For most of the Late Cretaceous, North America was two continents. A shallow sea, the Western Interior Seaway, flooded the middle of the landmass and split it into Laramidia in the west and Appalachia in the east. Nearly every dinosaur you can name came from Laramidia, because Appalachia's record is poor by comparison. New Jersey is where that gap gets filled.
The rock in this creek is the Navesink Formation, a greensand of glauconitic marl and clay laid down 66 to 70 million years ago. Chase Brownstein's 2018 survey of Appalachian dinosaurs identifies it as preserving the best known non-avian dinosaur fauna from the Maastrichtian of Appalachia. That fauna includes an ornithomimosaur, two possibly distinct tyrannosauroids, a nodosaurid, and hadrosaurs including Hadrosaurus foulkii, estimated around ten metres long.
The eastern continent ran differently from the west. Tyrannosauroids were the dominant large predators on Appalachia, as they were in Laramidia, but the supporting cast diverged: no ceratopsids, no ankylosaurids, no therizinosaurids or troodontids, and instead an assemblage of basal hadrosaurids, leptoceratopsians and intermediate-grade tyrannosauroids. Some of that fauna appears to have arrived from Europe rather than from the west.
Be realistic about what you will actually pick up. Dinosaur bone from these beds is museum-grade rarity. What the creek reliably gives is shark teeth, belemnites and ammonites, weathering out of the marl and collecting in the gravel where the current drops them, and that is a genuinely good afternoon for anyone within reach of central New Jersey.
The Navesink runs from Sandy Hook down to Pennsville and sits between 45 and 65 feet thick across that range, above the Mount Laurel Formation with a five-million-year gap between them. Big Brook is one exposure; Poricy Brook at Poricy Park in Middletown is another. Check the posted rules at whichever you visit, since the tool and digging restrictions are set locally.
Hand tools and screens only, no shovels into the bank, and the township posts the current rules at the lot.
Shark teeth, belemnites, ammonites · Late Cretaceous, ~75 mya
Open to the public for collecting, and what you find is yours to keep. Check the posted rules before you start: they can change with the season and the landowner.
Other places on the map within a couple of hours.
