Identification of Common Sedimentary Rocks Lab
For this lab, you will be making observations and interpretations about sedimentary rocks. I
recommend that you complete the data table provided as you analyze each sample. Then, use
your table to answer the questions at the end of the lab handout and transfer your answers to the
lab exercise on Canvas.
Sediment and Sedimentary Rocks
Sedimentary Rocks are formed by the consolidation of sediments. Sediments are solid materials
that settle from a state of suspension or solution in a fluid. For example, take sediment in a lake.
Sediment can be an accumulation of sand that has settled to the bottom of the lake. However,
sediment can also be salt that was at first dissolved in the water but is now precipitating out of
that solution and is settling to the bottom of the lake. These two types of sediment can be
lithified (converted into sedimentary rock) as different types of sedimentary rocks. Therefore,
there are two main types of sedimentary rocks, detrital (clastic) sedimentary rocks and chemical
sedimentary rocks.
A. Detrital (Clastic) Sedimentary Rocks
Detrital sedimentary rocks are composed of fragments of older rocks. Therefore, their formation
begins with weathering. Mechanical weathering breaks down rock into smaller particles of
sediment. Chemical weathering can decompose minerals (example: feldspars to clays). Now that
the original rock was weathered, erosion removes the grains from that location to a new location.
The grains of sediment get transported and eventually deposited. Over time, they get buried by
younger sediment. The pressure of overlying material causes compaction of the sediment. Fluids
passing through the buried sediment may precipitate minerals in the pore spaces between grains
leading to cementation. Therefore, lithification, which is the process of converting sediments into
sedimentary rock, involves compaction and cementation.
The texture and composition of detrital sedimentary rocks help us to make interpretations about
the formation of these rocks as well as the environment in which they are formed. Textural
details that help us to interpret detrital sedimentary rocks are grain size, sorting, and grain shape.
Grain size is simply a measurement of how large the grains are in a sedimentary rock. In order,
from largest to smallest, grain sizes include boulders, cobbles, pebbles, granules, sand, silt, and
clay. Analyzing the grain sizes of the sediment in a rock is important because grain size reflects
the strength of the process that transported the grains. For example, boulders can only be
transported by rivers during flood stages, mudflows, mass wasting (landslides), or some other
high-energy process. Therefore, if a layer of sedimentary rock has boulders in it, such as
conglomerate and breccia, you know that it was deposited during a time of flooding, or some
other high energy environment. Another example is a layer of sedimentary rock that has a lot of
silt and clay in it, such as shale, is deposited in low-energy environments where water is rather
still, allowing the grains to settle out of the water.
Detrital sedimentary rocks are classified primarily by grain size. Figure 1 contains common
detrital sedimentary rocks. Notice that they are named due to the size of the sediment. For
example, if it is made of sand, it is called sandstone. If it is made of silt, it is a siltstone. If it is
made of clay, it is a shale or mudstone. If it is made of grains larger than sand, it is either a
