Soil Types Triangle
All dirt is basically made of three different kinds of particles. These are clay, sand, and loam. Clay is very fine, small inorganic particles, worn down from rock. Sand is bigger, coarser particles, also from rock, often quartz or silica. Loam is organic particles (bits of dead plants). Geologists and archaeologists can describe any particular soil by saying where it lies in this triangle.
The best soils for farming have a lot of loam in them; they fall near the top of the triangle, like the blue spot. Down where the purple spot is, that would be good clay for making pottery. Over by the green dot, that's sand that you can melt down into glass. The orange dot marks a good soil with a lot of loam in it, , but it does have a lot of clay in it, and that makes it heavy and hard to plow.
You need animals to pull the plow. A lot of northern European soils are like that, and also the soil in river valleys like the Nile. The yellow dot marks soils that are more typically Mediterranean: a mixture of loam and sand, with only a little clay, light enough to plow by hand.
Learn by doing: check out the dirt in your yard or a park. Is it sandy? Clay? Loam?
More about geology and rocks
Archaeology : Uncovering the Mysteries of Our Past, by Richard Panchyk (2001). With twenty-five projects, like counting tree rings, and serializing cars from photographs. Includes a project on soil types.