The correct answer is - a very old and highly-weathered soil.
A soil that is very old, and has undergone a process of lot of weathering and erosion, will be totally stripped from its upper layers. With the upper layers, the fertile ones, being totally removed from the soil, on the surface will rise the lower layers of the soil. These lower layers that have been uncovered, are in fact the layers that have been the parent material for the formation of the soil. So by studying that material, it can easily be seen what was the basis of the soil, and how it was evolving further in time on that basis before being destroyed.