Rogen Moraines
Moraines > Classification and Formation of Moraines
Background Information
Rogen moraines, also known as Ribbed moraines, appear as successive ridges of till oriented perpendicular to ice flow. The name “ribbed” moraine is derived from their similarity to fish ribs, while “Rogen” was quoted by Lundqvist (1969) after analyzing a moraine near Lake Rogen in Sweden (Marich et al., 2005). In the figure above, ice flow would be in the same direction as the road running beside the ridges. One cannot decipher absolute direction of ice flow from Rogen moraines, but a general north-south, or east-west direction can be determined. However, Rogen moraines are typically not used to reconstruct glacial history due to a lack of knowledge regarding their formation (Hattestrand & Klemen, 1999). These types of moraines are concentrated in the central areas of the late Pleistocene ice sheets once covering North America; more precisely, the Keewatin, Quebec and Newfoundland portions of the Laurentide ice sheet (Hattestrand & Klemen, 1999). These areas of Rogen moraine formation have also been correlated with areas where the ice sheet was cold based; this could assist in the understanding of how they are formed (Hattestrand & Klemen, 1999). The theories on Rogen moraine formation will be discussed in more detail in the Formation section of this page.
Characteristics
Rogen moraines are typically found in rows of successive ridges, and the areas between them can be occupied by ponds or bogs due to their low-lying nature (Marich et al, 2005). Typical moraines range from 5-25 meters high, 100-450 meters wide and 0.5 km to 5 km long and are usually similar and evenly spaced (Marich et al., 2005). As for the sedimentology of Rogen moraines, there is high variability in their composition and sedimentary structures. Most are composed of till, with some having successive layers of till and supraglacial material on top (Lundqvist, 1996). Also, Rogen moraines have been found that were composed of glaciofluvial sediments with sedimentary structures that have been subsequently re-worked (Lundqvist, 1996). There is still much discussion on how these moraines are formed. It is widely accepted that these features are depositional; however the theories on how the till gets deposited differ greatly. The three forefront theories discussed by Marich et al. (2005) are: the bed-deformation model, the shear stack model and the subglacial meltwater flood theory.
Formation
The formation of Rogen moraines is a topic that is subject to much debate in recent literature. An article by Marich et al. (2005) examines ribbed moraines on the Avalon Peninsula in Newfoundland. Within this paper, three theories of ribbed moraine formation are discussed. The first of which is the bed-deformation model illustrated below in Figure 1.
The bed-deformation theory requires two separate glaciations in perpendicular directions or a change in ice flow direction by 90 degrees. The first glaciation, where ice is going from the bottom of the diagram to the top, creates landforms known as drumlins, which are another subglacially formed landform. The second ice flow direction, perpendicular to the original direction, re-works the drumlins and creates ribbed moraines. A requirement of this theory is that the drumlins be composed of alternating sections of more resistant and less resistant sediment. This will allow for the wavy shape of the ribbed moraines to occur. The more resistant sediment, shown by “x” in the figure, does not get deformed as far as the less resistant sediment. The resistant sediment cores “x” are also claimed to represent the stoss side of the subsequent drumlins that get formed over extensive ice coverage. A problem with this theory is the lack of knowledge on how drumlins are formed. Drumlins are another glacial landform where there is considerable debate on their formation. For more information on drumlins, and how they are formed, follow the link.
A second theory on how rogen moraines are formed relies on the underlying bedrock terrain and ice stagnation. Refer to Figure 2 below for the illustration of how they are formed.
In the first diagram in Figure 2, it shows ice flowing over undulating bedrock. As the ice moves over the down-ice side of the low area, it is compressed and shear planes develop. This is shown in the second diagram by the layers of sediment laden ice stacked up against the down-ice wall of the basin. When the glacier is no longer advancing (stagnation), the layers of sediment rich ice are lowered to the bed to form linear layers of glacial till. As the glacier retreats, these stacked up layers of till are revealed on down-ice walls of basins and form ribbed moraines, as shown in diagram four. Further up ice, areas of Fluted Terrain and Hummocky Terrain are also created in the basin. A problem existing with this theory is the explanation of ribbed moraines occurring where the bedrock is not undulating. Also, fluted and hummocky terrain is not always found up-ice from ribbed moraines.
The third theory on Rogen moraine formation involves radically different methods and parameters than the previous two. In the previous two methods, sediment was deformed in some way by the movement of the ice itself, whereas in this theory, the creation of Rogen moraines relies on subglacial meltwater in catastrophic magnitudes (Shaw & Fisher, 1992). This theory was proposed by Canadian professors of geography, John Shaw and Timothy Fisher in 1992. In their paper on Rogen moraine formation, it is proposed that an almost instantaneous event involving massive amounts of subglacial meltwater formed these successive ridges of till. Shaw and Fisher describe the process as subglacial meltwater carving cavities in the underside of the glacial ice, these large cavities are then infilled with till once the ice stagnates. Figure 3 provides a schematic of how the subglacial metlwater flood theory works.
As shown in Figure 3, the meltwater lifts the glacier off the bed and flows between the till and the basal layer of ice. Rogen moraines are shown on the margins of the glacial bed and are claimed to be similar to ripples in a stream bed. Shaw and Fisher (1992) perform a form analogy between Rogen moraines and river ice ripples to help support their argument. Another form of evidence used to support this idea is the existence of bedrock erosional marks caused by fluvial processes. Shaw (1983) has also claimed that drumlins form in the same manner. A persistent problem with this theory is the source of the water. The magnitude of water Shaw and Fisher (1992) are referring to is so large, it is difficult to provide reasonable explanations as to where it came from.