The Birkenstock Sierra Crosstown is the perfect sandal for all those who are on their feet all day and who expect a high level of comfort from their footwear. The sandal boasts a distinctive sole, which is reinforced in the heel area, and combined with understated straps. Sierra Sierra: a sporty two-strap model designed with all adventurous and active Birkenstock fans in mind. A high level of comfort for those on their feet all-day.