classification of detrital rocks      shale      siltstone      sandstone      conglomerate      breccia


Detrital (or clastic) sedimentary rocks are the most common rocks exposed at Earth's surface.   They form as sediment grains (detritus or clasts), weathered and eroded from pre-existing rocks, are deposited as layers in low-lying areas such as valleys, lakes, or an ocean basin.   With continued deposition, sediment grains become compacted tightly together.   As groundwater slowly percolates through open spaces (pores) within the sediment, minerals will precipitate onto the surfaces of the grains forming a natural cement.   Detrital sedimentary rocks can have distinctive colors derived from the minerals they contain, and they can weather into unusual shapes depending on the degree of cementation and composition of individual layers of rocks.

sed_archDelicateshadow_1.jpg (32186 bytes)  Delicate Arch, at Arches National Park in Utah, is composed of sandstone.      

sed_xbedsWalnutCanyon_1.gif (347836 bytes) A sandstone outcrop with well-defined cross beds in Walnut Canyon, Arizona.

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Classification of Detrital Sedimentary Rocks

Detrital sedimentary rocks are classified mainly by the size of their grains, and secondarily by the composition of the grains.   The list below compares sediment grain sizes to the rocks that form from such sediment.

sediment grain size

rock name







gravel, pebble, cobble, boulder

conglomerate (rounded grains)            breccia (angular grains)

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Descriptions of the Detrital Sedimentary Rocks

SHALE    Shale, the most abundant type of sedimentary rock, is composed of tiny, clay-sized sediment grains.   Such sediment typically settles to the ocean floor where it is deposited in thin, well-defined layers.   Also settling to the ocean floor is organic matter (plant and animal detritus) that becomes mixed with the clay grains.   Gradually, over a period of thousands of years, the clay and organic matter become compacted and solidified forming shale.   The carbon from the organic matter is dark in color, so shale is often dark in color as well.   Below are some images of shale.

           picture described in text  A mountain of shale in Utah.        picture described in text A closer look at a shale outcrop.


Below are hand samples of shale.   The two images on the left show the thin layering characteristic of shale.   The third image is a close up of shale.   The individual clay grains are too small to be seen.   

                               picture described in text                   picture described in text                   picture described in text      

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SILTSTONE      Siltstone is composed of silt-sized sediment grains, which are transitional in size between clay and sand grains.   So, siltstone represents natural depositional environments transitional between those of shale and sandstone.   For example, on the ocean floor clay dominates sediment deposition in the deep ocean far from shore, whereas sand is commonly deposited closer to shore in shallow water.   Silt would therefore be deposited in between these two extremes, on the continental slope.   Siltstone, when found within a sequence of sedimentary rocks, often represents such a transitional environment.   Below are some images of siltstone.   Note that it is very similar in appearance to shale, but it is a bit grittier.   Geologists working on an outcrop will often do a field test to distinguish siltstone from shale by dragging a sample across their front teeth.   Shale is smooth whereas siltstone is more abrasive.   Older geologists who have done this experiment hundreds of times will have very straight-edged front teeth.

sedrocksshaleDan3.jpg (17906 bytes) Dan, whose hands were tired, holding a piece of siltstone.                                 

picture described in text Siltstone is thin and platy like shale, but it tends to be a little harder than shale.   Siltstone has the texture of a very fine-grained sheet of sandpaper

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SANDSTONE     Sandstone forms as sand-sized sediment grains become compacted and cemented together.   Sand grains transported and deposited by wind will be arranged in tilted layers called cross beds, internal features within sand dunes.   Sand grains transported by water (either by streams or ocean currents) will settle out forming small cross beds, or graded beds if the sand is being transported along with larger sediment grains.   Below are some images of sandstone.

On the left is a hand sample of sandstone.   A closer view of the same sample, to the right, shows that the sand grains are mainly composed of gray quartz grains.

                                    picture described in text                                       picture described in text   

Below are some views of Kelso Dunes in California.   The Kelso Dunes are composed of sand-sized detrital grains derived from weathering of the San Gabriel Mountains approximately 90 miles to the west.   The sand is transported by the Mojave River, when it floods, out into the heart of the Mojave Desert.   Then, strong winds from the northwest pile the sand up into the dunes shown on these images.  Internally, the Kelso Dunes are cross bedded just like the petrified sand dunes shown in the fourth image below.   

DanKelsoDunesdistant2.jpg (36742 bytes) Dan and a great view of Kelso Dunes just north of a high desert peak.   

sed1KelsoDunes2.jpg (38780 bytes) An afternoon view of the Dunes when shadows deepen and nocturnal animals begin to stir. 

picture described in text Jill and Thomas camping near the base of the Dunes.            

picture described in text A cut away view of ancient dunes clearly show the large-scale cross bedding that is typical of sand dunes.   This photograph was taken in Walnut Canyon, Arizona.          

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CONGLOMERATE      Conglomerate sedimentary rock forms as relatively large, rounded detrital grains are compacted and cemented together.   Grains sizes range from the size of your little fingernail (gravel), to grains the size of a quarter (pebbles), too grains the size of your fist (cobbles), and even to grains the size of a bowling ball (boulders) or larger.   In general, these grains become rounded due to their transportation within a stream, and the resulting collisions with each other.   This process knocks of the sharp edges of the grains, making them rounder.   So, the further the grains are transported from their source area, the rounder the grains will become.   Below are some images of conglomerate.

sedDianeconglomerate2.jpg (42323 bytes) Diane next to a layer of conglomerate exposed in a side canyon of Death Valley, California.

sedclosehand1congl2.jpg (61957 bytes) A close up view of conglomerate.   Note the rounding of the large grains (also called clasts).   Most of the clasts are pebble-sized, gray-colored limestone, but the large black clast at the bottom is chert.   The matrix, composed of the smaller quartz grains in between the larger clasts, holds the rock together.   Most accurately, this rock should be referred to as a pebble limestone conglomerate.

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BRECCIA      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 stabilizes and solidifies, it can form a breccia containing huge, boulder-sized fragments.   Below are some images of breccia.    

sedvolcbreccia2.jpg (30525 bytes) Dan and Scott love this breccia outcrop composed of volcanic pebbles and cobbles.  The location is near Las Vegas, Nevada.

 sed_breccialimest_1.jpg (29709 bytes) 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.

 sed_brecciaclose_2.jpg (33473 bytes) Here is a breccia composed of chert pebbles, so it is called a pebble chert breccia.

  sedrockgravelbreccia2.jpg (90635 bytes) 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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