"they were called barrows"

Barrows are large mounds of earth and stone placed over a grave.  They were a common burial feature in Britain between 2900 and 800 BCE.  The dead laid to rest in barrows were usually buried with "grave goods", including treasure. 

The Sutton Hoo hoard is an example of such treasure: a collection of very well-preserved grave goods recovered from the barrow of a high-ranking man.

A barrow at Sutton Hoo
Creative Commons Attribution Share AlikeA barrow at Sutton Hoo - Credit: Geoff Dallimore
A reconstruction of a helmet buried with the dead at Sutton Hoo
Creative Commons Attribution Share AlikeA reconstruction of a helmet buried with the dead at Sutton Hoo - Credit: Gernot Keller