In a simple demonstration of this method of comparison, Figure 13-5 shows vertic
ID: 113446 • Letter: I
Question
In a simple demonstration of this method of comparison, Figure 13-5 shows vertical sections through two different rock formations (rock profiles). The letters in the layers represent specific index fossils found only in that layer. 8. Imagine that you were able to slide profile "b" up the page until the layer with the letter F is parallel to the letter F in profile “a , which letter represents the fossil layer that is the youngest in both profiles? In your imagination, slide profile "b" down the page until the layer with the letter P is parallel to the letter Pin profile "a.", which is the oldest rock layer? FIGURE 13-5. Index Fossils Embedded in Two Sedimentary Rock ProfilesExplanation / Answer
8. If we slide column b to match the layer with letter F of b with that of a we can find that the layer F becomes the common layer to both of them, and it resides the top, thus it is youngest.
Similarly if we slide column b down to match layer P from both columns, it becomes the common lowest layer in both.
9. The rock layers which are common in both the columns are namely, F, Z, B and P.
10. The layers are marked by the presence of index fossils. Some of them are present in both the columns and the others are only present in column a or b. The explanation is quite simple, the fossils which are only found in one column must be endemic of that area from which that particular column has been generated, whereas the other fossils which can be found in both the columns can be inferd to have a global distribution or at least present in those two areas.
11. Cretaceous should represent the thickest layer of rock strata.
Silurian will represent the thinnest layer.
It is not the abundance of fossil which causes the for the rock layers, it is availability of sediments and also the time of accumulation responsible for the thickness of rock layers. Thus the time period which has more span would have thickest rock layer whereas the the time period with shorter span will have thinner rock record.