Minecraft wiki entry for Altitude, associated text: Looking at water, the amount at layer 62 (sea level) is obvious. Moving down, the amount quickly decreases at layers 56 and 48, the usual depth of river and ocean biomes respectively.
There are corresponding peaks in the amount of clay beneath them.
Between layers 33 and 12, most water is falling down sub-ocean ravines, spreading out on the lava-filled bottom at layer 10 and producing most of the naturally-occurring obsidian.
Ores and gravel (not shown) usually occur as a fixed percentage of the amount of stone (also not shown), tapering off at the ends of their allowed generation range.
This is why coal and iron follow parallel tracks between layers 5 and 60.
The one exception is lapis lazuli ore, which has a linear progression up to a peak at Y = 15.