In this time of the clash of continents, an earthquake split the land. Herds were divided, families were cut in two. Littlefoot was separated from his grandparents.
— Narrator, The Land Before Time
The Great Earthshake occurred in the collision of two continental plates, and was far more severe than any earthquake ever encountered in the real world. It completely altered the landscape, sending pillars of stone jutting out of the earth and breaking open the ground to form the Great Divide.
When the Great Divide was opened up, it claimed many casulalties, as it split open underneath a large herd of dinosaurs. Sharptooth, among others, fell down the Divide, presumably to certain death (though Sharptooth somehow managed to survive the fall).At the bottom of the Divide is an area called the "Big Underground". Its geography, and inhabitants, remain unknown.
The Great Divide is arguably the most prominent land feature in The Land Before Time, possibly even more so than the Great Valley, simply due to its size. Like all landmarks in the Land Before Time no exact dimensions are given, but judging by the visuals provided in the first film, it is at least a half-mile across, and possibly several miles deep. The length of the Great Divide is not shown, as it stretches to the horizon, but it is extensive enough that there is no quick way around it, prompting Cera to travel through the Big Underground rather than walk along the edge of the Divide until she found a way across.
The Great Divide appears only in a part of the first film, and was referenced by Bron in The Great Longneck Migration. It can be seen in his flashback.