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Streamlined forms are characterized by elongated, straight, low-elevation ridges, composed mostly of diamicton. This generic term encompasses several types of morphologies, including drumlins and drumlinoids. They are probably the most studied glacial morphology in recent decades. There are more than 1300 contributions (articles, abstracts or theses) in the literature, including more than 400 scientific papers published since 1980 (Menzies, 1984; Patterson and Hooke, 1995; Clark et al., 2009; Stokes et al., 2011; Spagnolo et al., 2012).
Etymology
The term drumlin is derived from the Irish word druim which means “back” or “rounded ridge”. These forms were recognized in glaciated terrains of Ireland and named by Close (1866). The name was subsequently used by various geological surveys undertaking the study and mapping of ice forms in Britain, Ireland (Kinahan, 1874; Kilroe, 1888) and North America (Davis, 1884; Upham, 1889; Tarr, 1894).
Drumlinoid is a term related to drumlins, but with less restrictive morphological criteria.
In English literature, the terms fluted moraines, flutings or streamed line ridges are the best equivalents for drumlinoids (Boulton, 1976; Rose, 1987; Benn and Evans, 2010). There is no consensus on the use of the different terms and the French equivalent of some terms is unclear. In the Ministère’s publications, the term drumlinoid is a general term for streamlined shapes larger than drumlins, thus encompassing any fluting shape, or the term Mega-Scale Glacial Lineation (MSGL). The drumlins themselves have a separate designation (DU) in the Quaternary legend, as do flutes (RGL).
Description
A drumlin is a small, ellipsoidal, asymmetrically shaped mound characterized by a rounded apex side and a gently sloping streamlined side. The blunt apex is the highest point of elevation and is located at the glacial headwaters of the bedform while the streamlined side indicates the glacial flow direction (Benn and Evans, 2010).
Numerous studies have shown that their morphology can vary greatly (length, width, height, spacing, symmetry, wick or parabolic shape; Menzies, 1979; Coudé 1989; Mitchell, 1994; Smalley and Warburton, 1994; Benn and Evans, 2010). Analysis of clusters or swarms comprising tens of thousands of drumlins in Britain and Ireland led Clark et al. (2009) to establish mean morphometric parameters (lengths, widths and elongation ratios) of 629 m, 209 m and 2.9 respectively. In general, drumlins vary in length from 250 m to 1000 m, 120 m to 300 m in width and 0.5 m to 40 m in height (Clark et al., 2009; Spagnolo et al., 2012; Ely et al., 2018).
The elongation ratio, determined by the length of the shape in relation to its width, is generally below 3. Above this threshold, hectometric to kilometric shapes will generally be described as drumlinoid, and above 10, as mega-scale glacial lineations (MSGL; see Etymology; Stokes and Clark, 1999; 2002).
Several studies have demonstrated that the composition of drumlins can vary greatly depending on the substrate. Drumlins are predominantly composed of glacial sediments, although some specimens have a rock core (Menzies, 1979; Patterson and Hooke, 1995). The majority of drumlins are composed entirely of glacial diamicton (Newman and Mickelson, 1994), but may also be composed of a core of sorted material with a till shell on the surface (Hart, 1994; 1995). The sedimentary characteristics of sediments beneath the till may be evidence of a deposition which is earlier (Krüger and Thomsen, 1984; Boulton 1987; Boyce and Eyles, 1991; Menzies and Brand, 2007; Benn and Evans, 2010) or contemporaneous with the formation of streamlined shapes (Dardis and McCabe 1983; 1987; Dardis, 1985; McCabe and Dardis, 1989; Shaw, 1989; Benn and Evans, 2010; Evans et al., 2015). Some studies also mention the presence of glaciotectonic deformations in sediments during the drumlin formation process (upturned folds, faults and water escape structures) (Bluemle and Clayton, 1984; Stanford and Mickelson, 1985; Boulton and Hindmarsh, 1987; Hart, 1995; 1997; Stokes et al., 2011; Hermanowski et al., 2019).
Crag & tails develop downstream of an obstacle, commonly a rocky knoll. The orientation of the tail indicates the ice flow direction. According to the legend used by Géologie Québec, these structures are mapped independently of drumlins and drumlinoids (TMF).
Genesis
Over the past few decades, many hypotheses, ideas or conceptual models have been proposed on the mechanism of drumlin formation without a clear consensus within the scientific community (Hall, 1815; Davis, 1884; Boulton, 1976; Menzies, 1979; 1987; Rose, 1987; Shaw, 2002; Stokes et al., 2013). The debate on the origin of drumlins is ongoing and somewhat controversial (Schomaker et al., 2018). The main models suggested in numerous studies imply the formation of drumlins by erosion processes, deposition, or by the theory of environmental instability.
