SEDIMENTARY ROCK TEXTURES

 

    Grain Size
 

The Wentworth sediment size chart
 

Gravel
Sand
Silt
Clay

    

 

Toss a boulder in a river and wait. Eventually there will be a high enough flow of water in the river that the rock will start rolling and bouncing its way downstream.  It will likely collide with other rocks on its journey and start to break apart.  The farther your rock travels, the more it will break apart and the smaller it will become. Eventually you will be left with nothing by silt and clay.

Sedimentary rocks are classified in large part, by the size of the particles of which it is made. The most common scheme for classifying sediment size is the Wentworth Scale. There are specific size values assigned to the scale. For example, a coarse sand is defined as a grain of rock whose diameter is between 1mm and 1/2mm.  You could determine grain size if you spent the time to sieve your grains through well calibrated sieves.  Or you could simply visually compare the grains to one of the charts on the left. It's up to you.

 

 

 

 
  Grain Shape
  Examples of Grain shape 
 

Break a rock with a sledge hammer and the broken pieces will be rough and angular.  Toss the pieces into a river and watch them get swept away in the current.  As they travel downstream they will roll along the bottom of the river and the corners of your rock pieces will become rounded off.  The farther the sediment travels the more rounded the particles will become.  Take a close look at the grains in a clastic sedimentary rock and try to determine how well round the grains are.

 
 

 

    Grain Sorting
  Examples of grain sorting 
 

 

Put a pile of clay, silt, gravel and sand in the desert and sit back and watch geologic processes in action. When the wind kicks up some of your pile will become mobilized by the wind.  The very finest grained clay and silt may be completely carried away by the wind.  The sand may roll and bounce along the ground until it piles up in a hole or behind an object that shields it from the wind. The gravel will remain behind because it is too large to be moved by wind alone. Wind is very effective in sorting out sediment by grain size and will produce in a very well sorted sand, often times as large dunes we all enjoy playing on.  A glacier on the other hand will transport any size material imbedded in the ice.  When the ice melts, anything and everything will be deposited, resulting in a very poorly sorted sediment.

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