- The Sea of Beer and Earth
When you're on the ocean, you can see nothing but water.
Picture, if you will, standing in the middle of a small valley. There's nothing growing on the valley floor or on the sides of the hills as far as you can see. (This is not a sad thing: it's the end of winter, seeds are sprouting, but this is the day before they burst out of the ground.) All you can see is the earth. It's soaking wet mud and that's all you can see: a "sea of mud" or "sea of earth".
Now picture standing outside the brewery. There was some kind of disaster inside (no one was hurt of course) and all the brewing vats have split open and beer is running 10cm deep everywhere you can see. You are in the middle of a "sea of beer" or an "ocean of beer".
- The Thin White Trail
(as above) You spilled flour on the kitchen floor and the cat got his tail in it. Before you could catch the cat, he ran away and you've got to find him and get him outside before everything in the house is covered with flour. So you follow the "thin white trail" where his tail dragged on the floor.
Alternatively, you're following a path through the woods. Thick woods, narrow path. While the path is earth, all the rocks in the area are white (quartz), so the path you are following is a "thin white trail".
I don't think the phrases can be made any more clear. They don't refer to something in particular, they're just rather impressionistic descriptions of a plaid pattern.
BB,
NightWing