part of a mapping programme which used 10 m resolution Satellite Pour l'Observation de la Terre satellite imagery, Shuttle Radar Topography Mission and Canadian Digital Elevation Data. The map records the spatial distribution of ∼126,000 individually mapped glacial lineations and ribbed moraines and extends the number and spatial extent of each landform across the island. It is a new data set which has the potential to provide important insights into former ice sheet behaviour in this region.

Keywords:

• subglacial bedforms
• glacial lineation
• ribbed moraine
• Newfoundland Ice Sheet
• remote sensing
• palaeoglaciology

1. Introduction

Reconstructing the configuration and dynamic behaviour of palaeo-ice sheets is important because it provides new insights into how former ice sheets responded to both internal and external drivers in the past (Carlson & Winsor, 2012; Greenwood & Clark, 2009a, 2009b; Kleman & Borgström, 1996). This approach has shown that ice sheets can have relatively fast response times which raises concerns about the future stability of the Greenland and Antarctic ice sheets and how they will respond in a warming world (Khan et al., 2014; Pritchard, Arthern, Vaughan, & Edwards, 2009; Shepherd & Wingham, 2007). Of particular concern is the potential contribution that melt water from both ice sheets will have on global sea levels and the impacts this will have on coastal communities if sea level continues to rise (e.g. Brysse, Oreskes, O'Reilly, & Oppenheimer, 2013; Oppenheimer & Alley, 2004). However, predicting their future trajectory is difficult, given the long timescales involved in ice sheet evolution. In this regard, palaeo-glaciological reconstructions are important because they allow us to examine the dynamic behaviour of ice sheets through full glacial cycles. This information has added to our understanding of the internal and external forcing mechanisms that control former ice sheet stability (e.g. Boulton, Dongelmans, Punkari, & Broadgate, 2001; Clark, Knight, & Gray, 2000; Glasser & Jansson, 2005; Greenwood & Clark, 2009a, 2009b; Kleman, Hättestrand, Borgstrom, & Stroeven, 1997). The location of the former Newfoundland Ice Sheet (NIS) (∼28–10 ka BP) may be of significance in this regard. It was situated on the fringes of the North Atlantic Ocean and the much larger Laurentide Ice Sheet (LIS) which was centred over North America during the Wisconsinan glaciations (∼80–10 ka BP; Catto, Scruton, & Ollerhead, 2003). Palaeo-ice sheets in the Amphi-North Atlantic are now known to have been sensitive to changes in ocean circulation and sea level (Clark, McCabe, Bowen, & Clark, 2012; McCabe & Clark, 1998).Therefore, given its location and relatively small size (∼110,000 sq. km), it is likely that the NIS may also have responded to similar external forcing mechanisms.

During the last glacial cycle, the North American Ice Sheet Complex stretched across virtually all of Canada and into the northern parts of the USA. This ice complex comprised a number of ice sheets that included the LIS, the Cordilleran and Innuitian ice sheets, the Canadian Maritime Provinces Ice Cover and the NIS. These latter two, known collectively as the Appalachian Ice Complex, maintained independent ice centres through much of the last glacial cycle (Dyke, 2004; Dyke et al., 2002). The LIS formed the central parts of the North American Ice Sheet Complex and during its maximum extent (∼27 ¹⁴C ka BP and ∼24 ¹⁴C ka BP; Dyke et al., 2002) coalesced with the smaller regional ice sheets including the NIS with the convergence zone being along Newfoundland's northern and western margins (Figure 1; Dyke, 2004; Grant, 1989, 1994; Kleman et al., 2010; Stokes, Tarasov, & Dyke, 2012). A southward ice flow from Labrador crossed the Strait of Belle Isle onto northern parts of the Great Northern Peninsula, while further to the south, the LIS advanced as far as the Long Range Mountains before being deflected into the Gulf of St. Lawrence (Brookes, 1982; Catto, 1998; Grant, 1977, 1989; Putt, Bell, Batterson, & Smith, 2010; Shaw et al., 2006; Tucker, 1974). At this time, the NIS is understood to have maintained a complex of local ice caps with ice cover having advanced to the continental shelf (Dyke et al., 2002; Shaw, 2003, 2006; Shaw et al., 2006). At maximum ice extent, a number of large ice streams were well established in the marine sectors of the LIS and Atlantic Canada (Kleman & Glasser, 2007; Shaw et al., 2006; Stokes et al., 2012). Marine-based ice streams such as the Laurentian Channel Ice Stream, which was partially fed by terrestrial ice streams from the NIS is thought to have heavily influenced the configuration, dynamics and decay of the both the LIS and NIS in this region (Dyke & Prest, 1987; Kleman & Hättestrand, 1999; Shaw et al., 2006; Stokes & Clark, 2001; Stokes et al., 2012). Shaw et al. (2006) argue that major deep water calving events initiated in the Laurentian Channel Ice Stream just before 14 ka, and possibly correlated with Heinrich 1, resulted in early ice retreat and separation of the NIS from the LIS. By 12 ka, the NIS margins had become land based with the LIS having retreated into southern Labrador leaving the Strait of Belle Isle ice-free. These events caused significant configuration changes in the NIS with migration of primary ice centres and subsequent separation of the NIS into at least 15 isolated ice caps with final disintegration of the NIS occurring just after 10 ka (Grant, 1974; Shaw et al., 2006). Previous research on the glacial history of the NIS confirms that it was influenced by both configuration changes in the LIS and climatic events in the wider Amphi-North Atlantic over relatively short timescales making Newfoundland an ideal location for investigating how ice sheets respond to a range of external and internal drivers.

