Limestone and Sandstone
Here is a picture recently taken at Fountains Abbey. The right side of the wall is made of Limestone and right hand side is made of Sandstone.
The relevance is, if water passes over the Limestone it will pick up calcium and magnesium, making the water hard. Water passing over the Sandstone is probably going to be soft, acidic and may turn water brown / black.
Not a lot of people know that :)
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