Grain size

This episode, "Grain Size," delves into one of the most fundamental properties of sediments and sedimentary rocks: the size of their constituent particles. Building on our previous discussions of sedimentology, sediments, sedimentary environments, facies, depositional environments, and cross-bedding, we'll explore how grain size provides critical information about the energy of the depositional environment, the transport distance of the sediment, and the source rock. We'll examine the Wentworth scale, the standard classification system for grain sizes, and learn how to identify and interpret different grain size classes, from clay and silt to sand, gravel, and boulders. Understanding grain size is essential for interpreting Earth's history, as it allows us to reconstruct past environments, from quiet, deep-sea settings to high-energy rivers and beaches. This episode will equip you with the knowledge to analyze and interpret the information contained within the grains of sediment.

Check your understanding

These are the same multiple-choice questions you will see in the Quiz section after you listen to the episode. Use them here to preview or review the answers.

Which of the following grain size categories is the finest?

  1. Sand
  2. Silt
  3. Clay
  4. Gravel
  5. Cobble

What is the name of the standard classification system for sediment grain sizes?

  1. Mohs scale
  2. Richter scale
  3. Wentworth scale
  4. Beaufort scale
  5. Fujita scale

A sedimentary rock composed of predominantly sand-sized grains is called:

  1. Conglomerate
  2. Shale
  3. Sandstone
  4. Breccia
  5. Siltstone

What does well-sorted sediment indicate about the depositional environment?

  1. A high-energy environment.
  2. A low-energy environment.
  3. A consistent energy level during transport and deposition.
  4. A fluctuating energy level during transport and deposition.
  5. A very short transport distance.

Which technique is commonly used in the laboratory to determine the grain size distribution of coarse sediments like sand and gravel?

  1. Settling tube analysis
  2. Laser diffraction
  3. Sieving
  4. Microscopy
  5. Chemical analysis

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