Blocs erratiques des Dévens © Marlène Piron

Mysterious monsters from elsewhere: The erratic blocks from the Dévens

These blocks have witnessed the birth of a new scientific theory and the birth of the last glaciation. The rock of the blocks is different than the rock found on the hills.

Mysterious monsters from elsewhere: The erratic blocks from the Dévens.

These blocks have witnessed the birth of a new scientific theory and the birth of the last glaciation.
The rock of the blocks is different than the rock found on the hills.

“Not far from my house in the Dévens, on the northern flank of a little gypsum mountain called Le Montet, lies a block whose limestone comes from the mountains bordering the Avançon valley (…) As the block has no particular name, when I have to mention it, I talk about the Monster Block”
… they could not have possibly rolled all the way down to here on their own… it is mystery!

The Pierra-Bessa, a 61-foot high pyramid not far from my house, is vertically split from the top to the bottom. It is evident that the crack occurred when the rock fell in its actual place. However, there are no rocks above from which it could have come loose.
An enigma to geologists
An erratic block is a geological intruder: it is not made out of the type of rock that is found in the surrounding area.
How did these blocks come along here? Jean de Charpentier investigated…
False assumptions
At the end of the 18th century, several explanations were brought about to explain the movement of erratic blocks. Jean de Charpentier set them against his own field observations and refuted them all. Another theory had to be found…
What if it were the glaciers that moved the blocks?
To begin with, it is only to contradict the hypothesis of his friend, Ignatz Venetz, that Jean de Charpentier studies erratic terrain.But, as he progresses in his investigation, he sees himself forced to change his mind: a long long time ago, gigantic glaciers transported blocks from the Alps to the Jura!
The first presentations of the glacier theory were largely condemned but – thanks to, among other things, the book written by Jean Charpentier in 1841 – the scientific community would eventually be persuaded.
Witness protection
Erratic block are exhibits of evidence that allowed the development of glacial theory. Henceforth, protecting erratic blocks became a necessity as they were, at the time, often chunked into pieces to free some space or to be used as building material.
And that is the way, the blocks of the Dévens, described by De Charpentier, have survived to this day. The Monster Block, so called by De Charpentier, still bears the name of this honest and perceptive scientist.
17 000 years ago: The Chablais was under ice. The Rhone Glacier stopped near Aigle. Icebergs, ahead of its snout, floated on an extended Lake Geneva. Coming down from mountains of limestone and carrying rock debris, the glacier of Avançons was blocked and forced into a different path by the Rhone Glacier.It followed north the hill of the Montet (where you are at the moment) and carved on its way the little glen of the Dévens. When it melted, it left here many blocks (two of them were our two giant blocks).
The adventures of Monster Block and Pierra-Bessa.
1. Ice fills the valley in. Monster Block falls from the mountain onto the glacier and follows his friends.
2. Ice brings our heroes to the plain next to the Dévens.
3. The Glacier does not like hot weather: It returns to its mountains.
4. Monster Block and Pierra-Bessa settle at the foot of the hill, very near here.
Go and find them!

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