Figure 1. The mapped area encompasses the entire terrestrial landscape of Newfoundland (∼110,000 sq. km) which forms part of the Canadian Province of Newfoundland and Labrador. The landform examples mentioned in the text are labelled with numbered boxes showing the figure locations.

Mapping and interpreting the glacial geomorphological record is an established tool for reconstructing past glacial environments and provides the primary data set required for the glacial inversion approach to reconstruct the extent, configuration, dynamic behaviour and decay of former ice sheets (e.g. Boulton & Clark, 1990; Clark, 1999; Greenwood & Clark, 2009b; Kleman, 1994; Kleman & Borgström, 1996; Kleman et al., 1997). The essential prerequisite to this technique is the creation of a detailed glacial geomorphology map that provides information on the spatial distribution of individual glacial landforms across the former ice sheet bed. Maps that record individual landforms are essential because they allow users to identify and unravel the complex flow patterns and geometries, such as cross-cutting assemblages that are observed in palimpsest glacial landscapes (e.g. Clark et al., 2000; Greenwood & Clark, 2009a; Jansson, Kleman, & Marchant, 2002; Kleman, 1992; Trommelen, Ross, & Ismail, 2014). Currently, the only published glacial map that covers the entire island of Newfoundland is the 1:5,000,000 scale Glacial Map of Canada (Prest, Grant, & Rampton, 1968) which is available to download from the Geological Survey of Canada. Although this map provides a good overview of landform distribution, its scale restricts its use for identifying the complex ice flow patterns and regional scale variability which is fundamental for accurate palaeo-glaciological reconstructions (Kleman & Borgström, 1996).

2. Glacial landform mapping in Newfoundland

A long history of glacial research in Newfoundland, dating back to the 1800s, has used the geological and geomorphological record as a means to decipher former ice sheet behaviour (e.g. Bell, 1884; Milne, 1876; Murray, 1882). Despite this, there is currently no detailed glacial geomorphological map recording the spatial distribution of individual subglacial landforms across Newfoundland. This contrasts with the considerable detail of landform mapping and subsequent ice sheet reconstructions carried out for other palaeo-ice sheets, for example, the LIS (Boulton & Clark, 1990; Clark et al., 2000; Kleman et al., 2010).

Although detailed mapping of individual subglacial bedforms in Newfoundland has been carried out in a number of location, the study areas are spatially fragmented and therefore provide only snap shots of portions of the former ice sheet bed (e.g. Fisher & Shaw, 1992; Grant, 1970, 1989, 1994; Marich, Batterson, & Bell, 2005). Ongoing regional mapping programmes by the Geological Survey of Newfoundland and Labrador (GSNL), along the1:50,000 National Topographic System (NTS) map areas, are beginning to address this shortfall. This work combines surficial geology mapping using aerial photography and field verification, till geochemistry sampling and palaeo-ice flow mapping derived from striae evidence and supplemented by landform mapping (e.g. Batterson & Taylor, 2009; Batterson, Taylor, Bell, Brushett, & Shaw, 2006; Brushett, 2012; Smith, 2012; Taylor, Bell, Brushett, & Shaw, 2009). Landform data collected as part of these regional mapping programmes are available from the Geoscience Atlas maintained by the Newfoundland and Labrador Department of Natural Resources. This online resource provides links to detailed surficial geology maps, access to geoscience data sets, maps and reports with mapped landforms available for download in shapefile format (Honarvar, Nolan, Crispy-Whittle, & Morgan, 2013). Although this work by the GSNL has provided excellent detailed reports and surficial geology maps for many sectors, the current landform record still remains spatially incomplete. While this is an important resource, the partial subglacial bedform record limits its use when attempting to reconstruct the ice flow pathways and behaviour of the entire ice sheet bed.

