h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

According to the OED, the collective noun for mountains is a "gullible".

We'll believe you - thousands wouldn't!

Many of those duns, while Celtic, mean brownish grey as in donkey. My elder son is named Duncan which means brown warrior.His second name is Edward, meaning Spearcarrier. He was named after one of England's best ever footballers, the Busby Babe Duncan Edwards who died as a result of the Munich air crash.

The "brown" meaning seems more likely for names of people, than geographical features or places. There are more hills and heights than areas which are intrinsically browny-grey, but there are a lot of people with browny-grey hair.

At the end of the day, though, etymology's usually about deduction and inference. There's always a bit of guesswork involved - because we have lots of bits of evidence, but not a whisp of proof.