(c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Boudinage ("boo-dee-NODGE") is a process that happens to rock layers when tectonic forces deform them. Layers of harder rock, when stretched by folding or squeezing, tend to break in chunks while the softer rocks around them tend to flow. For example, a sheet of quartzite inside a body of schist will be stretched and break into ribbons. If the rock is then eroded in a cross-section of the ribbons, the quartzite will appear in the outcrop as sausage-shaped bodies. This process was first described in Belgium, where the Belgian French word for a sausage is boudin. Hence this process of "sausageization" is called boudinage.
Boudinage has affected this schist in New York City's Riverside Park, pulling apart the thicker quartz layer in the middle into boudins. Boudins may take various shapes, each of which is a clue to the exact forces involved in folding. The prominent veins were emplaced in a later episode of deformation, then folded themselves still later. Features like this really put a premium on the geologist's ability to visualize in three dimensions.