INTRODUCTION
The studied area, a part of Veryarc'h Beach (illustrated in Figure 1) is composed of ordovician formations. The area is divided into two parts; the first is the Schists of Postolennec, of middle ordovician age, and the second formatin is the Sandstones of Kermeur, of upper ordovician age. Figure 2 illustrates the cross section of the area.

I. POSTOLENNEC
Lithology and stratigraphy
The shales of Postolennec are composed of detrital terrigenous sediments and clay. The color of the deposits is dark due to the organic matter content. In these, we find many fragmented fossils, like trilobites and crinoids, along with laminations due to tectonics. Between important shale beds, there are lower layers of broken shells. This thin bed is composed of many fragmented shells called coquinas storm bed.

Interpretation
After analyzing the shales, it can be concluded that the clay and terrigenous sediments are deposited on the continental shelf with a low energy. However, during high energy events, such as storms, many shells in life postition are dispersed in many small heaps. After the storm, the shells are broken and dispersed regularly on this surface. That explains the thin beds which we see.

II. KERMEUR
Lithology and stratigraphie
The Sandstones of Kermeur are divided into two parts, the lower and the middle part. The stratum of the lower Kermeur is heterogenous and made of alternatively different beds. Here we notice two major beds. The formation is composed of thin sandstone beds and thicker, reworked beds and the lithology is not the same. The sandstone layers are beige and medium- to coarse-grained while the intermediate layers are grey and fine-grained. These beds contain complete fossils in life position and therefore have also been affected by bioturbidity. The tubes made by organisms are perpendicular as well as horizontal to the deposited beds. Figures 3 and 4 are illustrations of such bioturbidity.

The middle Kermeur formation is formed of thick, beige sandstone beds which are composed of coarse-grained sand. This formation has homogeneous stratum and is topped with a succession of thin sandstone beds, which are laminated and of the same lithology. There are also bioturbidity systems (Figure 5) and ripple marks (Figure 6, 7).

After analyzing the Kermeur formation, one can make a few conclusions. For the lower formation, it is evident that the deposition occurred under a low energy system. We propose the idea that the sediments were deposited in a lagoon or a zone which was protected by the marine environment as well as storms. The surface, however, has been reworked and marked by alterations of beige and grey sediments. This began the regressive phase.

The middle Kermeur formation contains sediments which were deposited during a transgressive phase. The ripple marks and the bioturbidity support this theory. Also, the sediments are laminated and deposited toward the coastline due to gravity and the sediment density.

III. THE BOUNDARY
The boundary between these two formations is rather sharp. On the Postolonnec side of the boundary, there are a few laminated layers of pebbles in a matrix. After extracting some samples and running tests, local universities have agreed that the pebbles are eroded pieces of the older Postolonnec formation that were later redeposited in the younger matrix near the Postolonnec/Kermeur boundary. Also, there are tiny fragments of silica throughout the laminated layers approaching the boundary. On the Kermeur side of the boundary, the sandstone layers appear instantly.
Analyzing the boundary, one may wonder if the pebbles in matrix are a local event. After some research, it is evident that this late Postolonnec erosion is not a local event, but has been seen in many other countries, such as Portugal and Morocco. This indicates that there was a large-scale regression. The observations made of the Kermeur formation indicate a transgression, which has also been seen in many countries.
CONCLUSION
The older Postolonnec formation of Veryarc'h Beach is composed of dark shale and coquinas storm beds and was the continental shelf during its deposition. At the boundary of the Postolonnec and the Kermeur formations, a large-scale regression, and therefore erosion, occurred. The lower Kermeur formation is composed of alternating sandstones and a mix of clay and sandstone. This is due to the depositional environment, the lagoon. And lastly, the middle Kermeur formation is composed of thin, laminated sandstones, due to the transgression.