The Serengeti Plains are mostly in the southern half of the Serengeti. This is where the migration of Wildebeest occur after the seasonal rains. When it's the dry season, seasonal water-holes dry up and the grass turns gold. The Serengeti plains are divided into three main categories: short grass, intermediate grass, and long grass. The "short grass" begins in Ngorongoro and goes into Serengeti National Park. The "intermediate grass" crescents the short grass plains. The "long grass" goes around Seronera. Before any of the plains existed, it was covered with volcanic rock and ash. The ash from the volcanoes contained certain salts that would wash down during the season rains into the soil, forming a hardpan layer. This layer keeps trees' roots from going into the soil, which makes it very hard for trees to grow and develop. The layer also keeps water close to the surface, helping plants and shrubs grow thickly.