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BRECCIA

classification of detrital rocks      shale      siltstone      sandstone      conglomerate      

Breccia is a detrital sedimentary rock composed mostly of large, angular clasts.   Breccia forms as streams transport grains, or clasts, only a short distance before depositing them within a stream channel or valley.   Because of this short distance of transportation, the clasts have experienced relatively few collision, and so still possess their original sharp edges and corners produced when they weathered from rocks in their source area.   Breccia can also form from landslide masses that became fragmented as they moved down slope.   Once such a mass stablizes and solidifies, it can form a breccia containing huge, boulder-sized fragments.   Below are some images of breccia.    

Dan and Scott love this breccia outcrop composed of volcanic pebbles and cobbles.  The location is near Las Vegas, Nevada.

  This is a hand sample of a pebble limestone breccia, composed largely of clasts derived from the weathering of limestone mountains outside of Las Vegas, Nevada.   The matrix is tiny grains of limestone held together by calcite cement.   Such a rock fizzes readily when it comes in contact with hydrochloric acid.

  Here is a breccia composed of chert pebbles, so it is called a pebble chert breccia.

  This sample of breccia is composed of gravel-sized clasts of varying composition, mainly rock (or lithic) clasts.   Observe that some of the clasts are rounded, but most are angular in appearance.   This is a gravel lithic breccia.

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