Clastic Rocks: Conglomerate to Shale

How transport energy sorts fragments into distinct rock types.

Intro Uni Geology
⏱️ About 18 min
Clastic Rocks: Conglomerate to Shale — illustration
Illustrative image (AI-generated).

Hold a handful of gravel and a handful of mud. One was dropped by a raging river; the other settled quietly in a still lagoon. The size of the pieces is a diary of the journey.

💡
The big idea: Transport energy sorts fragments by size. When those size classes are buried and lithified, they become distinct clastic rock types — each recording the energy of the environment that deposited them.
🎯 By the end, you'll be able to
  • Name the four main clastic rock types and their grain-size ranges
  • Relate grain size to transport energy and depositional setting
  • Use sorting and rounding to interpret transport distance and history
📎 Helpful to know first

The clastic family

Clastic (or detrital) sedimentary rocks are built from fragments — clasts — of pre-existing rocks and minerals. Every clast carries a story: what it was originally, how far it travelled, and how energetic its transport was. The simplest way to classify these rocks is by the size of their clasts.

Wentworth grain-size scale from boulder to clay Gravel Sand Silt Clay Colloid 256 mm 2 mm 1/16 mm 1/256 mm finer φ < −8 to −1 −1 to 4 4 to 8 8 to 12 φ > 12 Wentworth Grain-Size Scale Lithified equivalents: Conglomerate / Breccia → Sandstone → Siltstone → Shale / Mudstone
The Wentworth grain-size scale divides sediment into named classes based on particle diameter. Each class maps to a distinct clastic rock type when lithified.

Conglomerate and breccia

Conglomerate is the lithified version of gravel: rounded clasts larger than 2 mm, cemented together in a finer matrix. The rounded shape tells you the clasts tumbled for a long distance in a river or along a beach. Breccia also contains large angular clasts, but they are sharp and unrounded — a sign of short transport, such as a rockfall or landslide deposit.

Sandstone

Sandstone forms from sand-sized grains (1/16 mm to 2 mm). It is one of the most common sedimentary rocks on Earth. Geologists further classify sandstone by composition:

  • Quartz sandstone — dominated by quartz grains; very stable, mature.
  • Arkose — rich in feldspar; indicates rapid burial before chemical weathering could destroy the feldspar.
  • Lithic sandstone — contains rock fragments; immature, close to source.

The better sorted and more rounded the grains, the longer and more intense the transport.

Siltstone and shale

Siltstone is composed of silt-sized particles (1/256 mm to 1/16 mm). It feels gritty between your teeth but looks smooth to the eye. Shale is even finer: dominated by clay-sized particles (< 1/256 mm). Shale is famous for splitting into thin sheets along bedding planes — a property called fissility — caused by the alignment of flat clay minerals under gentle pressure during burial.

Clastic sedimentary rock classification by grain size Clastic Rock Classification by Grain Size Grain size Rock name Texture Energy > 2 mm (gravel) Conglomerate (rounded) / Breccia Coarse, clastic Very high 1/16–2 mm (sand) Sandstone Medium, clastic High–moderate 1/256–1/16 mm Siltstone Fine, gritty Low < 1/256 mm (clay) Shale / Mudstone Very fine, fissile Very low Grain size reflects the energy of the transporting medium.
Clastic rocks classified by grain size: conglomerate (gravel), sandstone (sand), siltstone (silt), and shale (clay). Each size class reflects a different transport energy.
⚠️ Sedimentary rocks do NOT form from cooling lava
A common misconception is that sedimentary rocks form when lava cools. That describes igneous rocks. Sedimentary rocks form by deposition and lithification of sediment, or by precipitation from water — never by the cooling of molten rock.
\[ \varphi = -\log_2(d) \]
The Wentworth phi (φ) scale converts grain diameter d (in millimetres) to a whole-number scale where each step is a doubling or halving of size. Coarser grains have lower (or negative) φ.
✏️ Practice: A grain has a diameter of 0.25 mm. What is its phi (φ) value? Use φ = −log₂(d) where d is in mm. (Round to one decimal place.)
φ
Solution
  1. φ = −log₂(0.25).
  2. 0.25 = 1/4 = 2⁻², so log₂(0.25) = −2.
  3. Therefore φ = −(−2) = 2.0 φ.
✏️ Practice: A grain has φ = 3. What is its diameter in mm? Use d = 2^(−φ). (Give the value in mm.)
mm
Solution
  1. d = 2^(−3).
  2. 2³ = 8, so 2⁻³ = 1/8.
  3. Therefore d = 0.125 mm.
📝 Worked example: You examine two sandstone samples. Sample A is poorly sorted with angular quartz and feldspar grains. Sample B is well-sorted with rounded quartz grains only. What does each tell you?
  1. Sample A contains unstable feldspar and angular grains — it was buried quickly near its source, with little time for chemical weathering or abrasion.
  2. Sample B is mature: quartz is the most durable mineral, and rounding plus good sorting indicate long transport by rivers or wind.
  3. Sample A likely formed in an alluvial fan or braided stream near mountains. Sample B likely formed on a beach or dune far from source.
✓ Sample A is immature, close to source, rapidly deposited. Sample B is mature, far-travelled, and deposited in a high-energy, winnowed environment like a beach or dune.
Comparison of poorly sorted angular grains versus well-sorted rounded grains Poorly sorted, angular Short transport, near source Well-sorted, rounded Long transport, mature transport Sorting and Rounding Record Transport Distance Angular, poorly sorted grains are immature; rounded, well-sorted grains are mature.
Sorting and rounding increase with transport distance. Angular, poorly sorted grains are fresh from source; rounded, well-sorted grains have travelled far.

Check your understanding

1. Which clastic rock forms from gravel-sized clasts?
Conglomerate is lithified gravel (> 2 mm). Sandstone forms from sand, siltstone from silt, and shale from clay.
2. Well-sorted, rounded quartz grains in a sandstone most likely indicate:
Quartz is durable; rounding and good sorting require long abrasion and winnowing. This points to mature transport, such as rivers, beaches, or dunes.
3. What is the phi (φ) value of a sand grain with d = 0.5 mm?
φ = −log₂(0.5). Since 0.5 = 2⁻¹, log₂(0.5) = −1, so φ = 1.
🔎 Related lessons
Previous: From Weathering to Sediment Next: Limestone, Chert, Evaporites — and Organic Rocks (Coal)
✅ Key takeaways
  • Clastic rocks are classified by grain size: conglomerate (gravel), sandstone (sand), siltstone (silt), shale (clay).
  • Grain size reflects transport energy: high energy carries coarse grains; low energy deposits fine mud.
  • Sorting and rounding increase with transport distance and reworking.
  • The phi scale (φ = −log₂ d) quantifies grain size in whole-number steps.
  • Sedimentary rocks form by deposition and lithification, not by cooling lava.
➡️ Not all sedimentary rocks are built from broken fragments. Some crystallise from water, and others grow from organic matter — a completely different origin story.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of sedimentary rocks & environments.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.