Traditionally, research in Newfoundland has used surficial geology, striation mapping and clast provenance studies to establish the former ice flow patterns across the landscape (e.g. Batterson & Catto, 2001; Catto, 1998; St. Croix & Taylor, 1991, 1992). The preferred approach taken by the GSNL to delineate former ice flow direction in Newfoundland has been to use striae evidence. Striae mapping programmes by the GSNL are ongoing, with an online database of ∼11,800 striae measurements currently available from the Geoscience Atlas (Taylor, 2001) with data being continuously updated as new work is completed. Although striae are important indicators of ice flow direction, in areas where striae measurements are limited, their use in identifying ice flow patterns at the system scale can be restricted as interpretation of ice flow patterns from spatially distributed striae sites may not be representative of regional ice flow patterns (Clark & Meehan, 2001; Clark et al., 2004, 2000; Kleman, 1990). This contrasts with the greater areal coverage provided by using the full landform record which can be mapped relatively quickly using a remote sensing approach (Boulton & Clark, 1990; Greenwood & Clark, 2009a; Kleman, 1994). The availability of this striae database is, however, a significant resource. In addition to providing supporting evidence of former ice flow direction, cross-cutting striae measurements recorded across the island can be integrated as part of an ice sheet reconstruction to aid interpretation of the relative chronologies of former ice flow events (e.g. Jansson et al., 2002; Kleman et al., 2010). Other research approaches have employed Shuttle Radar Topography Mission (SRTM) data to carry out landform mapping in the region. For example, Liverman, Batterson, and Bell (2006) carried out a broad scale assessment of regional ice flow patterns, while Blundon, Bell, and Batterson (2010) used SRTM to identify potential ice stream tracks. Others have used this approach at a reconnaissance level to identify ‘areas of interest’ prior to carrying out field research or aerial photography mapping (e.g. Batterson & Taylor, 2009; Putt et al., 2010).

This synthesis has highlighted that although there is a significant history of glacial landform mapping in Newfoundland with detailed local scale observations in many areas across the island, current landform information remains incomplete. The existing record has been based on a variety of mapping approaches from a number of researchers with mapping generally conducted for the purposes of drift prospecting and mineral exploration rather than for palaeo-glacial reconstructions. To date, no detailed systematic mapping approach has been taken across the entire island to identify individual glacial landforms that are essential to facilitate a comprehensive reconstruction of the spatial and temporal behaviour of this ice sheet throughout its evolution during the last glacial cycle.

To address this deficit, this paper presents a new glacial geomorphological map showing the distribution of individual subglacial bedforms across the island of Newfoundland (Main Map). A top-down approach using SPOT satellite imagery and Digital Elevation Models (DEMs) was employed to systematically map the glacial geomorphology of Newfoundland to provide a new primary database of glacial landforms for the region.

3. Methods

A number of data sets were used to inform the mapping programme which was performed entirely within a Geographic Information System (GIS). The following sections discuss the data used and the mapping approach taken to produce the final Main Map, which is located within meridians 60°W and 52°W and parallels 46°N and 52°N and encompasses the entire terrestrial landscape of the island of Newfoundland (Figure 1).

3.1. Primary data

The primary data used in the mapping programme included SPOT Satellite imagery and two DEMs, all of which provided full coverage of the island of Newfoundland (Table 1). SPOT orthoimagery was downloaded and false colour composites were constructed using 20 m resolution bands 1, 2 and 3 (green, red and near infrared bands, respectively). These were pan-sharpened using the panchromatic band to provide 10 m resolution multispectral data. The DEMs used were 0.75 arc second Canadian Digital Elevation Data (CDED) and 90 m SRTM data. The CDED DEM tiles were downloaded in ASCII format, converted to raster and mosaiced to produce a seamless DEM of the island. SRTM tiles covering the island were downloaded as GeoTIFFs and mosaiced to produce a second seamless DEM. The SRTM data were generally used to identify larger scale landforms (typically > 1 km) and was used in areas where poor-quality CDED data resulted in a stair-step effect in some areas of low relief. The Universal Transverse Mercator (UTM) projection in reference to NAD83 was chosen as the base projection for all the data. Although Newfoundland crosses UTM zones 21 and 22, the majority of the Island lies within Zone 21; therefore, all data were projected in UTM Zone 21.

Table 1. Properties of primary data used for landform mapping.

Data	Specification and spatial resolution	Coverage	Source
SPOT 4 and 5 Ortho Imagery	Minimum cloud-free (2% max.) Panchromatic band 10m. Multispectral – 3 bands used (Green, Red, NIR) 20 m	Island wide. 67 images – each covering area 60 km by 60 km (3600 km^2)	(‘Geobase Orthoimage 2005–2010 Canadian Council on Geomatics', 2010) GeoBase^® database – Canadian Council on Geomatics (CCOG) http://www.geobase.ca/geobase/en/index.html
CDED	Digital Elevation Data derived from 1:50,000 NTS map sheets. Stored at 0.75 × 0.75 arc seconds south of 68° Latitude (11–16 m in ground units in x (East to West); ∼ 23 m in y (North to South). Ground elevations recorded in metres relative to mean sea level based on NAD 1983 horizontal reference datum	Island wide. 339 tiles – each covering area ∼500 km^2, consisting of a western and an eastern section corresponding to half a NTS map sheet	GeoBase^® database – CCOG http://www.geobase.ca/geobase/en/index.html
SRTM DEM	DEM built by interferometry from shuttle mission. Horizontal resolution of 3 arc seconds (∼90 m) – absolute vertical accuracy of 16 m	Island wide. 33 tiles – each covering one degree of latitude and one degree of longitude	United States Geological Survey's Earth Resources Observation and Science Center http://eros.usgs.gov/

