The Image below is by Jordan Mallon showing a depiction of a sauropod dinosaur eating fronds and shoots. They also would have pulled out plants by their roots and with these would have been stones attached. It is often said that these stones were intentionally eaten by herbivore dinosaurs to aid their digestion. Gastroliths or stomach stones are often found near to a known dinosaur skeleton. When they are excavated they are slightly polished in nature (not from water or wind erosion) and smooth and this differentiates them from other stones that have not been in a dinosaur's stomach. Also the acid in the stomach would have had an effect and etched the surface of these stones.
Sauropod Dinosaur Gastroliths from the Morison Formation, Denver, Colorado, USA. These are 150 Million years old. Late Jurassic Age.
Jurassic Gastroliths are often referred to as Morrison stones because they are found in the Morrison Formation (named after the town of Morrison, west of Denver, Colorado), a late Jurassic formation roughly 150 million years old.