Columbia subject headings:
Harvard subject headings:
Delaware River Valley (N.Y.-Del. and N.J.)--Guidebooks.
Natural history--Delaware River Valley (N.Y.-Del. and N.J.)--Guidebooks.
Natural history--Pennsylvania--Guidebooks.
Princeton subject headings:
Delaware River Valley (N.Y.-Del. and N.J.)--Guidebooks.
Natural history--Delaware River Valley (N.Y.-Del. and N.J.)--Guidebooks.
Natural history--Pennsylvania--Guidebooks.