Genesis of a giant erratic

Scrambling about on the west side of Rødløga Storskar (in the Stockholm Archipelago) the other day I realised that the prominent rocky knoll I was climbing on, about 25m above the sea, was falling to bits. I have always recognized it as a well-developed roche moutonnee with a smoothed and moulded up-glacier side and a plucked and fractured lee side, facing south. The rocks here are Pre-Cambrian basement rocks, comprising pink and red granite and a wide range of metamorphosed igneous and sedimentary rocks which probably have some very complicated geological names.

The western flank of the roche moutonnee is breaking up, although many of the broken blocks on the cliffs are well smoothed by over-riding ice. So these blocks (some of them as big as caravans) have been entrained, moved and smoothed.  Most of them have not moved very far.

But then at the position shown in the satellite image above I realised that a deep crack about 30m long and at least 20m deep was the first sign of a massive shearing-off of rock on the south-western corner of the hillock.  In other words, the crack indicated the genesis of a giant erratic, since the block of granite to the south of it must have been bodily dragged away (by overriding ice) from a prominent fracture plane.  The gap is now in places up to 50 cms wide.  I calculate the size of the moved block to be about 40m x 30m x 30m -- which gives us a volume of c 36,000 cubic metres and a weight of around 100,000 tonnes.

That would have been rather a hefty erratic, had the Devensian Glaciation continued for longer.  It would have been moved southwards and no doubt broken up further, into lots of smaller erratics.  The large block is very heavily fractured and is petrologically varied too, so it could not have survived transport as a single super-erratic.  The photos below show the crack as it appears on the upper surface of this little promontory, and the broken or plucked southern face.  The bouldery beach is packed with erratics from near and far.