I just posted a query on the French site asking whether they had considered it could be a dragon's tooth anti-tank obstacle.
It may be down to distortion (or visual impairmanet on my part) but when I try to mentally fill in the bits to create a three-sided triangle the geometry looks a little irregular. Which got me thinking: if dragons teeth were being transported by sea and one was lost overboard, or jettisoned in bad weather, and it settled on the seabed sideways, corner down and buried halfway in sand and silt, then you’d end up with an irregular three sided pyramid with a pointed apex (because it’s not really a pointed apex, it’s a corner). Erm…I think that makes sense.
(I see Mikefule has already suggested a tipped object, but it would work for a four-sided pyramid, as well as a cube).
Although it’s not mentioned on the Wiki page I’m pretty sure that dragons teeth were also used as defence against shallow draft boats, landing craft etc. This one would clearly be too deep to be of any use, but it could explain why they may have been being seeded offshore.
And, once you are past the basic principle, there seems to be no hard and fast pattern to such a device – some are ramp shaped (much steeper on the threat side of the block than on the leeward), some cubed, and some are bullet shaped - like huge bollards; the very striking ones at
Cramond, on the Firth of Forth, are not much like the squat pyramids you see in most images (although, ironically, they look a lot more like I imagine dragons teeth to be).