The Rock - CONGLOMERATE

  • Sedimentary Rock Type: Clastic
  • Related to: Sandstone and breccias
  • Color: Variable
  • Texture: Rounded pebble to cobble sized grains usually in a finer grained matrix
  • Origins: River, ocean and glacier deposits
  • Common Minerals: Quartz, feldspars, micas, calcite and clays
  • Uses: Building material, decorative stones, tiles, tombstones, monuments, aquifers, petroleum reservoirs
  • Specimens:
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Conglomerate rock is a common sedimentary rock.  It forms in many different environments and settings where the energy of transport is high enough to move large grains.  The sediment from which it forms is much coarser than other clastic sedimentary rocks except for breccias.  The only difference between conglomerates and breccias is the roundness of the grains.  In conglomerates, the grains are rounded and usually indicate that they have been transported or worked more than the angular grains found in breccias.  Distinguishing between breccias and conglomerates is usually very easy as the grains are mostly large enough to see with the unaided eye.  If the rock has a smaller grain size (< 2.0mm) which is almost too small to see, then the rock is a sandstone.

Like sandstone and breccias, conglomerates are cemented by various minerals.  Normal cementing agents include calcite, quartz (silica), clays and gypsum.  When the sediment is first deposited there are lots of open spaces or pores. Cement can affect the amount of pore space that is left in a rock as it solidifies.  Conglomerates usually have significant pore space and they are generally a good rock to act as a reservoir for ground water, natural gas and petroleum.

Conglomerates form in environments that are generally not too far from the source of the sediments and high in energy.  The grains of a breccia are found even closer to the source of the sediments since they have not been rounded like the grains of a conglomerate.  If the deposit is farther from the source, then the sediment is more likely to be a sandstone with all the large grains left behind.  Prehistoric glacial deposits are a great source of conglomerates as are alluvial fans.  Anywhere that pebbles are found is a possible source of a conglomerate.  Generally conglomerates are made up of fragments of other rocks, but at times large quartz or feldspar crystals can also make up a significant percentage of the conglomerate's components.  These crystals are of course lacking in crystal faces and are just rounded grains.

Conglomerates with their interesting pebbled and fine matrix textures are often used as ornamental rocks for buildings, monuments, grave stones, tiles, and many other ornamental uses. However their irregular grain sizes contribute to less durability than that of sandstone and therefore fewer uses in building construction.

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