Complex sentence: At Saint Nicholas Place, the terrain drops off steeply, and 155th Street is carried on a viaduct that slopes down towards the Harlem River, continuing onto the Macombs Dam Bridge, crossing over (but not intersecting with) the Harlem River Drive.
Split into simpler sentences: At Saint Nicholas Place, the terrain drops off steeply, and 155th Street is carried on a viaduct that slopes down towards the Harlem River. The street continues onto the Macombs Dam Bridge, crossing over (but not intersecting with) the Harlem River Drive.