A 2.7 mm to 2.9 mm tanned leather. The extra thickness and durability of this leather is perfectly suited to stronger hill and mountain walking boots, where extra support and resistance to scuffing is important. Sherpa leather is treated with the HS12 tanning process which is durable and resistant to scuffing and stops the leather from wetting out so easily. We recommend the use from new of Scarpa's HS12 cream.