hardpan

noun

1. any layer of firm detrital matter, as of clay, underlying soft soil.Compare caliche, duricrust.
2. hard, unbroken ground.
3. the fundamental or basic aspect of anything; solid foundation; underlying reality: the hardpan of mathematical theory.

noun

1. a hard impervious layer of clay below the soil, resistant to drainage and root growth

1. A hard, usually clay-rich layer of soil lying at or just below the ground surface, in which soil particles are cemented together by silica, iron oxide, calcium carbonate, or organic matter that has precipitated from water percolating through the soil. Hardpans do not soften when exposed to water. Also called caliche