French geological maps have a curiosity compared to those in other parts of the world. They include the surficial geology, rather than just the bedrock. This surficial geology includes river gravels, glacial deposits and the like. All things which aren’t strictly rock, but could be one day. The reason for this is that these surficial sediments control the type of soil that lie above them, like most rock types do. Therefore a French geological map is a far better indicator of soil type (although interpreting structural features is far harder).
Why do the French do this? Because the soil type is of utmost importance to the French, for it is the crucial “la terroir”. And it has a great effect on the wines that are produced above, having large influences on flavour. After climbing Dun Mountain, and discovering lots of hard rock geology, we took a rest day and decided to take a close look at some surficial sediments and their influence. All in the name of science, of course.
We started the day with a trip to the Marlborough sounds. A network of drowned river valleys that owe a lot to a fortunate combination of a network of hills being at the right height that when sea-levels rose at the end of the last glacial period, a series of sounds and inlets were formed. This created a network of twisty peninsulas and islands that stretch over 50km. They were stunning, great views.
We then investigated the surficial sediments of the local beaches. We found them to be micaceaous schists, and so we decided to stay there for a while. Occasionally observing the rocks from the water, and sometimes, just spreading a towel over them, to get a real sense of what they feel like living down.
In the evening, we celebrated the field trip that was coming to a close, just one day left. We did so with dinner and a sampling of a few bottles of the local wines. As you might expect from the Marlborough region, the Sauvignon Blanc was excellent, and other wines merely very good. Good wine, good food and good company. Thank goodness for the terroir eh!