Minecraft wiki entry for Infdev_20100327_to_Beta_1.7.3, associated text: Top layer: This layer exists at the absolute top of the map. Occasionally there can be a lower area that is not shadowed (this is technically a dry layer). The lower area is where a majority of the trees and passive mobs can be found, as the top layer receives almost all of the sunlight. Due to the lack of space, the area at the absolute top cannot have trees or mobs. With mods in other biomes,[verify] they are most plentiful on the top as well.
The top layer tends to light incorrectly in day-night transitions. This is because the sunlight calculation does not work when the entire chunk is blocked at Y-coordinate 128 in early versions.
Dry layers: These generate slightly flatter than normal terrain and have grass, despite the darkness. At sea level, massive floating beaches can be found, which collapse if modified. Hostile mobs' spawn rate likely approaches the maximum due to being in the shadow of the top layer. Rarely, there are holes in the top layer that allow sunlight to reach these layers. Caves that have one of these layers as their "surface" can occasionally be carved out of dirt instead of stone. These layers have cave-like ceilings made out of stone, gravel and dirt.
Flooded layers: Like the dry layers, these generate somewhat flat terrain, but it is comprised primarily of stone. Sand and sandstone appear down here, even down to 30 blocks below sea level. Except for coal, all the ores can be found only in these layers.