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University of Toronto Scarborough
Environmental Science

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 unconformities. 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 surface. Nonconformity is a contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sediment
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