The Birkenstock Sierra is the right sandal for anyone who is on their feet all day and expects a high level of comfort from their shoes. The Adventure Crosstown variant is sporty for all adventure-oriented and active Birkenstock fans. The polyurethane sole is foamed in two layers on the footbed and is therefore very flexible, has a good grip and is particularly durable. The upper is made of particularly thick, oiled nubuck leather with an open-edged finish.