The Birkenstock Boston is a veritable classic that can easily be worn all year round. It has a legendary status thanks to its sophisticated. individually adjustable design. The sporty Adventure Crosstown version is designed for Birkenstock fans who are active and looking for an experience. The polyurethane sole is affixed to the footbed in two layers and therefore offers maximum flexibility. grip and durability. The upper is made from extra thick. oiled nubuck leather with an open-selvage finish.