The Thick is not an arbitrary demarcation line, rather it is the point at which visibility beyond the edge of an aerolith (or airship) drops to zero. Those on an aerolith can still see relatively well upon the surface, even at this depth, but the edge of the land seemingly runs into a curtain of white. This deep in the mists the constant moisture becomes a real problem, but it also allows for the growth of an enormous array of edible fungus.
The other important point about the Thick is that it is the line that the behemoths do not seem to be able to cross. Those that have come up this high have launched themselves up in pursuit of a meal, only to fall back down into the thick white mists of the Thick (much like a fish jumping out of water after an insect).
When seen from above, the Thick looks like the top of a churning white sea. Lighting flashes can sometimes be seen within its depths wherever Silvermist and Smoke meet, destroying each other in a burst of energy.