The Land Before Time VI: The Secret of Saurus Rock

Littlefoot's grandfather one night tells the protagonist children a story about "The Lone Dinosaur", a legendary longneck who once protected the Great Valley from the most ferocious sharptooth ever to live. A fight ensued, which led to the Sharptooth's death. However, the sharptooth left "The Lone Dinosaur" with a scar slashed across his right eye. Soon after the battle, a huge monolith that resembled a proud sauropod, having life-sized tyrannosaur teeth arranged around his neck, came out of the ground during an earthquake. The dinosaurs called it "Saurus Rock". The legend also states that if anyone damages the monolith, bad luck would descend upon the valley.A few days later when the kids are playing, Littlefoot accidentally falls off a cliff. Just before he hits the ground, a strange longneck dinosaur (a Diplodocus) rescues him. This longneck introduces himself only as "Doc" and gives no knowledge of his history. Littlefoot is intrigued by this newcomer, who is scarred across one eye and displays prior knowledge of the Great Valley's topography.For the preceding reasons, Littlefoot assumes that Doc is the Lone Dinosaur. He tells his friends this, narrating an apparently extemporaneous legend to support his assumption. Inspired, Cera's infant niece and nephew, the twins Dinah and Dana, go to Saurus Rock without anyone noticing. Later when the friends are playing, they notice that Dinah and Dana are missing. Recalling their talk of the day before, they go to Saurus Rock to find them.When they finally reach Saurus Rock, they see Dinah and Dana on the top. As they climb up to rescue them, Dinah and Dana fall off the top and land on Cera, causing the life-sized stone tooth on which she is standing to break off.As they walk home, an Allosaurus chases them. As they are pursuing them, they cross a gorge via a suspended log. As the Allosaurus follows them, one of them breaks the log and falls into the ravine below.When they get home, Cera is confronted by her father, who scolds her for losing the twins. Over the next few days, ill fortunes ranging from injuries to a tornado plague the valley. The adults blame Doc, in whose wake the misfortunes have apparently come, while Littlefoot blames himself and his friends, recalling the breaking of Saurus Rock.Eager to prove himself a solitary hero, as great as the Lone Dinosaur (whom he idolizes), Littlefoot attempts alone to take one of the Allosaur's teeth to replace the broken stone. He discovers, as he attempts to extract the tooth, that the Allosaurus is still alive.Littlefoot flees. Just as the Allosaurus is near to him, he is rescued by his grandfather. The Allosaurus is then joined by a Tyrannosaurus, which is then defeated by Doc. The two elders then combine their efforts and imprison the Sharpteeth.Littlefoot and his friends then complete Littlefoot's self-assigned mission. Doc departs, remarking as he goes that Littlefoot already has a hero on whom to depend, referring to Littlefoot's grandfather. Littlefoot and Cera later build a legend of their own based on this new paradigm, portraying Grandpa Longneck as a savior.

Question: What story did Littlefoot's grandfather tell the children?
Answer: The Lone Dinosaur