There are two types of sedimentary rocks that are dependent upon the way in which the sediments are transported. The two types are clastic and chemical sediments.

Clastic sediments - pieces of rock and minerals that are moved as solid pieces.

Chemical Sediments - material transported in solution and deposited.

Changing these sediments to rock is known as lithification. This process involves either cementation or recrystallation.

1. Clastic sediments

Gravel --- conglomerate

Sand --- sandstone

Silt --- siltstone

Clay --- shale

2. Chemical sediments - substances dissolved in water that precipitate.

• sediments are deposited in layers
• sequence of layers reveals relative timing of deposits

Layers in a sedimentary section reveal the relative timing of geologic events.

Uncomformities are defined as breaks in the sedimentation records which are a result of either changes in environmental conditions or erosion of the deposition record. There are three types of uncomformities.

1. Noncomformity: sediments overlying igneous or metamorphic rocks.
2. Angular Uncomformity: angular discontinuity between younger and older rocks.
3. Discontinuity: Irregular surface of erosion between two sets of parallel strata.

We introduced the term isotope when we began our discussion of minerals. An isotope is defined as atoms with the same atomic mass (same number of protons) but different mass number (different number of neutrons). All isotopes of a single element have the same chemical properties.

Some isotopes such as Carbon 14 underego radioactive decay or a spontaneous change to another element or isotope. This decay occurs at very regular time intervals and so if one can count the number of remaining initial atoms (parent atom) and the number of decay atoms (daughter atoms) then one can estimate the age of the rock in which these elements were found.

Using isotopes with long and short half lives one can date rocks of a wide range of ages. For igneous rocks in which radioactive isotopes are found one can determine an absolute date of formation.

Isotopic dating provides absolute dates for some rocks. Sedimentary stratigraphy provides estiomates of relative timing of sedimentary rocks. Combining these two measures provides a method for establishing detailed estimates of the timing of geological processes.