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.