9 Voyages oldest rock

Oldest rocks on this Voyage

Congratulations you have now finished the Wye Valley Voyage, once you have answered the questions for this site.

These oldest rocks on the Voyage are called the Brownstones. They have an age of around 400 million years. This closer view of piece of Brownstones, shows quite coarse grains (zoom the picture). This indicates the sand that now forms this rock was deposited by water moving fairly fast.

The rivers that deposited the Brownstones would make rivers like the Wye and Severn look like mere streams in comparison. These rivers would have have produced extensive flooding of the flat semi-arid plain they crossed. The flood waters would have arisen from monsoon rains over the higher ground which existed to the north and west. See the weather forecast for a day in Brownstones time in the ‘then’ section.

 

Not easy to find!

The picture shows the location of the Brownstone exposure. Most of the rock here is probably not in-situ having slid from above, but at least this small exposure allows you to see what Brownstones sandstone looks like.

The Brownstones occasionally contain small pebbles, but nothing on the scale you have seen in the Quartz Conglomerate.

 

Better Brownstones exposure

This exposure of the same type of rock, is located on the edge of Ross-on-Wye town. Notice the criss-crossing pattern of bedding You already know this is called cross-bedding and it tells us the sediments were deposited by flowing water.

Many of the beds in the Brownstones contain scattered white pebbles. For pebbles to be transported along with the sand, the rivers must have had a strong current. In places there are curved shapes, these are cross-sections of river channels. The channels were eroded out of previously deposited sediment, only to get infilled later as the channels of the river change course. These rivers were responsible for depositing thousands of metres thickness of sediment. Those now lithified (cemented into rock) sediments form the red sandstones widely seen in Herefordshire, Black Mountains, Brecon Beacons and beyond.

The picture above has some of the cross bedding highlighted. The best way to appreciate this feature is to go and have a look at it close-up. You can park nearby but beware of the traffic on the road which passes the cliff. Nearby postcode is HR9 6AH.

 

Devonian sandstones

Sandstones, similar to the Brownstones (these are called the Senni Beds, after Sennybridge), forming the Black Mountains on the western edge of Herefordshire. The layer upon layer of beds are the product of river deposits occurring over some 15 million years. This location is featured in the Cat’s Back Voyage.

As you move up through the beds of the Lower (older) Devonian sandstones, the sediment becomes finer and eventually dies out all together. There are no beds here at all from the Middle Devonian time (393 to 383Ma). Sedimentation starts again with the Quartz Conglomerate (365Ma). So there is an unconformity (time gap with no rocks to represent it) between the top of the Brownstones and the bottom of the Quartz Conglomerate. The river deposits becoming finer and eventually stopping, suggests the area was undergoing an overall uplift. The likely cause is something called the Acadian period of earth movements, which resulted from a ‘gentle’ tectonic collision between the Avalonia plate (you are standing on part of it now) and the Laurentia plate.

 

The rocks you have crossed

Summary of the rocks in this area with their dip of around 15 degrees towards the east. Because of this dip the youngest rocks are at the surface around the beginning of the Voyage (under the car park), even though that is lower in altitude than the top of Little Doward.

The eastward dip of the rock also explains why we get the Gully Oolite at the caves and also present at the much higher altitude of the limestone pavement. The land surface on top of Little Doward – the hill fort area – also follows this same eastward dip of around 15 degrees. Think about the sequence of rocks you would meet along this Voyage if the rocks were horizontal, or dipping towards the west.

 

Final Score

Once you have answered ALL the task questions, you should get your final score for the Voyage as shown here.

Please submit your final score. The anonymous feedback this provides will help the DeepTime team improve existing Voyages and build new ones for other locations around the country. Thank you.

If no final score has been produced, you will have missed one or more Task questions. None flashing blue dots on the ‘list all’ buttons indicate all the questions for those Tasks have been answered. Flashing indicates one or more questions missed. No blue dot means that Task has not been attempted at all. Answer any missed questions on your way back.

 

Where now?

To get back to the start of the Voyage, follow the route in reverse.

You might like to venture onto the upper part of Little Doward (picture above). Follow one of the red dotted routes shown on the map. On the way you could have a look at the impressive crags of Quartz Conglomerate (see map) or use the app to help identify the rock types exposed in the many small quarries on Little Doward.

 

Task Site 9 Questions

The rocks visible at site 9 are?

a) Brownstones about 400 million years old
b) Gully Oolite from the Devonian Period
c) Tintern Sandstone 200 million yers

 

What type of environment were the Brownstones deposited in?

a) Near a coast
b) Extensive desert
c) Vast river flood plains
d) Deep under the sea