Soil depth is best determined by digging straight down with a hand held post hole digger. If the soil is easy to dig, it is good. If the soil is so hard it cannot be dug, roots will likewise not grow in the hard soil. If there is a combination of two or more it is called a loam. A clay loam is mostly clay, but it also has sand and silt with it.
Loams are always better.
The sandy soils drain very fast, but have a very low water holding capacity. Clays have high water and nutrient holding capacity, but drain very slowly. Gravel or small rocks in a soil can help internal drainage which is good. From time to time growers will here someone talk about soil structure. It is easy to use the word and it is sometimes used with some understanding of what it means, but most of the time growers and scientists do not know much about soil structure, other than know it is their problem.