Transverse drift ridges - giant current ripples?
|There are several types of transverse ridges associated
with drumlins, including linear ridges of bedrock, and drift varieties
known as rogen moraine, and De Geer moraine. The moraine ridges sometimes
include stratified sand and gravel.
Are these ridges associated with drumlins, which trend normal to the flow indicated by drumlin orientation, actually the remnants of giant current ripples caused by rapid currents, when the flood waters retreated from the land?
Perhaps; this interpretation is suggested by my interpretation of drumlins as the effects of longitudinal vortices in former rapid currents, which streamlined bedrock and unconsolidated sediment, and by my in situ disintegration theory of the drift. If this interpretation is valid, some major evidence revealing the effects of the retreating floodwaters has yet to be thoroughly investigated.
Transverse VorticesSuppose sediments were formed during the flood, and at the end of the flood, these areas were rapidly uplifted. The flood waters would tend to flow away from the centers of uplift. Where the flow rate was very rapid, one would expect that there would be erosion of the sediment. There is lots of evidence for erosion; over much of the Canadian Shield, sediments were removed entirely. And the basins of the Great Lakes, and many other lake basins, are evidence for erosion by rapid current action.
The streamlining of the soft sediments or bedrock by longitudinal vortices in the currents formed drumlins and other streamlined landforms. [See the article Drumlins and Diluvial Currents.] Just as some drumlins are composed of bedrock, and the bedrock drumlins can be explained as the lithified expression of streamlining effects of the currents of the retreating flood waters on bedrock or sediments that subsequently became lithified, so there are transverse ridges associated with drumlins, that may be composed either of bedrock or drift. These would represent the effects of transverse vortices in the former currents, that acted as rollers which reduced friction at the water - sediment interface.
The transverse ridges composed of drift include rogen moraines and De Geer moraines. Examples of transverse ridges of bedrock occurring near drumlins, that I have interpreted as giant current ripples, occur in Ontario's Bruce Peninsula.
Many of the problems inherent in the glacial interpretation of drumlins, also apply to the question of the origin of the various kinds of transverse drift ridges in a glacial environment.
A Reinterpretation of Rogen MoraineRogen moraine is a particular type of landscape, sometimes referred to as "ribbed moraine"; it is even referred to as "rippled till". [Elson, p. 1217.] Its type locality is the region around Lake Rogen in central Sweden. The rogen moraine (pronounced "roogen") was described and identified by Jan Lundqvist.
The rogen moraine typically has a rounded, sinuous profile. Height ranges from 3 to 30 m. Spacing between crests varies from 100 - 300 m. It characteristically shows a gradual transition to drumlins. Over an extensive area, patterns of transverse rogen moraine ridges tend to change downcurrent to crescent shaped forms, which change downcurrent to typical drumlins; these changes in patterns may be repeated. Lundqvist says the rogen moraine ridges occur in concave areas, while drumlins occur in convex regions.
In the giant current ripple interpretation, during tectonic movements
that occurred at the end of the flood, as the floodwaters drained towards
the oceans, changes in the characteristics of the current flow would occur
where currents flowed over uneven ground. Because the surface of bodies
of water tend to become level, and because of the principle of continuity,
flow rate increases over elevated ground, and decreases over basins or
depressions. Changes in the type of vortices that existed in the former
currents at these varying flow velocities could perhaps explain the repeated
transition from ribbed or rogen moraine to drumlins.
A similar transition in bedforms can sometimes be observed where streamlined sand bars occur in the sandy beds of fast flowing streams and rivers; as the depth changes, there is a gradual transition from longitudinal forms, to transverse ripples or dunes. In the diluvial interpretation of drumlins, where there were different elevations of the ground, and if the current direction remained unchanged over the region, transverse ripples likely formed normal to the axial trend of drumlins in the region; these probably formed at the same time as drumlins.
As the land was uplifted after the flood, unconsolidated sediments and bedrock were eroded by currents, and the sediments were lithified. In my theory, this was the environment in which disintegration occurred; chemical processes in the sediments and rocks near the surface, driven by the rapidly changing conditions, caused the transformation of newly formed rock to pebbles and sand, or drift. This process of disintegration probably aided in the erosion of deep lake basins, such as the basins of the Great Lakes, and thousands of lakes in the Canadian Shield. Erosion and disintegration processes mutually enhanced each other.