3.2 Ancillary data

A number of additional data sets were used to aid landform interpretation. This included consulting previously published maps and using additional GIS database layers of bedrock geology, structural geology, and surficial geology (Table 2). These GIS layers were used to help differentiate between glacial lineations formed in bedrock and those formed in areas of glacial drift which are more likely to be drumlins as opposed to glacially streamlined bedrock landforms (Figure 2).The digital striation database also provided independent evidence of ice flow orientation which helped to reduce any potential misinterpretation, for example, flow transverse landforms being classified as flow parallel ones (Smith & Knight, 2011). Striae evidence was also used in areas of bedrock that had a streamlined appearance on the imagery to confirm that this region had experienced former ice flow in the orientation of the long axis of the streamlined bedrock (Roberts & Long, 2005) (Figures 3 and 4).

Figure 2. Demonstration of how surficial geology GIS layers helped to differentiate between glacially moulded bedrock and depositional glacial lineations, such as drumlins: (a) cross-cutting glacial lineations on SRTM data; (b) the same geographic extent as (a) with the 1:250 k surficial geology map draped on top. The surficial geology layer indicates that this area consists primarily of till, therefore suggesting that landforms mapped here are more likely to be of depositional origin and as such have been classed as drumlins (yellow arrows indicate the presence of cross-cutting drumlins with two separate ice flow events being observed, a north to south flow and a NNE to SSW flow); (c) a streamlined landscape with drumlinoid features evident on the SRTM data. The matched image in (d) shows the 1:50 k surficial geology overlaid on the SRTM data and indicates this region to be predominantly bedrock (exposed bedrock or bedrock concealed by vegetation). Glacial lineations mapped in bedrock areas are considered more likely to be of erosional origin and therefore have been classified as glacially moulded bedrock lineations. (For area locations, see Boxes 2ab and 2cd in Figure 1).

Figure 3. Both striation measurements and surficial geology data aided identification of areas of former ice flow and enabled categorisation of mapped landforms. The images display the same geographic extent of Fogo Island off the northeast coast of Newfoundland. The SRTM shaded relief image in (a) shows evidence of linear features across the island, which gives the landscape a streamlined appearance. Striae evidence in (b) indicates that this region experienced former ice flow (denoted by location of purple arrows with arrow indicating direction of flow). The 1:50 k surficial data in (c) allow for the identification of till-dominated or bedrock areas (Coastline has been inserted in black on the image to outline the coast as mapped on the SRTM imagery). (d) By using both striation evidence and surficial geology data, lineations mapped in exposed or concealed rock have been categorised as glacially moulded bedrock lineations, while those in areas of till are categorised as being of depositional origin, that is, drumlins, as illustrated in (d). (Box 3 in Figure 1). (Surficial geology and striation data from Newfoundland and Labrador Department of Natural Resources GeoScience Atlas OnLine: Available from: http://gis.geosurv.gov.nl.ca/).

Figure 4. In order to aid detection of subglacial landforms using a remote sensing approach, a variety of image manipulation approaches can be used. These can effectively enhance the representation of landforms within the imagery, thereby improving landform detection rates. The figure presents some key aspects which are of significance and the approaches which were taken as part of the mapping programme to aid detection and delineation of subglacial landforms. Integral to the remote sensing approach to landform mapping is the need to minimise the potential of misinterpretation error. While it may not always be possible to carry out field verification, certain practices can be used to enable correct identification and add support to interpretation during mapping. The figure describes some methods which were used to limit error in landform interpretation as part of the mapping programme.

Table 2. Ancillary data.

