Vineyard descriptions often mention periods of geological time. A famous example is the controversy over whether or not true chablis can be produced from Portlandian as well as Kimmeridgian soils. These two terms refer to intervals of geological time rather than to the actual nature of the material, what geologists call its lithology. For viticulture, it is really the lithology and its properties that are relevant, not in which remote geological time the bedrock happened to form. However, the age of the overlying soil, typically hugely younger than its parent bedrock, is relevant in that it influences soil depth and soil fertility.