7 Things are changing
Approaching Task Site 7
If you have background sounds on, underwater sounds have been replaced by the sound of water flowing – rivers.
Why do we have no bird sounds, insect sounds, animal sounds, traffic sounds?
None of these sources of sound existed on the planet when the rocks beneath you formed around 360 million years ago. On the land there were lifeforms like millipedes and spiders, but they are unlikely sources of much sound. Take a look at life in the ‘then’ section.
What is happening here? The lower beds are full of pebbles. The higher (younger) beds contain no pebbles. The upper beds, which are named the Tintern Sandstone, contain no pebbles. The lower pebbly beds are called the Quartz Conglomerate, which you will see a lot more of at the next Task site.
We have a definite change here from rocks formed in marine conditions to ones formed on land – terrestrial conditions.This change from terrestrial to marine conditions, took place over several million years. Very slow in human time, but reasonably quick in geological time.
These are slow rivers
The Tintern Sandstone was laid down by rivers, but the size of the particles or clasts being carried in the river was fairly small, compared with those you will meet in a moment. Smaller particles suggest rivers that are not flowing very fast.
The beds in this picture are just a couple of metres above the very pebbly beds which you will meet next. The rivers would flow slower if they were nearly at sea level. The sea level was rising at this time – remember you have just moved off younger marine rocks. The red colour suggests that a lot of the sand in these rivers was being swept in from the land where the iron mineral haematite was present. Haematite often forms in hot desert-like conditions. Check out the global mean temperature for this time (around 360 million years ago) – touch the clock (in the app) to get at that information.
The cross bedding feature, picked out in the overlay, is characteristic of sediments deposited from a moving water or wind. Sand deposited from wind forms cross bedding in sand dunes. In this example, we have cross bedding formed by deposition of sand from flowing water – see Task Site 8 for more on cross bedding.
Towards Task Site 8
Task site 8 is quite close to 7. As you walk between them you will get an ‘Environment Change’, with the background audio becoming loud, in keeping with the high energy environment around 365 million years ago.
Remember as you walk along this section of the Voyage you are moving back in time.
Task Site 7 Questions
Are the beds of Tintern Sandstone older or younger than the Quartz Conglomerate beds?
What can you say about the Quartz Conglomerate grain size compared with the Tintern Sandstone above it.
How similar is the dip of the beds here to the other locations you have seen so far?
a) Steeper and in the same direction
b) Steeper and in the opposite direction
c) About the same steepness and direction