In order to answer the question posed at the end of Chapter 4.1 we need to set up an arbitrary coordinate system where the x-axis is horizontal and the z-axis is vertical (Figure 6).
At point P the vertical stress will be the lithostatic stress that we calculated in Chapters 3.4 and 4.1, but lets round the calculated value off to 40 MPa for the sake of simplicity. Parallel to the x-axis is our modest tectonic stress which, following Davis and Reynolds (1996), we will assign a value of 20 MPa. In order to calculate the stress at point P produced by the given conditions we need to calculate the tractions acting on all possible planes that pass through this point! Though such a task seems daunting at first glance, with a little thought and the aid of a modern day computer, the task can be reduced to a few minutes of work.