Ochre mining - in galleries, or in open-cast quarries?

Ochre has been mined both in open-cast quarries, and in galleries.
This depends on the quantity of earth and rock, called the overburden, covering up the section to be mined.
This also depends on the fact that some ochrous layers are much richer than others, and are therefore more profitable to mine, but they do not necessarily lie on the surface, which is why they are sometimes mined in galleries!