Final answer:
Clay is desirable for a landfill site as it is cohesive, has low permeability, and can be molded into an impermeable barrier.
Step-by-step explanation:
Clay is desirable as an underlying layer for a landfill site due to its properties. Clay is a cohesive material that has low permeability, meaning it doesn't easily allow fluids or substances to flow through it. This helps prevent hazardous materials from seeping into the groundwater and contaminating it. Additionally, clay is plastic and can be molded, which makes it easier to construct an impermeable barrier for containing waste. Overall, selecting a site underlain by clay helps to minimize the environmental impact of a landfill.