Final answer:
In a sequence of undisturbed sedimentary layers, the oldest layer is at the bottom due to the principle of superposition. This is supported by the principle of original horizontality, which indicates that sediment layers were originally deposited horizontally.
Step-by-step explanation:
The question pertains to the identification of the oldest sedimentary layer in a sequence of rock strata. According to the principle of superposition, in undisturbed layers of sedimentary rock, the oldest layer is at the bottom, with newer layers progressively above it. Since the question mentions four straight layers, we can conclude that the oldest layer is the one at the bottom.
The principle of original horizontality supports this by stating that layers of sediment are originally deposited horizontally. Therefore, if there is tilting or folding present, then these post-date the deposition of the sedimentary layers. If the layers are now tilted, it is the bottommost layer of the tilt that would be the oldest.
Moreover, the principle of lateral continuity suggests that layers of sediment originally extended in all directions until they thinned out or were truncated by other geological features. This indicates that the sequence of layers would have been continuous and ordered by age at the time of their formation.
When studying sedimentary rock layers at specific geological sites, such as Zion National Park, geologists can observe the principle of superposition in the layers deposited from different geological periods. Investigating the types of sedimentary structures within these layers, such as stratification, can further aid in understanding the environment at the time of deposition and the history recorded within the rocks.