Sources	Description of sources consulted
Geological Survey of Newfoundland and Labrador, Geosciences Division of the Department of Natural Resources http://www.nr.gov.nl.ca/nr/mines/Geoscience/index.htm Earth Science Sector (ESS) of Natural Resources Canada Resources – Geoscan	Geofiles document repository containing published reports and maps, open file reports and unpublished reports and maps. Available at (http://www.nr.gov.nl.ca/nr/mines/geoscience/geofiles.html) GeoScience OnLine portal – access to GeoScience Atlas, Geofiles search, and GIS resources (e.g. Geogratis free topographic data, Canadian GeoScience database). Bibliographic database for scientific publications of the ESS http://geoscan.nrcan.gc.ca/geoscan-index.html and GeoGratis portal to access maps, data and publications (http://geogratis.cgdi.gc.ca/)
GSNL, Department of Natural Resources – Index to bedrock and surficial geology maps of the province http://www.nr.gov.nl.ca/mines&en/geosurvey/maps/	Surficial geology maps of Newfoundland, sorted by NTS map sheet area. Available at http://www.nr.gov.nl.ca/mines&en/geosurvey/maps/surfmap/surf_map/Full_Surf-NL.html Bedrock geology maps of Newfoundland sorted by NTS map sheet area Available at http://www.nr.gov.nl.ca/mines&en/geosurvey/maps/nl/html_nf/Full.html
GeoScience Atlas, maintained by the GeoScience Data Management section of the GSNL, Department of Natural Resources (Honarvar et al., 2013). GIS GeoScience layers. Available at http://gis.geosurv.gov.nl.ca/	Newfoundland and Labrador Geological Survey Regional Surficial Geology GeoScience Atlas OnLine. 1:250 000 scale Regional Surficial Geology derived from 1:50 000, 1:250 000 and 1:1 000 000 scale surficial geology maps, aerial photograph interpretation and variable field verification. Polygon layers – variables in alluvium; bog; colluvium; concealed and exposed bedrock; glaciofluvial sand and gravel; hummocky terrain; marine clay, sand, gravel and diamicton; ridge till; till blanket and till veneer. Available at http://gis.geosurv.gov.nl.ca/resourceatlas/viewer.htm
	Newfoundland and Labrador Geological Survey ‘Detailed Surficial Geology’ GeoScience Atlas OnLine 1:50,000 scale Detailed Surficial Geology derived from aerial photograph interpretation with a variable field checking (Original sources available from Geofiles). Layer provides mapped coverage of two-thirds of the Island of Newfoundland in a number of categories (Bog, Fluvial, Colluvium, Aeolian, Glaciofluvial, Lacustrine, Marine, and Glacial). Available at http://gis.geosurv.gov.nl.ca/resourceatlas/viewer.htm
	Newfoundland and Labrador Geological Survey. ‘Striation Database’ GeoScience Atlas OnLine. Striation database project initiated in 1989 by GSNL, Terrain Sciences Section of the Department of Mines and Energy. Individual striation measurements for Newfoundland compiled from various sources (e.g. published /unpublished reports) Available at http://gis.geosurv.gov.nl.ca/resourceatlas/viewer.htm

3.3. Mapping approach

The Main Map was compiled using a systematic province-wide mapping programme based on the visual interpretation of the satellite imagery and the DEMs, with the mapping being completed by one observer to reduce observer variability (Smith & Clark, 2005). Four types of subglacial landforms were identified and mapped. These were either streamlined in the direction of ice flow and included depositional glacial lineations (drumlins and crag and tails), glacially moulded bedrock lineations and drumlinoid hills with the fourth type being ribbed moraines which form transverse to ice flow. A table showing the criteria used in the interpretation and mapping of subglacial landforms is provided as Table 3.

Table 3. Characteristics and diagnostic criteria of subglacial landforms.

Landform type	Characteristics and diagnostic criteria	References
Ribbed Moraine	Aligned transverse to flow. Commonly found developed in large fields. Commonly found in close proximity to glacial lineations and superimposed by them. Generally composed of glacial sediment and formed under actively flowing ice. A wide range of morphological and spatial characteristics are recognised, with ridge lengths in the order of 10s metres to several kilometres	Lundqvist (1989); Hättestrand and Kleman (1999); Dunlop and Clark (2006)
Depositional glacial lineations	Drumlins – Aligned parallel to flow. Typically found in swarms. Generally composed of glacial sediment and formed under actively flowing ice. Many variations in form exist with classic drumlin being a smooth streamlined hill with blunt upstream (stoss) slope and tapered downstream (lee) slope, with symmetrical shape being most common form. Typical lengths range from 100 m up to a few kilometres. Considered part of a family of related landforms, collectively known as subglacial bedforms and have been suggested to form as a landform continuum (i.e. ribbed moraine, drumlins, MSGL. Crag and tails – aligned parallel to ice flow. Produced by selective erosion and deposition beneath an ice sheet, under active flow. Generally ovate in form with resistant bedrock crag at the head and tapered drift tail orientated in direction of ice flow. Can range from 10s metres to several kms in length	Rose (1987); Boulton (1976), Boulton (1987); Clark et al. (2009); Spagnolo, Clark, Hughes, and Dunlop (2011) Benn and Evans (2010)
Glacially moulded bedrock lineations	Aligned parallel to ice flow, moulded bedrock formed by glacial erosion. Observed in areas where surficial geology is predominantly of exposed or lightly vegetated bedrock. These bedforms are referred to by a range of terms, for example, rôche moutonnée, whalebacks, streamlined hills, rock drumlins	Benn and Evans (2010); Glasser and Bennett (2004); Glasser and Jansson (2005); Jansson and Glasser (2005); Roberts and Long (2005)
Drumlinoid hills	Streamlined landforms, characteristically drumlinoid in shape. These have been assigned to a separate landform category due to their much larger scale (height/width) than observed in neighbouring glacial lineations. No differentiation has been made to the depositional or erosional origins of these landforms on the map for cartographic purposes and in order to minimise detail

