St Joseph’s Section On the Lee Road between Inniscarra and Sunday’s Well it is easy to let one’s glance flicker between the dense forests and views over the Lee Valley. But along the way there is a car park directly opposite an impressive Catholic Marian grotto. The grotto offers some decent sedimentary rock exposures; however, it is the entrance to the Bon Secours Care Village that we will take a look at.
Approximately 30 metres from one end to another, the outcrop at St Joseph’s Section offers plenty to look at.
The cliff here is highlighted for both its extensive dimensions and fresh surfaces, but also because it shows the boundary between the Devonian and Carboniferous periods. These rocks belong to the Old Head Sandstone Formation, a suite of sandstones, siltstones and mudstones. The boundary itself is unfortunately not visible to the naked eye. Instead, scientists have studied tiny fossil spores preserved in this outcrop. Some spores were from the Devonian, while others belonged to the Carboniferous. These have then been used to pinpoint exactly when this transition between the two periods occurred here at the rock layers, and this transition can be traced globally. A few important things should be stated at this location. Getting close to the cliff can be dangerous due to falling rocks, so exercise care here. Further, this is the road to the healthcare clinic. Do not obstruct the entrance nor the road and be aware of any traffic coming in and out through the entrance. Facing the cliff, the rocks show a ‘marine transgression’ - a change from a land environment to a sea environment. Back at the end of the Devonian the seas started rising in the area and eventually what was once part of the floodplains became coast and eventually sea. This change is visible here. To the left, chunky red sandstone layers are visible (hinting at a land environment) and halfway and throughout we see alternating layers of thinner, dark, fine-grained sedimentary rock layers (hinting at marine environments).
28
Powered by FlippingBook