Figure 1. Same area as Figure 3 in the former post. One of the fan-like hollows (in Finnish: viuhkamurros) and two of the ice-flow directions are marked. Manamansalo, Kaivanto. (Map sheet Q5222D, © National Land Survey of Finland)
Recently, some Finnish (Mäkinen et al. 2017) and Swedish researchers (Peterson et al. 2017, 2018) have discovered a previously unknown glacial landform called Murtoo after a Finnish map name in Nokia (NGWM 2018). The type locality is obviously the surroundings of the lake of Kynäsjärvi in southwestern Finland. I have presented before the practically same terrain, too (Seppälä 2016a, figures 4-6, 2016b. cf. Figures 1 and 2).
The few readers of this site know that triangular morainic landforms are, strictly speaking, not "previously unknown to science," but I have presented them earlier in the blog (Seppälä 2015), in Geologi magazine (Seppälä 2016a) and in a scientific publication (2016b). However, I have dealt them in a broader context and with a different genetic interpretation (deformation or glaciotectonics): the so-called subglacial glaciotectonic hypothesis (SGH).
According to SGH, these would be mainly erosional forms, ie in the case of triangular ridges, this would be the result of the erosion of ridge spacings directly as a result of glacial activity. It may have preceded and followed also the subglacial meltwater activity, but in essence, it would have been, in my early interpretation, subglacial block-erosion. Glaciotectonic activity may have been not only plucking but also softer ductile deformation, less deforming or reforming, and even possible squeezing. These are alternative glaciotectonic processes for subglacial meltwater erosion and deposition.
Thus, I have formed a short description of the classification of the related glaciotectonic or deformational formations: Five types of mostly erosional glaciotectonic landforms were recognized: (1) plucked lee side, (2) hill-hole pair, (3) fanlike and transverse edges of plucked depressions, (4) sporadic plucked hollows, and (5) deformational or glaciotectonic mound or ridge." The Murtoo topography is essentially same as the class 3: fanlike and transverse edges of plucked depressions or diamond-shaped morainic topography or fan-like hollows (Seppälä 2016a, figures 4-6, Seppälä 2016b pp. 2-7).
Seppälä (2016b) described diamond-shaped morainic topography like this:
"3.3. Fan-Like and Transverse Edges of Plucked Depressions.
In northeast Manamansalo occurs one characteristic type probably plucked shape, which can be called a fan-like hollow
of glacial raft or fan-like edges and steps of plucked hollow
in the till bed (Figures 8–11). It has a punctuate proximal
ending from where the sides of the fragment open with a
fan-like manner in the distal direction. This is obviously a
predominantly brittle fracture of the frozen moraine block.
The opening angle is about 35 degrees, and the directions
of the fans vary, 120–150 degrees, which are more northern
than the main direction of the striations in the Oulujarvi ¨
region. The opening angle can still get a lot broader, too.
Sometimes both sides of the hollow can be seen; sometimes
only other side is in sight or formed. There are also variants
with wider round proximal endings. Dimensions can vary
significantly. The shapes can also be stacked and overlapped.
Representative quarries’ heads have a diameter of about 30 m
and an edge length of about 500 m. Often the detectable
maximum length and width are in the order of 50–200 m.The
depth of the shape is often 1–3 m and can be even 5 m at the
proximal end. The sides of the fan-like hollows are slightly
curved but roughly quite straight lines. Their steepness varies
slightly and in some places, they tend to rise to the low ridge.
In Figures 10 and 11 can be seen photos of local terrain and till
exposures in Kaivanto.
The edges of fan-like hollows are as a general rule oblique
in relation to the flow direction in which they are formed.
Plucked edges of fragments can also be more straight and
then often transverse in relation to the flow direction. Tightly
spaced fan-like fragment lines produce diamond patterned
moraine surfaces (Figure 12). In some places, faults produce
rectangular grid patterns. It can happen because transverse
faults are interleaved with fan-like faults. The faults in the
bedrock can also be evolved and confused with morainic
features." (cf. Figure 1).
While it is possible that my genetic interpretation is worse than the more meltwater-based, I dare to suppose that I still was the first one to describe and categorize these particular and related glacial landforms from DEMs.
Figure 2. Same as Figure 3 here.
This northwestern area of Sääksjärvi is partly the same as in the picture: Mäkinen et al. (2017): Fig 8. The area also shows the head and edges of the fan crags, open to the southeast, which is impressive and original (some are marked). However, their terraform formations or "triangular-shaped landforms" do not appear to be the result of glaciofluvial erosion, but rather of glaciotectonic plucking, which later may have been expanded and clarified by glaciofluvial erosion. (altitude model: © National Land Survey of Finland, open material 5/2017, TM35 map sheets M3413D, F)
References
Mäkinen J., K. Kajuutti, J.-P. Palmu, A. Ojala & E. Ahokangas 2017. Triangular-shaped Landforms Reveal Subglacial Drainage Routes in SW Finland. Quaternary Science Reviews 164, 37-53.
Peterson, G., Johnson, M.D. & Smith, C.A. 2017. Glacial geomorphology of the south Swedish uplands – focus on the spatial distribution of hummock tracts. Journal of Maps 13:2, 534-544.
Peterson, G., and Johnson, M. D. 2018. Hummock corridors in the south‐central sector of the Fennoscandian ice sheet, morphometry and pattern. Earth Surf. Process. Landforms, 43: 919–929. doi: 10.1002/esp.4294.
Seppälä. Matti V. J. 2015. Glasiotektonisista piirteistä ja pinnanmuodoista moreenimuodostumissa. (26.2. 2015)[http://glasialgeo.blogspot.fi/2015/02/glasiotektonisista-pinnanmuodoista.html]
Seppälä. Matti V. J. 2016a. Suomen moreenimuodostumat ja sekundaarinen deformaatio. Geologi 68 (3/2016): 98-106.
Seppälä, Matti V. J. 2016b. Lidar-Based Detection and Interpretation of Glaciotectonic Features of the Morainic Topography of Finland. Journal of Geological Research, vol. 2016, Article ID 4292806, 11 pages. doi:10.1155/2016/4292806
Seppälä, Matti V. J. 2017. Ensimmäinen sitaatti! Voihan hypoteesi. Alustavia huomioita ja selittelyjä.
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