Clearly visible in the walls of the Inner Gorge are spectacular light-coloured rocks, such as the pink granites,1 which stand out starkly against the darker, metamorphic rocks2 (figures 1 and 2).The latter are former sedimentary and volcanic layers that have been transformed (metamorphosed) by heat and pressure during intense geologic upheavals in the past.That is a six-fold difference, for samples that should be of similar age.
This means that the laboratory testing was precise.
Among these volcanic layers are distinctive dark-coloured rocks called amphibolites3 (figure 3).
These were once flows of basalt lava, up to tens of metres thick (figure 4).
The crystalline rocks of the Inner Gorge are below the horizontal strata of the main canyon walls (after Austin, ref. The problem is that we don’t know whether these assumptions are reasonable (because they are not provable), and it is especially awkward for metamorphic rocks.
Geologists overcome this problem by “interpreting” the result.