Can someone explain what I am observing.

Container Planters - fairly large volume. When it rains heavy or just prolonged moisture.....I do notice the coarse perlite tends to be more abundent on the top as it it has moved from the soil mix.

Is this

A- Perlite floating to the surface

or is it the latter...

B - Bark mix settling down more due to the heavy moisture and the light material, perlite is displaced to the top

Just wondering which one is the cause and effect.