Disintegration occurred from the surface downwards, forming drift gravels and sand. In many areas, both drumlins and transverse giant current ripples were converted to drift, or, perhaps the drift itself was shaped into patterns of ripples by the currents. These have been interpreted by geologists as deposits of the melting glaciers, but in this new interpretation, drumlins, flutings, and the transverse ridges associated with them may be giant current ripples and streamlined landforms; the effects of rapid currents of the floodwaters retreating from the land.
De Geer MorainePatterns of low profile, parallel ridges, sometimes referred to as "cross valley moraines", or "washboard moraine", but more generally known as "De Geer moraines", are present over vast areas, in Norway, Sweden, Finland, Maine, Alaska, NWT, Newfoundland, New Brunswick, Saskatchewan, Ontario, Quebec, and other regions; they also occur offshore on the Scotian Shelf. They were first described by Gerard De Geer in 1889, and the landform was named after him in 1959.
A large belt of De Geer ridges occurs in western Ontario, northwest of Lake Superior. Other major belts, with areas up to 43,000 km2, occur east of Hudson Bay and James Bay in Quebec.
The De Geer ridges are 1 to 10 m in height, with spacing 100 to 300 m. They are composed of drift materials, and are often associated with drumlins. Where drumlins are present, the ridges are generally oriented normal to the axes of drumlins, but in some cases they are offset, indicating they were not contemporaneous with drumlins. In some areas drumlins have become reworked into De Geer ridges. It seems then, that the patterns of De Geer ridges have been superimposed on a formerly streamlined topography, and they represent a modification of the landscape by altered conditions, immediately following the streamlining of the landscape.
The De Geer ridges probably represent effects of changes in the type of vortices present in the currents, at decreasing depths. As depth of the flood waters decreased, there would likely be a transition in the scale and type of vortices in the former currents, and a corresponding change in the bedforms produced, from the longitudinal vortices which built the drumlins, to transverse vortices (that acted as rollers) which formed patterns of ripples.
The composition of De Geer ridges could be explained by disintegration, or perhaps, some of the drift formed by in situ disintegration was transported and redeposited by the currents.
In the glacial theory of Louis Agassiz, the drift gravel and sand is attributed to the effects of hypothetical ice sheets of the ice age, and so the significance of drumlins and the rogen and De Geer ridges, and other evidence of the effects of retreating floodwaters, has gone unrecognized.
Subglacial FloodsInvestigation of the theory of catastrophic floods resulting from the sudden release of huge reserviors of meltwater that accumulated beneath the ice sheets has led John Shaw of the University of Alberta, Edmonton, and other researchers to conclusions about the origin of drumlins, and rogen moraine, that are quite similar to mine. Shaw wrote in 1994:
Shaw (1983a) suggested that large-scale meltwater floods were responsible for some drumlins. Later, erosional drumlins, bedrock erosional marks, tunnel channels and Rogen moraine were added to the forms resulting from catastrophic floods (Shaw and Sharpe, 1987; Sharpe, 1988; Sharpe and Shaw, 1989; Shaw et al., 1989; Kor et al., 1991; Fisher and Shaw, 1992). The meltwater flood hypothesis, like the concept of subglacial deformation, has the potential to change glacial geomorphology dramatically. Both necessitate radical revision of ideas on how ice-sheet form and behaviour are reconstructed and on how subglacial erosional and sedimentary processes are modelled.Hummocky terrain in south-central Alberta, southeast of Calgary, near the McGregor Lake Reservoir, was described by Munro and Shaw, and transverse tidges and depressions (resembling giant current ripples) were compared with features caused by the great floods of Lake Missoula in southeastern Washington; they wrote:
Some erosional forms scoured in basalt scoured by the Lake Missoula floods are morphologically similar to hummocky terrain (Baker,1978), indicating a strong possibility that hummocky terrain can be the product of meltwater erosion. Also, streamlined loess hills in the Channeled Scabland, Washington, maintain undisturbed internal stratigraphy (like the Travers-McGregor hummocks), despite erosion by the Lake Missoula floods (Bretz, 1969).Their conclusions about the environment of formation of these features, involving large scale rapid currents, parallels the interpretation I have proposed that involves currents of flood waters generated by differential crustal uplift of submerged regions at the end of the flood, when the waters retreated from the land.
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