GEOL 104 Dinosaurs: A Natural History

Fall Semester 2018
Pages in the Book of Time: Sedimentary Rocks

Crossbedded sandstones deposited by stream channels in the latest Cretaceous Hell Creek Formation, near Ekalaka, MT

Key Points:
•Rocks are the solid physical record of past environments. Our classification of rocks is based on the processes that produced them.
•Igneous (chilled from a molten state) and metamorphic (recrystallized by intense heat &/or pressure) do not contain fossils.
•In contrast, sedimentary rocks (those made by fragments of previously existing rocks transported and redeposited) often contain fossils.
•Sedimentary structures (such mud cracks, raindrop marks, ripple marks, crossbeds, and the like), and other features such as the size, sorting, and roundness of clasts, record the environments on Earth's surface (where living things live and die) at the time the rocks formed. •Because sedimentary rocks form by deposition of particles that were being transported, they naturally form layers (strata).

Fossils are contained in rocks, and therefore in order to understand dinosaurs one has to understand how rocks came to be and what information they contain. Rocks are our key to understanding environments of the past; how those environments (including position of the continents and composition of the atmosphere!) change over time; and to uncovering time itself.


The Rock Cycle: any rock can be transformed to any other major class of rock, because rocks are classified by the process in which they are formed. So if you melt an igneous, metamorphic, or sedimentary rock, and it cools down, you form a new igneous rock; if you recrystallize an ingneous, metamorphic, or sedimentary rock, you form a new metamorphic rock; and if you erode an igneous, metamorphic, or sedimentary rock and deposit the sediment from it, you form a new sedimentary rock.

Because sedimentary rocks form where animals and plants lived and died, these are the rocks in which fossils are common. One of the main categories of information sedimentary rock contain is the paleoenvironment (the conditions that existed when that rock was formed). The different environments of deposition represent different paleoenvironments. Some of the clues to discover paleoenvironments:

Of course, another main bit of information that sedimentary rocks contain are fossils, the subject of the next lecture.

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Last modified: 9 August 2018

Precambrian granite weathering into fragments, Wind River Canyon, Wyoming.