Oh, I know what that 'brown' stuff is

Whenever I went to the Highlands and drank from the mountain burns, you could always tell you were in Scotland ......... most of the stones and other bric-a-brac in the streams were covered in this substance - it's called Scottish tarnish, and arises because the natives are too mean to buy, and use, a descaler

I've a feeling there's something similar in Welsh mountain streams - assume it happens due to the acid/peat soils, as opposed to the chalk/limestone in the south of the U.K., where our water is very hard in some places.