Well, who knew...
Herdwick sheep are widely considered to be the most hardy of all Britain's breeds of hill sheep. Probably 99 per cent of Herdwick sheep are kept in commercial flocks in the central and western dales of the Lake District. These fells run to over three thousand feet and facing the westerly rain bearing winds they record the country's highest rainfall. Herdwicks have a well justified reputation for foraging abillity even in the most difficult terrain. Many of them live their lives without receiving any supplementary feed.
The name "Herdwick" is derived from the Old Norse herdvyck, meaning sheep pasture. Though low in lambing capacity and perceived wool quality when compared to more common commercial breeds such as Merino Sheep, Herdwicks are prized for their robust health, their ability to live solely on forage, and their tendency to be territorial and not to stray over the difficult upland terrain of the Lake District.
The wool quality of a Herdwick has unique qualities relating to durability. Thick bristle type fibres will often protrude from garments forming a protective barrier layer in blizzards. Most likely the same qualities that protect the sheep in similar conditions. They have been known to survive under a blanket of snow for three days while eating their own wool.
~from Herdwick Sheep Breeders Assc.
The Lake District