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S-forms by water erosion

Have the s-forms been sculpted by subglacial waters or abrasive action at the base of glaciers? The answer becomes obvious when reading all the publications on the subject. Subglacial waters are likely the cause. In fact, most of the detail publications of the last decades support this thesis. Field observations corroborate laboratory experiments, flash floods, jökulhlaup, and the 2004 tsunami. A scientific approach is legitimized when a comings and goings persist between physical theory, experimentation, and observation of modern events.

S-forms Thematic.jpg

Two contemporary events have reconfirmed that s-forms (sichelwannen, spindle flute, muschelbruch) are sculpted by catastrophic hydraulics:


• Flood of Katherine Gorge in Australia (Baker and Pickup, 1987)

• The 2004 tsunami (Bryant, 2014)

Surprisingly, some geoscientists still ignore hydrodynamic researches on s-forms. There are even faculties that ignore the debate in disfavor of the subglacial water thesis. For their part, hydrodynamic engineers describe this attitude as unacceptable. The notions of turbulent flows and vortices explain all types of s-forms in every detail. So, why some academics resist this idea? The reason is that the model would have two implications:

  1. The troubling aspect of the s-forms is their omnipresence in the territories covered by the ancient ice sheets. Wherever the bedrock outcrops we find these s-forms. This ubiquity suggests that large amounts of water swept under the ice sheets. If we accept that the s-forms are of hydro-catastrophic origin, there is only one step left to explain the genesis of the mysterious drumlins since these two morphologies are closely associated. In addition, several features specific to drumlins are not unlike those of s-forms.

  2. All these s-forms were not erased by subsequent glacial abrasion. When the ice sheet reattached to the bedrock the ice age was over. The water had probably warned down drastically the thickness of the ice. 

Drumlins vs s-foms.jpg

Within the scope of the subglacial water theory... here a 3D animation of these processes under the ice sheet. 

A glaciologist can stipulate that the s-forms could have several origins (polygenic). But so far, those who argue that ancient s-forms have been carved by the abrasive action of glaciers have produced no laboratory experiments, no study of modern occurrences of s-forms and no physical/mathematical models. In short, they produced no scientific publications other than their descriptions of ice age s-forms.

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With a polygenic approach, a glaciologist gives himself the right to insinuate that most of the s-forms of the ice age were the result of glacial abrasion. He does not realize that accepting that vortices can also carve identical shapes implies both; a singularity and an aberration of nature. In my opinion, the proponents of such a position are just unaware of all the publications in favor of the hydrodynamic origin of s-forms.


Before going further on reading this page I invite you to watch this short video of 9 minutes to awaken your intuition.



Omitted cavitation and water hammer, water as such has no abrasive power if not particles of sand and gravel it carries. In subglacial environment the water is overloaded with particles. At high flow the flow becomes turbulent since the slightest obstacle generates vortices or whirlpool. Inevitably vortices and cavitation when combined multiply greatly the erosive power of water.


These vortices dissipate naturally but before fading they will hit the rock digging s-forms for a brief moment:


  • When a vortex strikes the rock obliquely it flares upward, which carves a muschelbruch, a cavity in the form of a mussel.

  • If the vortex brushes the base with a lower angle a flute spindle will appear.

  • When the vortex is deviated laterally by an oblique surface a comma forms will carve a comma.

Play Video

If a vortex persists parallel to the rocky surface it can dig a long furrow called cavetto, a shape that evokes the concave molding called cavetto in architecture.

Small Cavetto Puddick island.JPG

At French River there are giant overhanging cavettos like this one, 2 meters high and 15 long:

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How can a stream of water trigger the appearance of vortices?


  1. The mere presence of an obstacle

  2. A rising ramp in the topography


When the current goes up a rising ramp, one or two cylindric rolls develop perpendicularly to the current lines. The rolls will curve and split in many vortices digging a hierarchy of sichelwannen (sickle cavities in German,).

The sichelwannen seem independent of any punctual obstacle. We know this from field observations like French River. In fact, the sichelwannen organize themselves arranged en echelon. This kind of area is described as a sort of hierarchy of s-forms. These s-forms fields can cover an area of several thousand square meters. They are they self-organized. We notice that the bedrock is quite homogeneous depleted of any punctual obstacles unlike Cantley in Quebec.

