Stratigraphy
Stratigraphy is a complicated name for the study of rock relationships. To understand the geological history of the earth, geologists make maps of the rocks that we can see on the surface. The types of rock within an area are identified and their ages determined. How rocks of different age formed and how they relate to the older and younger rocks around them are also investigated and recorded. Geologists then build a picture called a stratigraphic column using colours, textures and symbols to communicate the local geological story.
A stratigraphic column is usually drawn vertically with the oldest rocks at the bottom.
Using the colours from the book, a stratigraphic column for the current surface geology found throughout south-east lutruwita/Tasmania looks like this:
A stratigraphic column is usually drawn vertically with the oldest rocks at the bottom.
- Colours are used like a code to group rocks by age and type
- Textures are used to indicate things about a rock – perhaps it is coarse and sandy or it has visible layers; perhaps it is made of crystals.
- Symbols are used to mark points of interest, like fossil beds or relationships like faults.
Using the colours from the book, a stratigraphic column for the current surface geology found throughout south-east lutruwita/Tasmania looks like this:
This column is based on mapping information compiled by Mineral Resources Tasmania available online at www.mrt.tas.gov.au
Over time, the geology of an area can become quite complicated as changes occur such as the deposition of new rocks, erosion of existing rocks or movement of rocks due to faulting. The early pages of Before the Mountain Had a Name show layers building up as sediments are deposited in the Permian, Triassic and Jurassic. Basalt erupts in the Jurassic as well, alongside the intrusion of dolerite, which is why this time period has two stratigraphic colours (pink and black). By the Cretaceous faulting has caused different locations, even ones quite close to each other, to have different surface heights. Erosion removes rock rapidly from the higher places and layers like the pink Jurassic sediments are stripped away - almost completely disappearing - exposing the older rocks beneath. At the same time, new rocks are deposited in river valleys which then get extra layers - ones that are not found on the nearby mountain tops. Depending upon where you stand on the surface, the rocks beneath your feet may include some or all of the rocks shown in the column.
It may also occur to you to wonder - what about the time before, Before the Mountain had a Name? What lies underneath the Permian sediments? There are definitely older rocks down there but, unfortunately, in southeastern Tasmania, these older basement rocks are completely covered by younger layers. The only way to access them is by drilling a hole down through the overlying rock. This has been done at a small number of locations with results suggesting that deep beneath the mountain are rocks that look similar to older layers found further to the west. Exactly how old the mountain's basement rocks are, and exactly what their history is, is not known (yet). It is something we need future geologists to keep working on in order to find out.
Over time, the geology of an area can become quite complicated as changes occur such as the deposition of new rocks, erosion of existing rocks or movement of rocks due to faulting. The early pages of Before the Mountain Had a Name show layers building up as sediments are deposited in the Permian, Triassic and Jurassic. Basalt erupts in the Jurassic as well, alongside the intrusion of dolerite, which is why this time period has two stratigraphic colours (pink and black). By the Cretaceous faulting has caused different locations, even ones quite close to each other, to have different surface heights. Erosion removes rock rapidly from the higher places and layers like the pink Jurassic sediments are stripped away - almost completely disappearing - exposing the older rocks beneath. At the same time, new rocks are deposited in river valleys which then get extra layers - ones that are not found on the nearby mountain tops. Depending upon where you stand on the surface, the rocks beneath your feet may include some or all of the rocks shown in the column.
It may also occur to you to wonder - what about the time before, Before the Mountain had a Name? What lies underneath the Permian sediments? There are definitely older rocks down there but, unfortunately, in southeastern Tasmania, these older basement rocks are completely covered by younger layers. The only way to access them is by drilling a hole down through the overlying rock. This has been done at a small number of locations with results suggesting that deep beneath the mountain are rocks that look similar to older layers found further to the west. Exactly how old the mountain's basement rocks are, and exactly what their history is, is not known (yet). It is something we need future geologists to keep working on in order to find out.
