They were carved from the tough granites and gneisses of the Armorican Massif, the roots of once tall mountains that have been remobilised (ie grew and were eroded again) a full three times. The first was the Cadomian half a billion years ago followed by a complex series of tectonic events as the supercontinent Pangaea assembled during the Devonian and Carboniferous between 400 and 280 million years ago. Armorica started off life as the northern shore of Gondwana, before rifting off and colliding with Laurussia during the Hercynian event. The assembly of Pangaea when Euramerica collided with Gondwana in the Variscan orogeny completes this continental dance of mountain creation.
Each event was accompanied by extensive metamorphism, melting of both sedimentary and volcanic rocks into granite as they were buried under the pressure and heat of rising mountains and subduction related volcanism as ancient oceans closed. Since then of course Pangaea has split up, and the once proud peaks have been ground down by erosion, so that the tallest rise in the region is a mere 480 metros.
Legend states that they are a Roman legion, transformed into stone by the wizard Merlin, though they date from the Neolithic period around 4,500-2,000 BCE, long before the Celtic peoples moved west from their probable heartland in the Danube. Dating their erection is difficult, since little organic matter was buried under them for C14 data but the peak is thought to have been around 3,300 BCE. They are the largest assemblage of menhirs in the world, spread out over a wide area.
There are three main alignments related to the solstice sunrise and sunsets at both ends of the year, known as Menec, Kermario and Kerlescan in the local Breton language, though they may have formed as single grouping as stones have been removed over the millennia for building and other purposes. All of them have taller stones to the west (the traditional direction of death) and shorter ones to the east (that of birth).
Previous posts in this series:
The geology of newgrange: http://tinyurl.com/qhj3e4v
The geology of Stonehenge 1&2:http://tinyurl.com/kec8tu3 andhttp://tinyurl.com/nbaq8v5
Image credit: Jim Brandenburg via wallpaper downloader