The Clue to the Location
This is an area of the Great Valley which has Sinking Sand, in sight of the sheltering grass. It is also dino talk for a tar pit. It is seen in The Land Before Time II: The Great Valley Adventure, early in the movie, when the gang try to make their across as they wish to play in the sheltering grass, despite having been forbidden to go there by their parents. Soon they are in danger of drowning in the sand, before their parents hear their pleas for help and rescue them.
Later on in the same movie, Chomper is found by Littlefoot playing in the Sinking Sand, jumping between the stepping stones. Littlefoot calls to him, causing Chomper to return to the shore. This is a reflection of the first sinking sand scene, but with Littlefoot as the grownup and Chomper as the child who disobeyed orders to stay where he was.
In the Land Before Time book When We Grow Up, before Chomper left the group, the Sinking Sand appears again. When the gang are mad at the grownups for not letting them go on a big adventure, Cera runs off saying that a three-horn can do whatever they want to. She runs towards the Sinking Sand, but doesn't see it, and falls in. Spike suggests to try a different approach than the last time, holding a branch out for Cera to grab. The plan works out at first, but Cera slips in farther. Littlefoot's Grandpa comes to the rescue, and tells the children if they don't learn to follow simple rules, they would never understand all the secrets of the Great Valley. The children know they all grew up a little that day, and ride Grandpa Longneck home.
The first part in The Land Before Time II: The Great Valley Adventure, the tar pit in the Sinking Sand was gray when Littlefoot, Cera, Petrie, Ducky, and Spike got stuck in it. However, in the 2nd appearance of it, the tar pit in the Sinking Sand was blue when Chomper was on the stepping stones chasing butterflies.
Chomper got stuck in it in the episode The Mysterious Tooth Crisis, losing his tooth trying to escape it.
The Sinking Sand appeared in two parts: one, the kids tried to make their way across the Sinking Sand to play in the sheltering grass which they have not been listening to their parents and got stuck. Also, the families rescued their loved ones to safety and are mad at them for not listening.
Later on after the incident, Littlefoot was spotted playing there by Chomper, jumping between the stepping stones as Littlefoot calls to him, the Sharptooth returned to the shore as a cause of reflection to the first sinking sand scene ever which Chomper fell in, Littlefoot came to the rescue as the grownup and the sharptooth was the one, disobeying Littlefoot to stay where he was in LBT II ( The Great Valley Adventure).
Number two, the quicksand appeared in the second episode of the LBT TV series called 'The Mysterious Tooth Crisis' where Chomper, for the second time, fell in and tried to escape from it as Littlefoot, Ducky and Petrie helped him out but Chomper lost his tooth.
The Mysterious Tooth Crisis is the second episode in the LBT TV series between the first episode on air was 'The Cave of Many Voices' and the third one, 'The Star Day Celebration'.