Engineering geology

Shale, Mudstone, or Siltstone?

It is common to apply the term shale to all fine-grained sedimentary rocks, especially in a nontechnical context. However, be aware that there is a more restricted use of the term. In this narrower usage, shale must exhibit the ability to split into thin layers along well-developed, closely spaced planes. This property is termed fissility (fissilis = that which can be cleft or split). If the rock breaks into chunks or blocks, the name mudstone is applied. Another fine-grained sedimentary rock that, like mudstone, is often grouped with shale but lacks fissility is siltstone (see Figure 1).

Figure 1 – Particle size categories Particle size is the primary basis for distinguishing among various detrital sedimentary rocks.

As its name implies, siltstone is composed largely of silt-size particles and contains less clay-size material than shale and mudstone.

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