According to the Peterson Field Guide to Geology of Eastern North America
The Palisades are eroded cross section of a diabase sill in the Triassic Basin that dips gradually northwestward
The sill is 1,000 feet thick (Unusually thick for such an intrusion).
Dated 192 million to 189 million years ago, which puts it about on the border between Early and Middle Jurassic in age.
