The terroir of Margaux is perhaps the most diverse of any of the major Bordeaux appellations due to its size. Because of its size, the terroir naturally varies from chateau to chateau. Like every appellaiton, the best wines are most often made from the best terroir, which in Margaux, is the terroir with the deep gravel deposits on the elevation here Chateau Margaux has the best part of its vines planted. In the Margaux appellation, the terroir features soils with large deposits of gravel, limestone with pebbles, stones, rocks, white gravel and only a minor amount of clay in the vineyards. The gravel is paramount to helping determine the quality of the wines in Margaux. The gravel helps to provide natural drainage, reflect sunlight, retain heat and force the vines to dig deep into the soils. Of course there are exceptions to every rule and some estates do have more clay content in their terroir than others in the Margaux appellation. Those chateaux with heavier clay deposits plant more Merlot in their vineyards. Because the soils in the Margaux region are often thin and cannot easily hold water, the vines are forced to dig deep into the soil for nourishment.
The comparative lack of depth in the terroir of Margaux allows the soils to heat up faster. In turn, this transfer of heat from the soils to the vines helps the grapes mature before the other Bordeaux appellations in the Left Bank. Those thin soils are also why most of the estates in the appellation do not do so well in the very dry, drought vintages. That is because the soils do not always retain as much water. The specific terroir is what gives Margaux wines their elegance and intense, beautiful, floral character, which is often compared to violets. To get an idea on how the soil and terroir of Margaux compares to the other important appellations in Bordeaux; The terroir and soil of BordeauxClick here for more info
With thanks to Jeff Leve from http://www.thewinecellarinsider.com, for the invaluable information