The Ornithischian Paddock existed as one of six separate paddocks on Isla Nublar. This paddock housed the Hypsilophodon and Othnelia in Jurassic Park, and consisted of a grassy plains. This Paddock was located between the Visitor’s Center and the Dilophosaurus Paddock, and the jungle river ran through it as well. The Jungle river led into the waterfall, which had a recess behind it which led into a tunnel, meaning that worker access to the paddock would have been possible without having to physically enter it from the surface. It is known that a Juvenile Velociraptor and an adult Tyrannosaurus Rex managed to breach the paddock at separate points in time.
Jurassic Park by Michael Crichton page 141 (First Edition, Thirty Fourth printing)