1. Impermeable Rock Layers:
* Act as Barriers: Impermeable rock layers, like clay or shale, are dense and do not allow water to pass through easily. They act as barriers, preventing groundwater from flowing downward.
* Force Groundwater Upward: When groundwater encounters an impermeable layer, it is forced to move horizontally and eventually find a path to the surface.
2. Permeable Rock Layers:
* Storage and Transport: Permeable rock layers, like sandstone or gravel, have spaces (pores and fractures) that allow water to pass through easily. They act as both storage reservoirs and pathways for groundwater.
* Water Source: Permeable rock layers often hold groundwater that has infiltrated from rainfall or snowmelt. This groundwater eventually flows downward, encountering the impermeable layer.
Formation of a Spring:
* Water Pressure: As groundwater is forced upward by the impermeable layer, it builds pressure.
* Surface Outcrop: If the permeable rock layer reaches the surface, the pressure will force the water out, forming a spring. This is typically where the impermeable layer intersects the surface topography.
* Spring Location: Springs often form at the base of hills or slopes, where the impermeable layer intersects the surface.
In Summary:
* Impermeable layers: Block downward flow, forcing groundwater upward.
* Permeable layers: Store and transport groundwater.
* Combined Action: The combination of these two types of layers leads to the buildup of pressure that eventually forces the water out, forming a spring.
Example:
Imagine a layer of sandstone (permeable) overlying a layer of clay (impermeable). Rainwater infiltrates the sandstone, filling the pores and fractures. As the water moves downward, it reaches the clay layer and is forced to move horizontally. If the sandstone reaches the surface, the pressure will force the water out, creating a spring.
