These are common in New England and probably have some value for glacial movement.
Crescentric fractures are caused by the jerkily sliding of larger rock particles on hard rock surfaces under pressure. Maisch et al. (1993) explained such forms through the temporary freezing of the glacier ice on bedrock and subsequent break out of the rock fragments. Maybe the added sketch (redrawn from Maisch et al, 1993, Lebendiges Gletschervorfeld-Führer und Begleitbuch zum Gletschervorfeld Morteratsch. – Geogr. Inst. Univ. Zürich, 138 S., Zürich) can help you. Unfortunately in German … ;-)
Article Gletscherschliffe und -schrammen am Schwerzberg nordöstlich von Halle
The best explanation is generally thought (as Jan-Michael Lange said above) to be the stick-then-slip pressure-impact cause by embedded rocks in the ice pressing down on the bed. Crescentic fractures and glacial striae are indeed used to deduce glacial-flow directions. The main knock against them is that they can be time-transgressive as the flow centers of the ice change. Though very labor intensive, direction change studies (via cross-cutting relationships) can also be useful. The article Kenneth Towe linked above is a good one. Most introductory texts have some information on process (e.g. Ehlers, 1996, Quaternary and Glacial Geology). The attached photo shows both fractures and striae on columnar basalt near Haifoss in Iceland. Note the ice flow is toward the viewer in the center of the photo but to the left in an older set of striae in the middle left of the photo.
For the crescentic fractures (not the gouge and lunates) my interpretation was that when you have a focus point, such as a debris in the sole of the glacier, differential pressure builds up from the contact point between the debris and the bedrock and its surroundings. In particular, directly up-ice the pressure difference will be higher. In some case it will be able to "compact" the bedrock itself and create this little normal offset fractures. The alinement of the successive crescent is indicating the progressive motion of the debris. Indeed differential pressure helps enhancing freezing/thawing around the clast and promotes the dismantlement of the bed-rock. But I believe this is mainly a local compaction of the rock itself with a localised rotation of part of the rock down-ice which is accommodated the easiest way possible by creating a listric fracture up-ice of the compacted area.
This phenomenon is also seen on sands which behaves sort of similarly than crystalline bedrock when high under pressure. There are some nice observations from ancient glacial surface (pre-Quaternary). Not sure it interests you but I can dig out some field pictures and maybe papers if you want.
Dear Julien,
I think that you have hit the nail right on the head. Interesting hypothesis. I vote you up on your answer. Sure would like to see the photos.
Best wishes, Paul.
Many thanks Keven for the photographs. These are not the same crescentric fractures that I was referring to. Mine have essentially no relief, the shape is more like a bow (as in bow and arrow), and they very symmetrical. The radiuses of the bows are of the order pf several centimeters.
Thanks for your help Paul.
Hi Ken,
Thanks for the very valuable reference on crescentric fractures. Very valuable indeed. They indeed give you the direction of ice flow.
Paul.
Hi Paul,
Here is a link to an interesting paper on the soft substrate deformation
https://www.researchgate.net/publication/279183157_Denis_et_al_%282006%29_Africa_Geoscience_Review._Typology_of_Hirnantian_pavements_in_the_SW_Djado_basin_%28NE_Niger%29
Then I attach a picture of one of these pavement where in a groove, you can observe well pronounced crescent structures. I had other pictures, but I cannot get my hands on it. this one is from my Phd.
Cheers,
Julien
Research Denis et al (2006) Africa Geoscience Review. Typology of Hir...
Hi Paul,
Here is a link to an interesting paper on the soft substrate deformation
https://www.researchgate.net/publication/279183157_Denis_et_al_%282006%29_Africa_Geoscience_Review._Typology_of_Hirnantian_pavements_in_the_SW_Djado_basin_%28NE_Niger%29
Then I attach a picture of one of these pavement where in a groove, you can observe well pronounced crescent structures. I had other pictures, but I cannot get my hands on it. this one is from my Phd.
Cheers,
Julien
Research Denis et al (2006) Africa Geoscience Review. Typology of Hir...
Hi Paul,
Here is a link to an interesting paper on the soft substrate deformation
https://www.researchgate.net/publication/279183157_Denis_et_al_%282006%29_Africa_Geoscience_Review._Typology_of_Hirnantian_pavements_in_the_SW_Djado_basin_%28NE_Niger%29
Then I attach a picture of one of these pavement where in a groove, you can observe well pronounced crescent structures. I had other pictures, but I cannot get my hands on it. this one is from my Phd.
Cheers,
Julien
Research Denis et al (2006) Africa Geoscience Review. Typology of Hir...
Hi Paul,
Here is a link to an interesting paper on the soft substrate deformation
https://www.researchgate.net/publication/279183157_Denis_et_al_%282006%29_Africa_Geoscience_Review._Typology_of_Hirnantian_pavements_in_the_SW_Djado_basin_%28NE_Niger%29
Then I attach a picture of one of these pavement where in a groove, you can observe well pronounced crescent structures. I had other pictures, but I cannot get my hands on it. this one is from my Phd.
Cheers,
Julien
Research Denis et al (2006) Africa Geoscience Review. Typology of Hir...
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