Exactly. This is hard to understand when living in a flat in a modern city but it's immediately clear in any other case. My figure of 1 mm is about the compact

Exactly. This is hard to understand when living in a flat in a modern city but it's immediately clear in any other case. My figure of 1 mm is about the compacted result of decaying and layering. It may vary a lot according to the configuration of the ground.