This section of cliff cuts through one of the old river channels that would have existed in the area some 40 million years ago. The base of the cliff is a very dense mudstone and shale.
You can clearly see a line between this and the upper layers of medium to coarse grained sands. This line, or “unconformity” is the ancient river bed, where water has eroded down into the bedrock. While the water was flowing, large blocks of the bedrock were torn away from the river bed and deposited in the channel. These are called “rip up clasts” and can be caused by flood events or the sides of the banks of the river collapsing.
These flood events and bank collapses would have brought in mats of vegetation, perhaps plants that were growing along the riverbank, which were buried by the sands. Overtime, these turned into the thin seams of lignitic coal we see in this section.
The photo to the right shows the rip up clasts in more detail from this section. You can see that they are long, wedge shaped pieces of rock that seem to overlay each other like scales on a fish. This is what we call “imbrication”, and this can help geologists tell the direction of the current in the old rivers.
The clasts also form spaces and traps on the river bed where the current slows down and as such material can build up. This is likely why we see the small lenses of coal / plant material around these clasts. In the main sands of the channel fill, plant material is extremely broken up and finely distributed with a lack of volume to allow coal formation.