Practising science: Reading the rocks and ecology
Practising science: Reading the rocks and ecology

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Practising science: Reading the rocks and ecology

1.7.3 Explaining the observations

Having made and reviewed our observations, we are now in a position to interpret them – why are the rocks the way they are? The sedimentary strata that we see in Figure 16 were likely to have been deposited in essentially horizontal layers, so why is one set tilted and the other horizontal? To answer this we need to think about the processes that account for each feature and the relative timings of these processes. So, the strata exposed below the cliff have undergone deposition and tilting. The strata exposed high in the cliff have undergone deposition, but have not been tilted. The rocks with the more complicated history must be the older of the two sets of strata.

A sequence of events that accounts for the observations is as follows.

  • 1. Deposition of sediments as horizontal layers. (Details about the environment in which these strata were deposited could be discovered by closer study of the rocks, using observations of the nature of the grains, sedimentary structure or fossils.

  • 2. Compaction and cementation of the white sediments to form solid sedimentary rock.

  • 3. Tilting of the white strata. This could have been accomplished by powerful movements in the Earth, involving the rocks being compressed from the sides, to produce folds (Figure 18).

Figure 18
Figure 18 Diagram showing how initially horizontal strata (a) may become folded and hence tilted (b). The directions of the arrows in (a) show that compression is taking place

Activity 10

Assuming that the rocks were not completely overturned, in which direction (left or right) would you walk to find the youngest tilted strata?

Answer

The youngest layers lie on top of older layers (the principle of superposition introduced in Section 1.4.3), so these will be to the far right on Figure 16.

  • 4. Weathering and erosion to expose the tilted white strata at the Earth's surface, prior to deposition of the younger strata.

  • 5. Deposition of sediment onto the uneven eroded surface of tilted white strata. The hummocky contact between the two sets of strata that we spotted at the end of Section 1.7.2 is therefore the original land (or sea floor) surface when the younger strata were first deposited.

Activity 11

What term is used to describe the contact between the tilted and horizontal strata?

Answer

It is an unconformity (Section 1.4.4).

  • 6. Compaction and cementation of the younger sediments to form solid sedimentary rock.

  • 7. Formation of vertical joints in the horizontal strata.

  • 8. Erosion to produce the present land surface and coastal exposure.

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