Keith,
If the soil you have is similar to what is mapped and described (i.e. silty clay) you should have no problem compacting and sealing it.
Remember that the description you see in the soil survey is based on the "Typical Pedon" that the soil survey crew may have had to search and search for. In order to map that series in your county the crew had to find a soil profile (usually 3) that classified correctly in order to map it. If the survey is an older one from the 60's or before, the majority of the mapping may have been done with shallow sampling (30" or so). Many of the old soil surveys were completed with attention to shallow conditions as they affect agricultural uses. Your idea to dig is a good one.