In order to improve landform detection and to avoid the known problem of azimuth biasing (Clark & Meehan, 2001; Hillier et al., 2014; Smith, Clark, & Wise, 2001), various shaded renditions of the DEMs, including non-azimuth images, were consulted during the mapping process. A vertical exaggeration of 3x and 5x was used for all shaded reliefs with the exception of zero azimuth renditions with solar elevation being set at 30° as low angle solar elevation is considered the most effective for aiding landform detectability (Smith et al., 2001). Visualisation of the data was carried out either on a stand-alone basis or by combination viewing, for example, using the SPOT data on its own, viewing DEMs on their own or by combination viewing in which the SPOT imagery was draped onto the DEMs. Draping SPOT data over shaded relief images provided topographic context and tone and texture variation to the 2D satellite imagery, which helped to identify subtle, or low-profile landforms (Jansson & Glasser, 2005). This approach can alleviate some of the limitations of mapping by shaded relief alone, for example, differentiation of bedrock and drift landforms (Greenwood & Kleman, 2010). A variety of adjustments in contrast were tested and applied consistently to enhance both landform detectability and delineation. A repeat pass approach at a range of scales between 1:6,000 and 1:150,000 was employed systematically and province wide. This ensures that mapping is frequently re-evaluated, enables consistent cartographic homogeneity, reducing mapping bias and achieving a good representation of landform patterns, including more subdued and smaller scale landforms which may be significant in the detection of subtle fluctuations in ice sheet geometry (Clark & Meehan, 2001; Jansson & Glasser, 2005; Smith & Wise, 2007; Smith et al., 2001). Systematic island-wide quality control checks, using ancillary data sources (Table 2 and Figure 3), were employed throughout the mapping programme, for example, surficial geology data to differentiate depositional glacial lineations from glacially moulded bedrock lineations; and striae data to verify flow orientation. A synopsis of protocols used to aid identification and interpretation of subglacial landforms using a remote sensing approach is provided in Figure 3. All landforms identified on the data were digitised on-screen directly into the GIS as single lines along ridge crests (Figure 5b).

Figure 5. Matched pair (a) and (b) show classic downstream curving ribbed moraines with image (b) highlighting mapping style, with ribbed moraines mapped as vector lines across ridge crests (Box 5ab in Figure 1). Images (c), (d), (e) and (f) show other examples of mapped ribbed moraines and are named according to the Dunlop and Clark, (2006) classification. Broad arcuate ridges are shown in (c) (Box 5c in Figure 1), (d) jagged ribbed moraines (Box 5d in Figure 1), (e) hummocky ribbed moraine (Box 5e in Figure 1) and (f) cross-cutting ribbed moraines (Box 5f in Figure 1). All images are displayed using 10 m pan-sharpened SPOT satellite imagery. Yellow arrows indicate ice flow direction.

4. Map description

The Main Map records the spatial location and distribution of a total of 125,798 individual subglacial bedforms across the entire island of Newfoundland. Four subglacial landform categories are shown on the map. Three of these are streamlined glacial lineations that are aligned parallel to ice flow and include depositional glacial lineations, that is, drumlins and crag and tails, glacially moulded bedrock lineations and much larger drumlinoid hills. The fourth category is ribbed moraine which forms transverse to ice flow (Table 3).

This map, presented in A0 size at a scale of 1:750,000, extends the known distribution of subglacial bedforms across the island of Newfoundland (Main Map). The number of landforms mapped in the four primary landform categories and their associated sub-groups are as listed in Table 4. Ribbed moraines are shown in yellow, glacially moulded bedrock lineations in blue, drumlinoid hills in brown and the depositional glacial lineations of drumlins and crag and tails as green and purple, respectively. A brief discussion of the mapped landforms is considered below.

Table 4. Quantities of glacial landforms mapped in each landform category.

Primary landform category	Sub-groups	Number mapped	Total
Ribbed moraines		101,363	101,363
Depositional glacial lineations	Drumlins	16,787	17,291
	Crag and tails	504
Glacially moulded bedrock lineations	Streamlined bedrock	7056	7056
Glacial lineations	Drumlinoid hills	88	88

