Task Site 6
Task Site 6 is a good place to view the landscape features produced by past ice ages. Task 6 does not have to be done in any specific location, but after this site it is well worth venturing further towards Black Hill and beyond.
Quaternary Ice Age
You will recall that three factors control the landscape of the Olchon Valley. After the Devonian geology, the second most influential factor is the much more recent events of the Quaternary Ice Age and its aftermath, 400 million years later. The diagram above shows the maximum extent of the most recent glaciation, the Devensian peaking around 25,000 years ago.
The second diagram, shows the much greater extent of the earlier Anglian glaciation. The Anglian glaciation which peaked around 450,000 years ago, was probably the glacial event responsible for carving out the valleys of the Black Mountains.
The name ‘Devensian’ of the most recent glaciation has nothing to do with the name ‘Devonian’ given to the solid rocks of the area. Devonian is named after Devon, where rocks from this period were first studied. The name Devensian is derived from the Latin Devenses, people living by the Dee (Deva in Latin), a river on the Welsh border near which deposits from the glaciation are particularly well represented.
The Quaternary Ice Age consists of a series of glacial and interglacial events. The Anglian and Devensian are two glacial events and currently we are living in an interglacial. Whether there will be another glacial event in the future and to what extent human created global warming could counter this cannot be determined. Overall the Quaternary Ice Age has lasted some 2.6 million years, with glacial events lasting around 100,000 years and the interglacials for a much shorter time. The cause of these cycles is considered to be variation in the solar radiation. Prior to the Quaternary, the planets climate had been relatively stable for tens of millions of years.
In common with other valleys in the Black Mountains, the upper Olchon Valley has a distinctive U-shape profile, indicating erosion by former glaciers.
Most geologists interpret the Cat’s Back as a glacial arête (narrow ridge), created by converging masses of ice in the Olchon and Monnow valleys. This glaciation is likely to have last occurred at least 450,000 years ago. It is known as the Anglian Glaciation. The image below shows how the two glaciers may have appeared.
Towards the head of the Olchon Valley there is a distinct lump protruding from the side of the valley. This feature is a glacial moraine, a pile of boulders and finer material left by the glacier that once occupied the Olchon Valley.
A glacial feature present in the Olchon Valley, is the extensive ‘solifluction’ deposits mantling the valley slopes, and giving a distinct, smooth, concave-upwards profile to the valley sides.
These solifluction deposits were formed by soil creep and flow under very cold periglacial conditions (the kind of conditions found nearby a glacier or shortly after it has retreated – very cold with frozen ground, but the surface ice has gone), probably about 20,000 years ago in the most recent episode (Devensian) of the Ice Age. They can be metres thick, covering the bed rock below.
Streams cut into periglacial deposits
Notice how the streams on the lower slopes have incised (cut deeply) into these periglacial deposits, which are deepest at the foot of the slopes and are even softer than the mudstone bedrock below.
View from near Black Hill
Southern view along Cat’s Back clearly showing the hard beds of sandstone in the Senni Formation and the calcrete of the Ffynnon Limestone.
Active evolving landscape
The main phase of landslip activity seems to have occurred towards the end of the last Ice Age, although intermittent slippage, rock falls, and slow flowage is still observable today as small scale features within the Olchon Valley. It is still an active, evolving landscape.
Final Score – applies only when using Voyager app
If you answer all the task questions, you will receive a screen like the one shown here, along with a button to optionally submit your score. If the screen does not show, it is because you have missed one or more questions. Task Sites with incomplete answers will display a flashing blue dot on the respective Task buttons, displayed when you select ‘list all’.
You can have another go at all the questions by selecting ‘reset Voyage’ under the ‘reset Voyage’ options in the ‘help’ section. Second or more attempts will incur a reduced maximum score. Even if you delete and then download the Voyage again, the app will remember you have already had a go at the answers for that Voyage.
Task Site 6 Questions
The shape of the Olchon Valley indicates it was formed by:
a) desert conditions
b) glacial action
The Devensian glacial occurred:
a) 25,000 years ago
b) same time as Devonian rocks
c) 400 million years ago
d) 400,000 years ago
Geologists describe the Cat’s Back as?
a) a steep climb
b) a narrow ridge
c) an arête
d) a spine ridge