What is a hard layer of earth material that is often clayey called?

The term "hardpan" refers to a hard layer of earth material that often has a clayey composition. This layer can develop in the soil as a result of various processes, such as compaction from heavy rainfall or the accumulation of minerals that create a dense barrier, making it difficult for roots to penetrate and for water to drain. The hardness of this layer restricts root growth and can impact agricultural productivity, as it limits the effective depth of the soil that plants can access for nutrients and moisture.

In contrast, other options refer to different types of soil or layers with distinct characteristics. Subsoil, for instance, is generally softer and located beneath the topsoil, while caliche specifically denotes a type of hardened calcium carbonate deposit, not necessarily clayey. Transported soil refers to soil that has been moved from its original location by natural forces like wind or water, lacking the distinctive hardness associated with hardpan. Thus, "hardpan" aptly describes the hard, clayey layer described in the question.