4.1. Ribbed moraines

Ribbed moraines are the most abundant subglacial bedform on the island of Newfoundland, comprising 81% of all landforms mapped, with a total of 101,363 ridges. These regularly spaced ridges, aligned transverse to ice flow, are widely accepted to have formed subglacially and are located across considerable areas of the former Laurentide, Fennoscandian, British-Irish and Newfoundland ice sheets (NISs) (Aylsworth & Shilts, 1989; Bouchard, 1989; Clark, 1999; Clark & Meehan, 2001; Dunlop & Clark, 2006; Hättestrand & Kleman, 1999; Knight & McCabe, 1997; Lundqvist, 1969, 1989; Marich et al., 2005; Prest et al., 1968; Stokes, Lian, Tulaczyk, & Clark, 2008). Ridge lengths range from 100 m to 3873 m, with 63% of landforms having a typical length range of 300–1200 m (mean 447 m), which is consistent with typical lengths reported in a large representative database by Dunlop and Clark (2006). The distribution of mapped ribbed moraines extends the previously documented population of ribbed moraine in the Glacial Map of Canada (Prest et al., 1968) with ∼28% of mapped ridges recorded in areas not previously categorised as ribbed terrain. The mapped ribbed moraines commonly display similar morphological characteristics to those reported in the Dunlop and Clark (2006) classification and include, for example, classic downstream curving, broad arcuate, jagged and hummocky ridges (Figure 5). Similar to other ribbed moraine terrain, there are spatial associations with the glacial lineations mapped on the island (Dunlop and Clark, 2006). While drumlinisation of the main ridges has commonly been reported in other regions (Dunlop & Clark, 2006; Trommelen et al., 2014), it is observed less frequently in Newfoundland and, where it does occur, it tends to be poorly developed. Up-stream, downstream and lateral transitions occur across various parts of the landscape (Figure 6) and interestingly, cross-cutting ribbed moraines can be observed in the southwest of the island. This is a rarely reported phenomenon, but is important, as it provides clear evidence of former ice divide migrations in the region (Clark & Meehan, 2001) (Figure 5f and Figure 7).

Figure 6. This figure shows examples of the spatial connection between ribbed moraines and glacial lineations observed in Newfoundland. In (a), clear lateral transition is evident with ribbed moraine to the right of the image while drumlins are observed towards the centre. Image is displayed using SPOT imagery overlaid on top of a zero azimuth shaded relief. (Box 6a in Figure 1). Image (b), on the Avalon Peninsula, shows a field of ribbed moraines in the centre of the image, with transition to streamlined bedforms towards the coast. (Box 6b in Figure 1). Arrows indicate ice flow direction. All images are displayed as SRTM shaded relief which have been overlaid with pan-sharpened SPOT imagery.

Figure 7. Transitional and palimspsest subglacial landform relationships have been observed in many areas across Newfoundland; (a) illustrates a simple transition from ribbed moraine terrain (left) to streamlined terrain (right) (Box 7a in Figure 1). (b) Complex bedform patterns have been recorded, with this example showing a palimpsest landscape with overprinting of glacial lineations by ribbed moraine (top of image), while to the bottom right, a group of crag and tails are observed (Box 7b in Figure 1). (c) Two crag and tails are observed on the right of the image and adjacent to these is a field of hummocky ribbed moraine (Box 7c in Figure 1). (d) Illustrates transition of ribbed moraine to drumlinised ribbed moraine to streamlined terrain dominated by glacial lineations (Box 7d in Figure 1). (Yellow arrows indicate flow direction). All images are displayed as SRTM shaded relief which have been overlaid with pan-sharpened SPOT imagery.

4.2. Glacial lineations

The term glacial lineation tends to be used to refer to depositional subglacial bedforms (drumlins, flutings, mega-scale glacial lineations (MSGLs)). However, glacial landscapes reflect processes of both deposition and erosion and leave a record in the form of both depositional and erosional glacial lineations. Existing palaeo-glaciological reconstructions using the inversion technique to reconstruct ice flow patterns have commonly used evidence obtained from glacial depositional landforms, such as drumlins (Boulton et al., 2001; Clark, 1999; Clark & Meehan, 2001; Kleman et al., 1997) as opposed to using landforms modified by glacial erosion, such as glacially moulded bedrock lineations. However, with appropriate quality control strategies, such as verification of flow orientation using striae evidence (Figure 3), these landforms can add important information to aid full landscape interpretations (Glasser & Bennett, 2004; Gordon, 1981; Lindstrom, 1988; Roberts & Long, 2005). In order to provide completeness of information and facilitate a comprehensive interpretation of ice flow patterns and dynamics of the ice sheet, both depositional and erosional bedrock glacial lineations were mapped (Table 4).

