In loaming, Depending on the depth to bedrock, or what used to be bedrock and has weathered to a white, yellow, grey or red clay, some areas have almost no topsoil while others may have half a metre or more topsoil and several metres of clay, depending on the composition of the original bedrock, The loam sample should be taken from bottom of the ''B'' horizon . ''A'' being topsoil made up of plant material and lighter sands etc. ''B'' being made up of clays or gravels, rock fragments, Hill Scree, ''C'' being decomposed soft bedrock and true bedrock.
If you are tryng to learn to loam, pay attention to what the old timers did to locate reefs or lodes in you area. They may be no more than shallow scrapes or as in my neck of the woods in N,East Victoria, trenches cut across the side of a hill at least half a metre deep.
Maybe try loaming in reverse. Try collecting loams from below a known rich primary deposit/ mine and test at various depths until you get a positive result. just remember that the surface outcrop may not be anywhere near the main shaft or Adit. :|