Ribbed moraines are so named because they resemble the ribs of a rib cage. The name was popularized by Lee (1959) and Hughes (1964). The term “Rogen moraine” is also used, based on the typical example of Lake Rogen, Härjedalen, Sweden (Hoppe, 1959), where they were first described (Lundqvist, 1937; Mannerfelt, 1945; Hoppe, 1959). Historically, the two terms refer to slightly different moraines, but their use in recent Ministère literature is interchangeable. Initial observations of Rogen or ribbed moraines referred to a purely descriptive type of field morphology. Rogen moraine was then referred to as an area of undulating ridges; the term moraine being used in its lithological sense (Hughes, 1964; Bouchard, 1980; 1989; Benn and Evans, 2010).
Ribbed moraines are defined as undulating ridges with a boudinaged and possibly arched morphology. The main orientation of the ridges is perpendicular to the ice flow direction. Typical dimensions are 300 m to 1200 m long, 100 m to 300 m wide by 10 m to 40 m high (Hoppe, 1959; Lundqvist, 1969; Hättestrand and Kleman, 1999; Dunlop and Clark, 2006; Benn and Evans, 2010). There is an almost regular spacing between the moraines (Hättestrand and Kleman, 1999).
Several studies report an asymmetrical ridge profile, with the downstream slope being steeper than the upstream one (Hoppe, 1959; Shilts, 1977; Boulton, 1987; Shilts et al., 1987; Aylsworth and Shilts, 1989). They are typically arc-shaped with the ends pointing down ice (Bouchard, 1986; 1989). Dunlop and Clark (2006) demonstrated that these characteristics are too restrictive and that ribbed moraines exist in many different forms.
Ribbed moraines are generally composed of a massive or laminated diamicton incorporating a large number of boulders, but gravel, sand and silt may also be present. Sedimentary facies generally display evidence of glaciotectonic deformation in the form of shear planes and folds (Shaw, 1979; Fisher and Shaw, 1992; Hättestrand, 1997). In this type of moraine, the till fabrics indicate a clast orientation transverse to ice flow (Lundqvist, 1937; Cowan, 1968).
Areas of ribbed moraine are generally spatially associated with areas of streamlined till (Prest, 1968; Sugden and John, 1976); the latter are characterized by drumlinoids oriented at right angles or strongly oblique to moraine ridges. A gradual transition zone between these areas is common and is characterized by ribbed moraines displaying a streamlined surface morphology (Prest, 1968; Lundqvist, 1969; 1989; Markgren and Lassila, 1980; Aylsworth and Shilts, 1989). Aylsworth and Shilts (1989) note that lateral transitions between moraine and drumlinoid areas are generally steeper than transitions up and down ice of a ribbed moraine field. Moraines with a transitional or combined morphology are those that were originally referred to as “Rogen” (Lundqvist, 1969).
The genesis and mechanisms controlling the formation of ribbed moraines are still a matter of debate. Several studies suggest that their formation is associated with a single glacial event (Bouchard, 1989; Aylsworth and Shilts, 1989; Hättestrand, 1997; Hindmarsh, 1998a; 1998b; 1999) or fluvial event (Shaw, 1979; Fisher and Shaw, 1992), whereas some authors rather propose a polyphase formation (Boulton, 1987; Lundqvist, 1989; 1997; Möller, 2006; Sarala, 2006).
The morphological superimpositions observed in transitional zones could suggest polyphase genesis in space and time between ribbed moraines and certain drumlinoids (Lundqvist, 1969; Bouchard, 1986). Boulton (1987) proposed that ribbed moraines are part of a morphological continuum of linear bedforms resulting from the evolution of a deformable bedrock at the glacial base. A change in the direction of ice flow could thus cause undulatory deformation of drumlins, drumlinoids or terminal moraines (Lundqvist, 1969; 1989; Boulton, 1987; Clark, 1994; Möller, 2006; Finlayson and Bradwell, 2008). This process cannot be the origin of all the observed bedforms and is probably not the only one responsible for the formation of ribbed moraines.
A second model explains their formation by compressive ice flow that causes shearing of the pre-existing sedimentary layer or generating a new basal accumulation of sediments (Lundqvist, 1969; Bouchard, 1986; Hättestrand, 1997; Hättestrand and Kleman, 1999; Sarala, 2006; Lindén et al., 2008). Debris embedded at the base of the ice would be superimposed in response to compression and shear during ice flow. This compression would occur in topographic depressions or at the boundary between a warm- and cold-based glacier (Sugden and John, 1976; Shaw, 1979; Sollid and Sorbel, 1984; Bouchard, 1986; 1989). In this model, the transition from drumlin forms to ribbed moraines is explained by a change in the flow regime from compressive to extensive.
Another theory explaining the genesis of ribbed moraines is that of fracturing and extension of pre-existing frozen sediment layers (Boulton, 1987; Lundqvist, 1989; Hättestrand, 1997; Hindmarsh, 1998a, 1998b, 1999; Hättestrand and Kleman, 1999; Sarala, 2006). During deglaciation, two layers of sediment are superimposed: a first, superficial layer is formed of frozen sediment; a second, underlying layer is unfrozen and deformable. The interface between these two layers would allow for the formation of a decollement plane, generating an extensive flow regime. The resulting shearing would fracture the frozen sediment layers, creating an interdigitated pattern.
The latter two formation types are consistent with the gradual transitions observed between the streamlined and morainal areas. The authors invoke only one flow direction, suggesting that drumlinoids and ribbed moraines are contemporaneous and caused by the same event.
The global spatial distribution of ribbed moraines corresponds to ice-covered areas of the Laurentide and Greenland inlandsis and the Fennoscandian and Anglo-Irish ice sheets (Hättestrand, 1997; Clark, 1999; Clark and Meehan, 2001) in Sweden, Finland, Ireland, Scotland, Canada and the USA (Lundqvist, 1981; Hättestrand and Kleman, 1999; Dunlop, 2004; Dunlop and Clark, 2006; Finlayson and Bradwell, 2008). Typically, ribbed moraines develop within 50 km to 350 km of glacial margins as the recessional margin approaches these areas (Shaw, 1979; Shilts et al., 1987; Bouchard, 1989; Kleman and Hättestrand, 1999).
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