Foliations or Layers?

Sandstone - Wirral Cheshire

What can be read from the rocks is a very variable thing.  The presence of any sort of lineation across the surface of a rock can imply all sorts of things and the unwary could be caught out by the subtleties.  The brick in a building in Cheshire shown above has two very specific lineations - 1 natural, 1 man made.  Dealing with the man-made first, there are regularly spaced diagonal lines running steeply bottom left to top right.  These are the marks made by whatever instrument (A saw?) cut this block of sandstone to its desired size.  Man made, neither a foliation nor a layer, but the unwary may be taken in by the defined regularity - regularity tends to be the invention of man rather than nature.  The lineations that interest the geoscientist are the less regularly spaced ones that for the most part run in the opposite plane to the saw marks. These are layers and are of varying thickness and colour, and closer inspection will reveal differences in grain size too.  But perhaps the most striking thing about this brick is that the plunge (slant) of the layers undergoes an abrupt change to a less steep angle about two fifths of the way down the brick.  After this the layering seems to gradually peter out.  Like I said, this is nature - it does not feel the need to conform to the human desire for regularity.  The rocks this block came from were sedimentary, and this rock speaks of the ebb and flow of tides and deltas.  These rocks were formed in rivers depositing mainly sand and gravel detrital material in channels to form river terrace deposits during the Triassic Period (252-201 million years ago).  Just in these few inches of rock, the depositional environment seems to have gone from a relatively still tranquil one to a more aggressively tidal one with graded layers stacking in the characteristic herringbone pattern known as cross stratification.  The current would have been from left to right in the top section of the brick.  

Foliated Dalradian rock from Dunkeld, Scotland

So looking at this second piece of rock, layering seems abundantly clear - the exposed and weathered right hand edge seems to give us a clue - the layers can be seen as clear as pages of a book.  Can they though?  Like the one above, this rock was once a sediment.  This rock however has been through process' that the first has not.  Buried, heated, contorted, folded, pressurised, and only now raised back to the surface for our inspection.  It is far older than the previous one and everything it has gone through, including its original deposition is etched into it.  The red arrow indicates the planes of foliation - minerals in the rock have aligned themselves in this plane in response to extreme pressure.  The blue arrow on the other hand indicates the more subtle lineations of the original layers - Barely discernible, but there for all that.  Look long enough and they are apparent.

Garnet Mica Schist - Strath Ardle

This rock has a sort of lineation too.  The lineations flow like silvery glinting  flakes of filo pastry around the rounded raisins of the Garnets.  Don't try eating this though. This rock has been through even more brutal burial, heating and pressure than the previous one.  There are no sedimentary layers left, only foliations, and the minerals themselves have altered, metamorphosed to give it its correct title, into new minerals that are stable in such extreme conditions.  Mica, the shiny flaky mineral is quite common and there were probably some microscopic flakes of it in both the two previous rocks - here it is the dominant mineral.  The other mineral garnet is also a high pressure specialist starting to nucleate in the metamorphic process even before the flakes of mica - hence the mica appearing to have flowed around them.

Cross Stratification in Quartzite

This rock brings us full circle - well sort of.  This is Quartzite - the result of sandstone like in the first photo, being heated and subjected to pressure.  The rock produced is white/grey, very hard, but no longer composed of grains like the sandstone.  It has a slightly sugary texture and close up turns out to be tiny interlocking quartz crystals.  There is often a degree of foliation - a pointer to the direction of pressure, and just occasionally there is what we have here, just feintly visible - cross stratification - a testament to its sedimentary origins.  


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