A
mountain of shale in Utah. 
A closer look at a shale outcrop.TITLE INTRODUCTION DETRITAL SEDIMENTARY ROCKS CHEMICAL ROCKS BIOCHEMICAL ROCKS ENVIRONMENTAL CLUES CREDITS
SHALE
classification of detrital rocks siltstone sandstone conglomerate breccia
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.
A
mountain of shale in Utah.
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.
