Sedimentary Rocks Sedimentary rocks are unique and varied in nature. They form in one of three ways: (1) from

the compaction and cementation of sediments as a clastic rock, (2) from chemical precipitates

in solution or evaporates as a crystalline rock, and (3) from compacted plant or animal remains

as a bioclastic rock.

Clastic rocks are probably the most abundant type of sedimentary rock. Examples include:

conglomerate, breccia, sandstone, siltstone, and shale. Each of these 5 rocks are uniquely

identified by the sediments that they contain and their characteristics (for more on the 6 types

of sediments, turn to page 2). Clastic rocks form when water surrounding sediments evapo-

rates and the dissolved minerals in the water act as a cement locking the sediments together.

When chemical reactions or evaporation occurs, crystalline rocks may form. Many of the ma-

jor rock salt deposits found in New York State are the result of evaporation of ancient sea-

water. Limestone and Rock Gypsum are two additional examples of common crystalline rocks.

Bioclastic rocks form from some sort of biological process. A major source of energy for hu-

mans (bituminous coal) formed from compacted plant remains and is used to power many

power plants across the country.

Most sedimentary rocks (with the exception of crystalline rocks) have the potential to form

fossils within them and are favorites amongst collectors.

IN THIS ISSUE

Sedimentary Rocks P.1 Bioclastic Rocks P.1

Sediment and Clastic Rocks P.2 Crystalline Rocks P.2

Bioclastic Rocks

Bioclastic rocks are wholly or partial-

ly comprised of compacted plant or

animal remains. Bioclastic rocks will

often have fossils within them upon

discovery. A classic example of this

can be found in samples of coqui-

na— a bioclastic rock made up most-

ly of shell fragments. This indicates

that the environment the rock

formed in was likely a very shallow

warm-water sea.

SEDIMENTARY ROCKS THE EARTH SCIENCE JOURNAL

ISSUE 10 NOVEMBER 2013

Crystalline Rocks

Crystalline sedimentary rocks are

composed of crystals having formed

from a chemical reaction in a solu-

tion or from evaporation. The crys-

tals can vary in size from very fine

(you cannot see them with the na-

ked eye) to very coarse. An example

of the crystalline rock rock gypsum is

pictured below. When crushed and

processed it is used to produce dry-

wall.

Sand is the primary ingredient in the sedimentary rock sandstone, and ranges in size from

0.006 cm in diameter up to 0.2 centimeters.

Sand

Commonly found in the sedimentary rock conglomerate, pebbles range in size from 0.2 centi-

meters in diameter up to 6.4 centimeters in diameter.

Pebbles

Cobbles are the second-largest sediment size and range from 6.4 centimeters in diameter up

to 25.6 centimeters.

Cobbles

Boulders are the largest of sediment sizes and are 25.6 centimeters in diameter and larger but

are found in relatively few clastic rocks.

Boulders

Sediment and Clastic Rocks

THE EARTH SCIENCE JOURNAL

Created By: Dave Burgess

Rochester STEM High School

I do not own any rights to the images in this journal.

All images used for educational purposes only.

Silt and Clay

Silt and clay are the smallest sediments before one considers dissolved minerals in water. Silt

particles range in diameter from 0.0004 cm to 0.006 cm and clay is less than 0.0004 cm in di-

ameter. Both appear to make a stream appear “muddy”, and will remain transported in water

with very low velocity.