The tumulus forms what is almost an artificial hillock of more than 30,000 m3 (125 m long, 60 m wide and 10m high). It stands on high ground and a panoramic view can be seen from the top. The place was named after the archangel Michael long ago, probably in the Middle Ages. The monument located to the south below the tumulus and studied by J. Miln dates from the same period. The existing chapel is the latest in a long line of successive places of worship, it was restored in 1927.
The tumulus belongs to the "Carnacean barrows", a small group of monuments linked to the more general giant barrows group.
Inner Structure of the Saint-Michel Tumulus
Longitudinal section of the Saint-Michel Tumulus drawn by R. Galles, showing the structure then discovered and the location of the well dug to reach the central tomb.
The first excavations of the Saint-Michel tumulus were carried out by Galles and Lefebvre from 1862 to 1864. They started to bore a gallery from the west side and dug vertical pits from the top. One of the wells ended up in a burial-chamber filled with beautiful artefacts, in the centre of the monument.
The next excavations were led by Le Rouzic from 1900 to 1907. A series of mine galleries led to several burial-chambers surrounding the main tomb as well as a passage chamber close to the eastern end.
These galleries were consolidated to allow tourist visits but had to be closed down a few years ago for safety reasons. Given the sheer size of the monument, observations are still somewhat incomplete, hence a rather hypothetical internal structure:
- the different burial-chambers seem to be built into a long, narrow central rocky core,
- a thick layer of "mud" (more precisely, greyish silt collected in a neighbouring wetland area) covers the entire structure, including the eastern "dolmen",
- a superficial rocky hardpan completes the structure and protected it efficiently against erosion. The present terrace appears to have been added and is not simply the result of the top having been levelled.
The central burial-chamber is a typical "Carnacean barrow" crypt, with rough "cyclopean" dry-stone walling and a roof made of one capstone. It was placed directly on the ground and measures 2.4m x 1.4m, giving an inner headroom of 0.9m.
The artefacts found included 11 large polished stone axe heads (19 to 37cm in length and a smallish one of 97mm made of pyroxenite), 25 fibrolite axes, 97 disc beads, 10 variscite pear-shaped pendants and about forty disc bone beads.
Human and animal bone remains lay in different burial-chambers and ceramic sherds from the Middle Neolithic were found by the eastern "dolmen".
The age of the monument, and the chronology of the construction of the central burial-chambers and outlying dolmen have been the subject of much speculation. Ancient samples were radiocarbon dated, but the results were too disparate to be significant. Recent excavations (Locmariaquer, Erdeven) point to this large tumulus being constructed in several stages but in a rather short lapse of time, around the middle of the 5th millennium B.C..
The association of sealed burial-chambers with a passage grave could be functional or the two may simply have been built at different times.
The excavations conducted by Z. Le Rouzic in the entrance to the monument. The left-hand gallery reaches the dolmen, the right-hand gallery reaches the central burial-chamber.
Map and axial section of the central burial-chamber drawn by R. Galles showing the rough walling.