A Natural Resource Management Guide for the County of Morris A ...

Rock Talk
Rock, the parent of soil, contributes dramatically to the fate of rainfall. General classifications include:
Consolidated - openings consist of fractures, e.g. sandstone, limestone, and granite;
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Unconsolidated - openings consist of pores, e.g. granular material such as sand, gravel, & clay.
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In consolidated rock, ground water movement takes place primarily in intersecting fractures and fissures.
These fractures and fissures, which have become enlarged by weathering, may bear considerable water. An
example illustrating this dynamic is limestone dissolution. Weathering results in the creation of water soluble
channels, which have the potential of developing into tunnels, sinkholes, and caves. These natural phenomena
are both problematic and wondrous.
Unconsolidated deposits are underlain by consolidated rocks, which most often contribute disintegrated
material to the overburden. Ground water generally moves with ease through the spaces among the grains
(e.g., sand and gravel). Silt and clay, however, with very tiny spaces between tiny particles, detain water.
Overall, a greater proportion of recharge and ground water movement occurs in porous unconsolidated
formations (e.g., sand and gravel), than the consolidated formation (e.g., gneiss and shale).
These water-related characteristics, reflect holding capacity and mobility. Porosity reflects the maximum
amount of water that a rock is capable of containing within its void spaces. Although fine-grained materials
tend to have the largest porosities, that doesn't necessitate mobility. Permeability, or hydraulic conductivity,
indicates degree of mobility achieved by size and interconnectedness of the voids.
Primary permeability refers to the hydrogeologic character of the original formation. Consolidated rock (e.g.,
shales, limestone, many sandstones and most volcanic and metamorphic rocks) have few interconnected pore
spaces, and thus a very low primary permeability. Unconsolidated rock, with its generous pores, has good
primary permeability.
Secondary permeability is caused by the folding, faulting, weathering, compaction, dissolution and decementing
of the original formation. These actions accentuate the existing fractures, increasing porosity and
permeability. Examples include limestone and shales which crack along bedding planes, and sandstone which
is de-cemented by water flow.
When Is A Rock An Aquifer?
Although nearly all geologic formations could be considered to be aquifers, the definition has been honed
specifically for human purposes. "Aquifers are geologic formations which have the capability to store and
transmit water which is recoverable in sufficient quantity to be economically useable" (U.S. Army Corps of
Engineers, 1972). 2
The two types of aquifers typical to this region are bedrock and surficial aquifers. Bedrock aquifers consist
primarily of sedimentary and igneous rocks; surficial aquifers consist primarily of glacial and post-glacial
stratified sediments. Although the surficial aquifers are more productive they are not as abundant as the
bedrock aquifers.
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Ground Water