A trip around the three Lakes

This peculiar peat bog environment can be seen from the path to the Petit Maclu. The few trees and bushes able to grow in these difficult conditions (waterlogged soil, lack of oxygen, etc.) are birches, willows and alder buckthorn. In the spring, the white cotton grass bobbles - a plant typically found in these environments - can be seen above the grass. The lake used to be bigger. Little by little, aquatic plants have grown, and the permanently humid conditions have not allowed dead vegetation to decompose. By building up, all of these plants have formed what is called peat. In this peat bog, the peat isn’t very thick, but in the peat bog between the Grand Maclu lake and the Ilay lake, it has reached over 1.5m.

These diverse wetlands attract many species of insects, including dragonflies. Over half of the species in mainland France are located around ponds, lakes, rivulets and smaller pools on the site. The four-spotted chaser is easily recognisable with its very visible dots on the middle of each of its four wings. Adults of this large dragonfly species can be seen from May to September. Its wing can measure up to 4 cm long!  

Widely present in the marsh area between the Petit Maclu and the Grand Maclu, the asp viper is a poor swimmer and seeks sunny areas to ensure its metabolism can function properly. Like many reptiles, it is fearful and has a talent for detecting vibrations. To avoid bothering this species and being bitten, stamp your feet and be careful not to squash it, it will hide well before you even see it!

The waters of the Petit Maclu and the Grand Maclu feed the lake of Ilay. Each occupies a shallow basin dug out by the glaciers around 20,000 years ago. These lakes are mainly surrounded by a wide diversity of vegetation, called low-marshes, wet tall-herb and fern fringes or peat bogs. Some reed beds also develop there. Insects flourish here due to the high quality of the environment, in particular dragonflies who vary in colour. The Eurasian coot is the most faithful bird to these lakes. Not very discreet, it is recognisable by its trumpeting cries of warning.

The Franche-Comté conservatory for natural areas manages the wetlands on the 4 lakes site in partnership with the owning municipalities and local farmers. This site is fill of species specific to open environments which must be preserved. Once used by farmers, these zones have gradually been abandoned. This is why managers have made maintaining this type of environment a priority by reinstating extensive grazing. Highland Cattle cows are able to move around on waterlogged soil and find the food that they need. The amount of cattle and grazing periods are adapted depending on the species and environments in question. A site sheet dedicated to the wetlands on the 4 lakes site is available online: cen-franchecomte.org/document.php?iddoc=63dyrcgmw7yq42pg

Centred between the banks of the lake of Ilay is an island: the island of La Motte. Archaeological excavations have revealed that is was home to three successive religious establishments. An initial church was erected around the year 1,000 and seems to have burned down. After being rebuilt, a vestibule was added to it by a monastic congregation that ended up abandoning the place. Of the third establishment, we know only that it was abandoned during the 15th century.  A wooden bridge provided access to the structure from the bank and the marshes that are now present were once orchards and pastures.

Near lakes, there are not only humid areas! Where limestone is predominant, water immediately filters through, making life difficult for plants. The soil is not very deep, and is dry and nutrient-poor... These are dry grasslands. Vegetation is neither very tall nor very dense. The species living in these grasslands adopt special strategies to survive in these difficult living conditions: greasy leaves, parasitism, strong odours, etc. For example, Breckland thyme avoids “sweating” by synthesising essential oils. Its smell is easily recognisable, and this variety of wild thyme is excellent when used to clear airways.

Among the various environments that make up humid areas, reed beds play an essential role: they stabilise the bank and create an area where the plants present filter the water. It is a safe place and a reproductive area for many species, including birds, such as the Eurasian coot. Throughout the year, their short and powerful cries can be heard here.

The Petit Maclu and the Ilay lake provide 10 municipalities with drinking water, including Frasnois. 67,000 cubic meters are used up every year. The lakes on the Frasnois plateau are natural. The icebergs that covered the Jura 20,000 years ago have “planed” the earth and created the necessary depressions for the lakes to form. The limestone and moraine deposits from the movement of icebergs are watertight and are able to retain the water. To best preserve the quality of the water, a prefectural order bans the use of thermal engines.

The Jura massif provides a long geological history. The limestone rocks formed during the Mesozoic era, between -170 million years and -90 million years. Much later, the pushing of the Alpes led these rocks to fold, between -11 million years and -3 million years. Lastly, the erosion caused by the icebergs, the freezing, the water and the wind, have progressively shaped the landscape. And Man has made the final contribution through farming and development.