4.3 The Regional-Scale View

By zooming out further (Figure 28), the three-dimensional, regional, often layered, geologic units containing groundwater systems come into view. The geologic layers occur because: 1) sediments are laid down by water or blowing wind; and 2) sedimentary rocks, which cover about 73% of the upper continental crust, are consolidated so they retain their original layered structure. This layered structure is significant, because both the volume of groundwater storage and ease of groundwater flow differ depending on the porosity and permeability of the layers, creating a sequence of aquifers and confining units.