The river must have been meandering on a flat landscape and then begun cutting downward. Landscapes can be rejuvenated at any time. But the valley bottoms are so young that valley widening has scarcely occurred. The Delaware River cuts right through a ridge. This is the Delaware Water Gap.
New Jersey is on the left, Pennsylvania on the right.
Note how flat the ridge crest is. Another water gap seen from the ground. Note the very flat ridge crest, and that the distant ridge is about as high as the nearer one. The ridge crests are interpreted to mark the original land surface before the Appalachians were gently uplifted perhaps 10 million years ago. The ridge on the skyline is the Cumberland Mountains, an impassable barrier in colonial days and still formidable.