The erosional model is explained by differential erosion of pre-existing diamictic, fluvioglacial or bedrock material from the top to the bottom of the glacier, leading to the individualization of drumlins (Menzies, 1979; Boulton, 1987; Hart and Boulton, 1991; Hart, 1995; 1997; Knight; 2010; Eyles et al., 2016). One branch of this model, the subglacial catastrophic flood hypothesis as the origin of drumlin formation (Shaw, 1983; 1989; 1996; 2002; 2007; Shaw et al., 1989; Shoemaker, 1992; Rains et al., 1993), has been contested since its inception in the 1980s (Kehew et al., 1990; Forsström and Shaw, 1990; Clarke et al., 2004; 2005; Sharpe et al., 2005; Evans et al., 2006; Benn et al., 2007; Evans; 2010; Ó Cofaigh et al., 2010; Shaw, 2010a; 2010b; Shaw and Young 2010).
Drumlins could also form by deposition of loose sediment downstream of an obstacle (rocky hummock, consolidated till, competent sediment; Fairchild, 1929; Boulton, 1987; Menzies et al., 2016), or by continuous sedimentary accretion to the tail of the streamlined bedform (Dardis and McCabe, 1983; Dardis et al., 1984; Hart and Boulton, 1991; Dardis and Hanvey, 1994; Patterson and Hooke, 1995; Fowler, 2009; Knight, 2010; Barchyn et al., 2016; Hart et al., 2018). In contrast to the erosional model, drumlin formation in this case would be by vertical accretion and therefore “bottom-up” (Eyles et al., 2016).
The theory of environmental instability to explain the formation of drumlins and other subglacial bedforms was introduced in the 1990s (Patterson and Hooke, 1995; Hindmarsh 1996; 1998; 1999; Fowler, 2000; 2009; Schoof, 2007; Dunlop et al., 2008; Clark et al., 2009; 2018; Clark, 2010; Stokes et al., 2011; 2013; Spagnolo et al., 2012; Fowler et al., 2013; Hillier et al., 2013; 2018; Eyles et al., 2016). This theory predicts that slight variations in local relief (instability) are sufficient to induce a positive feedback loop in erosion and deposition processes. Through this loop, the instability of the medium increases exponentially until a specific wavelength is reached, resulting in the formation of a multitude of shapes with similar morphology and spacing. This mechanism is commonly observed in the organization of aeolian and fluvial sediments, for example in the repetitive undulations of ripples on a sandy beach. In a subglacial context, the variation in basal ice conditions generates a rheological instability which is large enough to cause deformation of the water-saturated diamictic substrate, and the subsequent formation of subglacial bedforms, particularly streamlined forms.
Spatial Distribution
Drumlins are a ubiquitous landform found in most areas that were glaciated during the last ice age (Flint 1971; Embleton and King 1975; Menzies 1979; 1984; Clark et al., 2009). They are often grouped in fields that may reach several thousand (Benn and Evans, 2010). In North America, the largest drumlin swarms are in New York (10 000), New England (3000), Wisconsin (5000) and Nova Scotia (2300; Menzies, 1979).
Fields of streamlined bedforms are generally formed in depressional areas, in this case in lowlands and valleys, where subglacial basal stress is low, pore water pressure is high and sediment occurrence is high (Mitchell, 1994; Kovanen and Slaymaker, 2004; Mitchell and Riley, 2006). However, streamlined bedforms are not only confined to topographic depressions. Indeed, their presence has also been described on some plateaus, indicating an emplacement which is independent of topographic controls (Patterson and Hooke, 1995). As opposed to what has been previously suggested (Smalley and Unwin, 1968; Francek, 1991), recent studies tend to indicate that the distribution of drumlins is rarely random within fields and that they are emplaced at regular intervals between 100 m and 1200 m (Fowler, 2000; Clark, 2010; Clark et al., 2018). Finally, the nature and properties of the substrate also play a role in their distribution (Menzies, 1979; Boulton, 1987; Patterson and Hooke, 1995).
Because of their uniformity of characteristics within the same field (similar orientation, proximity and plurality of shapes, similar morphologies, etc.), groupings of streamlined bedforms are generally used to define flow-sets (Clark, 1994; 1999; Clark et al., 2000). These fields tend to appear in proximity (~80 km) to the ice margin (Patterson and Hooke, 1995); thus, the study of their distribution is essential for understanding and reconstructing the dynamics of former ice sheets (Clark et al., 2000).
References
Other Publications
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Contributors
|
First publication |
Simon Hébert, GIT, M.Sc. Simon.Hébert@mern.gouv.qc.ca (redaction) Olivier Lamarche, P. Geo., M.Sc. olivier.lamarche@mern.gouv.qc.ca (critical review); François Leclerc, P. Geo., Ph.D. (template and content compliance); Simon Auclair, P. Geo., M.Sc. (editing); Céline Dupuis, P. Geo., Ph.D. (English version). |
29 mars 2021