Final answer:
An aquifer is a layer of permeable rock that holds groundwater, often beneath an impermeable layer or caprock, and is a vital part of the hydrologic cycle.
Step-by-step explanation:
A layer that sits above the main water table where a lens of impermeable rock blocks and collects water infiltrating into the ground is known as an aquifer. This impermeable layer, often referred to as caprock, prevents the water from seeping further down and forces it to accumulate within the porous and permeable rock beneath it.
The water table itself behaves dynamically, fluctuating with rainfall and drought conditions, and mimics the topography by rising with hills and descending with valleys. The aquifer is a crucial component of the hydrologic cycle, serving as a reservoir for groundwater that can persist for millennia.
The pore spaces within the aquifer's rock or sediment act as storage for the water, often being tapped through wells for purposes such as drinking or irrigation.
However, it's important to understand that groundwater is not composed of underground lakes or streams, but rather is a result of water slowly seeping from one pore to another within the aquifer's rock or sediment.