The Main Map displays a total of 24,435 glacial lineations with 17,291 classed as depositional glacial lineations, 7056 classed as glacially moulded bedrock lineations and 88 drumlinoid hills. Two sub-categories have been mapped in the depositional glacial lineation category; these are 16,283 drumlins and 504 crag and tails. Drumlin ridge lengths range from 60 to 4602 m (mean 781 m). Although mean ridge length exceed that reported by Clark, Hughes, Greenwood, Spagnolo, and Ng (2009) in the British and Irish Ice Sheet (629 m), 68% of the mapped drumlins in Newfoundland have similarly reported ridge lengths in the range of 250–1000 m. Greater abundance of depositional glacial lineations is observed at lower elevations east of the Long Range Mountains, where elevations generally do not exceed 300 m. The majority of drumlins show ice flow commonly towards the coast with convergence and attenuation noted in a number of areas, for example, into the Bay of Exploits and Fortune Bay. Highly attenuated lineation patterns are used to identify fast ice flow events (Dowdeswell, Cofaigh, & Pudsey, 2004; Stokes & Clark, 2001, 2002) and as such these patterns may point to former ice stream locations (e.g. Blundon et al., 2010). Crag and tails are found generally within and adjacent to drumlins fields, although some isolated groups are evident on the map south of Red Indian Lake in central Newfoundland. The highest abundance of streamlined glacially moulded bedrock lineations is found in coastal regions. Good examples occur on the Avalon Peninsula showing convergence of ice flow from the surrounding sub-peninsulas into Conception Bay and an eastward ice flow offshore on the east coast of the Avalon Peninsula. The third class of linear landforms mapped are drumlinoid hills. Although drumlinoid in form, these are generally significantly larger in height and width than those of other neighbouring glacial lineations and have ridge lengths ranging from 1470 to 7880 m (3517 m). Although these are observed scattered and isolated across lower elevations of the island, a relatively greater number are located in central southeast mainland Newfoundland. On the Main Map, these display a broadly radial pattern, associated with eastward-oriented ice flow towards Bonavista Bay, a south-eastward flow towards Placentia Bay, a southeast flow towards Placentia Bay and a southward flow towards Fortune Bay.

4.3. Landform relationships

The Main Map shows that many of the subglacial landforms across the NIS bed have strong spatial relationships. Some of these are relatively simple and show transitions from one landform to another; for example, ribbed moraine terrain evolving into more streamlined terrain containing glacial lineations, as can be observed on the landscape north of Fortune Bay (Figure 2a). However, there are other more complex relationships whereby different sets of landforms are either superimposed or cross-cut one another, creating a palimpsest landscape (Figures 5f, 6a and 7). Such landform relationships provide an important record of temporal changes in former ice flow and can be used to create a relative chronology of shifting ice flow phases during the evolution of the ice sheet (Clark, 1997). For example, superimposition and cross-cutting landforms record significant shifts in ice flow dynamics during different bedforming events. These can be accounted for by either ice divide migrations which causes deflection of primary ice flow (Clark and Meehan, 2001), ice stream activation which can result in a new suite of landform development overprinting older ice flow patterns (Stokes et al., 2008) or may occur during deglaciation where increased topographic control causes shifts in ice flow orientation which can cut across older flow patterns (Greenwood & Clark, 2009a). Because the map records the spatial position of individual landforms, it can be used to investigate these relationships more thoroughly which will greatly improve our understanding of the dynamic behaviour of the NIS throughout the last glacial cycle.

5. Conclusions

The Main Map builds upon previous landform mapping in the region (e.g. Prest et al., 1968) by being the first map that documents the spatial distribution of individually mapped subglacial bedforms across the entire island of Newfoundland (Figure 8). It offers a new record of almost 127,000 individual subglacial bedforms and is a comprehensive new landform database that builds upon previous landform research in the region. The mapping extends the known spatial extent and quantity of both ribbed moraines and glacial lineations across the island of Newfoundland and reveals a more complex subglacial bedform record than has been discussed in the existing literature. This is an important new data set from which the ice flow pathways and dynamic behaviour of the former NIS can be reconstructed and which has the potential to provide important new insights into the behaviour of the former NIS during the last glacial cycle.

Figure 8. (a) The flow indicators of subglacial lineations (black lines) and ribbed moraine (yellow polygons) contained within the Glacial Map of Canada for the Island of Newfoundland are redrawn (Prest, V.K., Grant, D.R. and Rampton, V.N. 1968: Glacial map of Canada. Geological Survey of Canada. Map 1253A). This map by Prest et al. (1968) records the ice flow indicator glacial lineations at relatively low density while ribbed moraine terrain is presented as generalised polygons. (b) Comparative image of the same geographic extent as (a) to illustrate detail of the new mapping completed in Newfoundland. Individual glacial lineations are represented as black lines while ribbed moraine ridges are illustrated as yellow lines.

Software

Software used included Erdas Imagine 2011 and Esri ArcGIS 10.1. All processing of satellite imagery and DEMs (e.g. creation of shaded reliefs; false colour composites; pan-sharpening, geocorrection, mosaicing) was carried out in Erdas Imagine 2011. ArcGIS 10.1 was used for visualisation, interpretation and digitising the landforms. All cartography was performed using ArcGIS 10.1

Acknowledgements

This mapping forms part of Maureen McHenry's Ph.D. research project which is entitled ‘Reconstructing the Newfoundland Ice Sheet through the last glacial cycle’, and is based in the Quaternary Environmental Change Research Group, School of Environmental Sciences, Ulster University. Her Ph.D. is funded by a Department for Employment and Learning (DEL) Ph.D. scholarship. The authors are very grateful for the thorough reviews provided by Martin Batterson and Krister Jansson which have improved the final manuscript, and to the cartographer Chris Orton for his valuable advice in finalising the map.

Disclosure statement

No potential conflict of interest was reported by the authors.