Chablis' vineyards lie within the north-east quarter of France, in northern Burgundy. They are centred on the small town that gives them their name, and extend around 19 neighbouring villages and hamlets within the Yonne departement. This is by far the most northerly outpost of Burgundy, and is actually closer to southern Champagne than to the Côte d'Or. Roughly-speaking it follows the line of a small river, the Serein, over a length of 20 kilometres. At its widest, it stretches for 15 kilometres. The climate here is semi-continental, with considerable temperature variations. Summers are hot, and winters long and rigorous. Both sunshine hours and rainfall fluctuate greatly from one year to the next, causing considerable quantity and quality variations. Spring frosts are a particular danger, and for many years posed a constant threat on the future crop. From March to May, the young buds and shoots are especially vulnerable, and the history of Chablis is lined with disastrous vintages. From the 1960's onwards, various techniques have been used to combat frost, both heating the air and spraying water on the vines. They are both tricky to use and expensive, but nevertheless efficient, stabilising crop levels within reason and protecting vinegrowers from potential disaster. In geological terms, Chablis lies along the southern extremity of the Paris basin, which stretches across northern France and the Channel to southern England, including the village of Kimmeridge in Dorset. These are sedimentary soils formed from an Upper Jurassic sea-bed about 180 million years ago. Lime-clay soils are dominant here, but the geological originality of Chablis resides in its sub-strata, often quite near the surface, of kimmeridgean limestone, formed by billions of tiny fossilised shells of exogyra virgula. This layer has a variable thickness of 50 to 100 metres, with alternating layers of clay, limestone and marl. The Grand Cru vineyards form a homogenous entity on slopes from which emerge outcrops of this specific type of limestone. Linked to optimum sunlight exposure, this is a key ingredient in the production of specific flavours in the chardonnay grape. Chablis' vineyards lie within the north-east quarter of France, in northern Burgundy. They are centred on the small town that gives them their name, and extend around 19 neighbouring villages and hamlets within the Yonne departement. This is by far the most northerly outpost of Burgundy, and is actually closer to southern Champagne than to the Côte d'Or. Roughly-speaking it follows the line of a small river, the Serein, over a length of 20 kilometres. At its widest, it stretches for 15 kilometres.