Lyell referred to Hutton’s concept that geologic processes operating at present are
the same processes that operated in the past as the principle of uniformitarianism.
Dating based on radioactivity, allows us to determine a rock’s numerical age
(absolute age) in years or other units of time. Geologists working in the field or in
labs with maps, are more concerned with relative time, the sequence in which
events took place.
Contacts are the surfaces separating two different rock types or rocks of different
ages. (useful for finding geologic history of an area)
Four basic principles to determine relationship of geologic events: original
horizontality- states that beds of sediment deposited in water formed as horizontal
or nearly horizontal layers. Superposition- states that within a sequence of
undisturbed sedimentary or volcanic rocks the layers get younger going from
bottom to top. Later continuity- states that an original sedimentary layer extends
laterally until it tapers or thins at its edges. Cross-cutting relationships- states that
a disrupted pattern is older than the cause of disruption.
Principle of inclusion states that fragments included in a host rock are older than
the host rock.
An unconformity is a surface (or contact) that represents a gap in the geologic
record, with the rock unit immediately above the contact being considerably
younger than the rock beneath. 3 types: disconformity, nonconformity, and angular
In a disconformity the contact representing missing rock strata separates beds that
are parallel to one another.
Angular unconformity is a contact in which younger strata overlie an erosion
surface on tilted or folded layered rock. It implies the following sequence of events:
1. Deposition and lithification of sedimentary rock. 2. Uplift accompanied by folding
or tilting of the layers. 3. Erosion. 4. Renewed deposition on top of the erosion
Nonconformity is a contact in which an erosion surface on plutonic or
metamorphic rock has been covered by younger sediment