Keriaval at Carnac is an illustration of a sophisticated dolmenic tomb with a complex funerary area.

There is a general consensus that the first Breton passage tombs were single-chamber tombs with round or polygon-shaped chambers and a relatively simple architecture. 
These were soon extended. Square-shaped chambers appeared and, in the largest groups, monuments with a particular status were erected, differing from the others by the presence of a very long passage and a very high 
The need for greater diversification and specialisation of the funerary space led to the creation of new subdivided-chamber tombs.

The end of the 5th millennium BC and the beginning of the 4th marked the peak of this period.

During the 4th millennium, an opposite trend towards "de-differenciation" began, with the distinction between the chamber and the passage becoming less clear and the need for monumental architecture fading as the funerary surface increased.

At the end of this millennium, chambers began to be extended, resulting in the creation of the small specific group of "L-shaped" or "elbow" tombs on the Morbihan coast.

In other parts of Brittany, the length of the passage and the size of the monument decreased to create the 3rd millennium gallery-type tombs ("V-shaped" tombs, lateral entrance tombs, "gallery graves", reflect the influence of the Parisian basin and even of the Netherlands).

In Western Brittany, the use of megalithic tombs seems to have come to an end rather suddenly when the first Bronze Age settlers arrived in the area, around 2200 - 2000 BC, and developed the "Tumulus armoricains" culture.

Evolution of neolithic monumental funerary architecture, in western France

Today, there are two interpretative models for "sealed" tombs:
  • a "linear" model (suggested by C. Boujot and S. Cassen): 
This model sees the origins of Breton funerary megalithism in modest buried cist graves with no monumental superstructure, like those dating back to Late Mesolithic. Some examples can be seen at Teviec. 
With the "Neolithisation" process, these tombs were rapidly given more elaborate structures and large superstructures. 
The elongated "tumulary mound" is thought to have been dominant (on the basis of monuments like the one located at Erdeven, which has been recently studied). Some of the monuments, like the carnacean barrows, were huge. 
At the end of the 5th millennium, however, the need to stay in contact with the dead would have led to the creation of passage chamber graves, which may have soon become communal graves.
Eventually, the evolution of society must have reduced the importance of the monument in favour of the funeral rite. Thus, the later gallery-type tombs were often smaller than the ones built at an earlier period.
  • a "complementary" model
Although it has been levelled, the structure of this small dolmen, with its chamber and passage, is clearly visible
"Open" and "closed" tombs reflect almost opposing visions of death. It is doubtful whether one kind of tomb derived from the other, but quite possible that these tombs were in fact complementary.
Cist-tombs had undoubtedly been in existence in the area for a long time, especially in Morbihan. However, as tumulary mounds began to develop, new concepts emerged, leading to the creation of the first passage tombs. This may have occurred in the middle of the 5th millennium in areas with no cist-tomb tradition, as in northern Finistère.

Although it has been levelled, the structure of this small dolmen, with its chamber and passage, is clearly visible.