Education and Careers

Q: I am involved in a treasure hunt type activity known as geocaching (http://geocaching.com) and am trying to set an Earthcache (www.earthcache.org) at this location, but am not finding it as straight forward as I had hoped. Although I have a basic grasp of how sandstone is created and what gives these cliffs their distinctive colour (iron oxide I believe), I am struggling to find any information specifically relating to the following questions:

1. I know that sandstone is a sedimentary rock, but are you able to say how it got here and how long ago it was formed?

2. What causes the obviously harder layers of rock between the sandstone layers and why are they sloping?

3. How would the iron oxide which causes the red colour come to be present here?

4. At a number of locations along its path locally, the River Almond is vertically sliced by igneous dykes, causing the river to narrow and forming ideal places to build bridges across it. Can you explain briefly how these dykes are formed?

1. I know that sandstone is a sedimentary rock, but are you able to say how it got here and how long ago it was formed?
   
   Yes. The rocks you have here are Devonian in age, 416-359 Million years old. They were laid down on the edge of the Strathmore Basin, a precursor of the modern Midland Valley, bounded by the same big faults that were on the whole more active then than now. I am not sure exactly where the photos were taken (my map to hand is not that detailed!) but I can tell you that they are fluvial sandstones, consisting of mudstones and coarser sandstone layers that are more resistant to weathering. The sandstones may be laterally discontinuous (they may pinch out as you trace them sideways). These rocks were laid down by a braided river system draining the Highlands, which were then very high mountains.
    
2. What causes the obviously harder layers of rock between the sandstone layers and why are they sloping?
   
   See above – the hard layers are coarser materials and are cemented by percolating fluids after deposition. They are sloping (geologists say that they “dip”) because after deposition (when they would have been essentially horizontal) they were involved in earthmovements that bent and folded and faulted them across a wide area. Many of these sandstones, when looked at under the microscope, will be seen to contain fragments of volcanic ash. This reflects the existence of local volcanoes in the vicinity at that time.
    
3. How would the iron oxide which causes the red colour come to be present here?
   
   The red sandstones and mudstones were laid down at a time when the UK was situated near the equator. The environment would have been extremely hot and (mostly) arid, characterised by flash floods from the older mountains to the north, creating these sediments in an outwash fan – rather like the environments we see today fringing the Himalayas. Vast barren floodplains traversed by braided sediment-choked streams would have presented a harsh and oxidising environment for minerals, and would have resulted in the release of iron oxides in large quantities. This is what gives these rocks their red colour.
    
4. At a number of locations along its path locally, the River Almond is vertically sliced by igneous dykes, causing the river to narrow and forming ideal places to build bridges across it. Can you explain briefly how these dykes are formed?
   
   There are several sorts of thin, sheet-like igneous bodies running through these rocks. These are all more resistant than the sands and muds and as you say, form constrictions in river courses. There was contemporaneous volcanic activity in the area (see above) so some of these are dykes and sills (vertical/subvertical and horizontal/subhorizontal respectively) intruded into the sediment after deposition (sometimes before it became hard) and also some lavas. Lavas of course were laid down at the same time as the sediment, so are concordant with bedding and have clearly differentiated tops and bottoms.