This animation takes the previous video (from time  2:14 to 2:42) to show the development of this process. A current rises a ramp and generates vortices. In hydrodynamics, it is a transverse. As these forms of erosions have never been filmed it is still difficult to know their development.

A hierarchy of s-forms was reproduced in the laboratory by pouring a slightly acidic water on a homogeneous base made of plaster of Paris. As this experiment was not filmed, the details of the development of the s-forms remain unknown. Geodoxa and its partners plan to reproduce the experience while filming the process with vortex staining.

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Experiment made on plaster of Paris eroded with slightly acidic water whose Reynolds number = 1.10 x 10^4. Muschelbruch: m; Sichelwannen: S; Ridge: r; Spindle flute: sp; Lateral furrow: lf.



S-forms are often limited by sharp edges. Only the vortices of a fluid (water or air ...) can carve this kind of sharp edges. The plastic deformation of the ice will sand the bedrock with rounded edges unless the rock breaks under the action of freezing and thawing as in the downstream slope of the roches moutonnées.

Studying the basement of flooded rivers, Richardson and Carling (2005) elucidated the following principle:

In all cases, sharp rims of furrows reflect simultaneous erosion of the surrounding bed by the primary flow and of the scours by secondary vortices.

1-Sichelwannen with sharp edges. Photo by John Shaw.

2- Mega obstacle mark at Cantley.

3- Obs = obstacle; the red dashes indicate the ridges sharpened by the vortices.

4- Snow dunes with sharp ridges.

5- Sand dunes with sharp ridges.



The obstacle marks types of s-forms are sculpted when a rapid flow of water encounters an obstacle. The created vortex will split in two arms digging furrows in front and beside the resistant obstacle. The horseshoe furrows reflect well the vortex shape generated during the erosion.


Cantley, Quebec, is a world-renowned site for its obstacle marks. A careful study of the Cantley outcrop reveal well the hydrodynamics origin of these types of s-forms.  Before reading ahead we invite you to watch this short video:

This video was presented at the CANQUA / AMQUA 2018 conference in Ottawa to introduce the fieldtrip to Cantley site. The fieldtrip was guided by David R. Sharpe of the Geological Survey of Canada and the video prepared by Geodoxa.

Cantley mable.JPG

In Cantley, the bedrock is composed of a marble spotted with a multitude of volcanic inclusions of all sizes ranging from millimeters to a few meters in diameter. These inclusions are composed of an intrusive volcanic rock much harder than the marble. During the subglacial flood these inclusions resisted better than marble.

These hard obstacles slowed the erosion of the marble on the downstream slope preserving a ridge.

These crests look like comet tails and have different names depending on their shape: rat tail, hairpin...

Lab obstacle forms.png

Rat tails were also reproduced in the laboratory on plaster of Paris encrusted with iron particles by the erosion of a slightly acidic water (Reynolds number = 1.10 x 10^4). Flow from right to left. Notice the furrows carved on the front of the obstacle.

In front of each obstacle will appear a front furrow carved by a jammed vortex. This front furrow is observed


• In laboratory experiments

• At all scale in Cantley

• Sculpted in sand and snow by the wind around an obstacle

• On the upstream front of the famous drumlins.


Under no circumstances ice abrasion would sculpt these front furrows.




Allen, J. R. L., 1982, Sedimentary structures: their character and physical basis, Volume 2, New York, Elsevier Scientific Publishing Co., p. 253-291.


Baker V. R. and Pickup Q., 1987, Flood geomorphology of the Katherine Gorge, Northern Territory, Australia: Geological Society or America Bulletin, v. 98, p. 635-646.


Bryant E., 2014, Signatures of Tsunami in the Coastal Landscape. In: Tsunami, pp 35-61, Springer, Cham


Kor, P. S. G., Shaw, J. and Sharpe, D. R. (1991). Erosion of bedrock by subglacial meltwater, Georgian Bay, Ontario: a regional view. Canadian Journal of Earth Sciences, 28, 623–642. Get pdf >>


Richardson, K., and Carling, P.A., 2005. A Typology of Sculpted Forms in Open Bedrock Channels. Special Paper 392. Geological Society of America, Boulder Colorado, 108 pp.


 Sharpe, D.R. and Shaw, J. (1989). Erosion of bedrock by subglacial meltwater, Cantley, Quebec. Geological Society of America Bulletin, 101, 1011–1020.Get pdf >>

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