Features such as crag and tails only occur because the crag was there first. Roches moutonnees start out as rock knolls. Small-scale rat tails are destroyed by the abrasional processes that produce striations and destructive wear by clasts in overriding ice (attached: intact rat tails compared with those destroyed by overriding boulders). Bernard Hallet shows that glacial abrasion removes bumps on a subglacial rock surface. Therefore, rock drumlins can only form if there are preexisting protrusions with the same spatial arrangement as the drumlins. Since the drumlins are commonly en echelon (Kor and Cowell, s-forms at French River attached), the same en echelon pattern required of the initial rock protrusions calls for a remarkable coincidence.
https://www.youtube.com/watch?v=7L2rnpWE3jA
A very insightful question. If the spacings are not imposed by the character of the substratum then it implies there is spatial structure to the ice flow dynamics that produces greater abrasion in some locations than in others. I am not a glaciologist, but this is akin to the problem of identifying coherent spatial structure in waterflow.
Paul - what I believe John (and Kor and Cowell) are proposing is that there is a spatial structure in a sub-glacial sheet flood to produce the erosional forms. This clearly assumes some stability in the flow pattern during the event to produce the en echelon forms.
Yes I know. I am happy to accept this is a valid argument. I was also arguing objectively that the regular spacings could indicate regular structure to ice flow.
Perhaps one does not need to invoke spatial structure in the sheet flow at the scale of the sheet flow, and, simply, similar flow structures occur throughout because similar conditions occur throughout.
Thanks Marco,
Transverse asymmetrical drumlins at Livingstone Lake are skewed in opposite directions on either side of the drumlin field. As well, drumlins of different shapes, parabolic, spindle, and transverse asymmetrical, appear spatially in subgroups in different parts of the field--transverse asymmetrical on the flanks, and huge parabolic drumlins down the centre of the field, on the highest land. I think this clustering of drumlins with similar form suggests that conditions were not similar throughout. Someone with your ability in morphometry and GIS could base a hypothesis on this description. It would be refreshing to see statistics used to test a hypothesis. I have any number of airphotos which you are welcome to keep, I won't ever use them again and my wife would be delighted if they were out of the house.
You see the same forms, formed entirely by abrasion, in fault systems. There is a paper in 1994 by H.D. Tjia on kinematic analysis of striated fault systems.
Thanks Roger,
I am not sure what your question means. Striations on fault planes are the same as striations on a glaciated rock, because they form by the same process--brittle shear. They are not the same as obstacle scour marks, rat-tails, erosional drumlins, crag and tails, yardangs, parting lineation etc which relate to flow separation in turbulent flow. Flow separation and vortices are responsible for the hairpin scours wrapped around the stoss side of these bedforms. While, separated flow features have smooth, sharp edges, brittle shear striations have rough, jagged edges at the microscopic scale. We have to be prepared to look at the detail. If we use analogy indiscriminately, we would conclude that yardangs form by brittle shear. We must be wary of what Karcz referred to as the "lethal lure of geometric similarity." If we are not careful, we end up calling a horse a dog because they both have four legs!
Thank you John.
Attached is a visualization of the Livingstone drumlin field region based on a ~10 m DEM. It includes the 1:250 000 bedrock geology outlines, for whatever they may be worth.I see the drumlin morphology variation you mention.The yellow arrows are ice flow direction interpretations (ambiguous). Perhaps part of the transverse asymmetric forms are the result of the combined diachronic action of relatively somewhat-off ice flow directions (?), and/or of the streamlining of positive-relief flow-transverse features (?). In places, the transverse component of the forms is ~perpendicular to the lineations the flow direction interpretation was based on. The partial coincidence between the largest drumlins with the top of the 'plateaux' is interesting.
Thank you John! Awesome pics! Had no idea of how they looked like in the ground. I am very keen on comparing morphological analogues. You probably have some great ideas on this.
And this interchange is pretty amazing. Hopefully more and more people will start participating in discussions of this type. A great place to learn.
Don't be so quick to dismiss abrasive erosion features John, even in fault settings...
Roger, I don't dismiss glacial abrasion, striations on rock surfaces and on boulders demonstrate that abrasion occurs. Faceted boulders, especially those in boulder pavements, show that abrasion can wear away about half of a boulder volume. However, where there is cross-cutting striation, loss by abrasion has a maximum depth less than the depth of the striae. Otherwise, you could only preserve the youngest striation (Chamberlin 1888).
Your original question was: "How can glacial abrasion create positive relief features?" There are lots of different ways (differential erosion with rat tails, etc.)... My point was using an analogy whereby pure abrasion was doing exactly that.
That's OK John... for the record, we have never been on the same wavelength, but I always did enjoy your company and perspective when having beers with Ralph Stea at CANQUA meetings and on field trips. Glad to see you still active in your retirement.
- Badges
- Science topic
- Similar topics
- Geoscience
- Cartography
- Mapping
- Maps