Fluting, Lapiez and Solution pits at Wairere Boulders,
now a Nature Park in Horeke, Hokianga   
Copy of News Letter Article, Institute of Geology and Nuclear Science

Summary:  The “Wairere Boulders” at Horeke is the name for a Nature Park created in a part of the Wairere valley. There is an agglomeration of hundreds (if not thousands) of huge basalt boulders covering an area which is 1800 m long and can reach a width of around 250 m.  The valley is not only of geomorphic interest and stunning to view,  but also the weathering on the basalt surface shows flutings or lapiez and solution pits of outstanding size and beauty.


The Formation of The Valley

The Horeke basalt flow which sourced its lava from the lake Omapere region in the Kaikohe/Okaihau area, covered the land (around 2.7 mio. years ago) which forms now the catchment area of the Wairere stream which discharges its waters into the Hokianga Harbour, about 1 km to the west of Horeke.  There are still existing remnants of the basalt layers at the headwaters of the Wairere stream.  Also, numerous boulders scattered all over the farmland are the testimony of the extent of the original flow.

The “Magik Rock” (see picture on right, Picture 1, and Picture 4), which can be seen from Taheke Road, demonstra- tes clearly the altitude (ca. 180 m) and thickness (ca. 12 m) of the initial flow in the area.  Parts of the original flow, which broke off,  were gravity transpor- ted towards the valley cen- tres where they accumulated into a “chaotic” mass.  The most accentuated area of this agglomeration,
Picture 1:  "Magik Rock", indicating original location and thickness of lava flow known as Horeke Basalt. Erosion on slopes will progress and cause boulders to roll towards the middle of the valley.  This boulder will come….  The question is:  When?
or of this “gathering” is the valley floor of the Wairere stream, the bottom end before it reaches the tide (see aerial photo on left, Picture 2). 
Picture 2, top: Extent of the “Wairere Boulders”. Aerial photo covers about
1800 m x 350 m

Picture 3, right:  View downstream the Wairere Boulders towards the Hokianga Harbour.  A “chaotic” mass of boulders “littering” the valley floor. The stream is hidden “underground” but can be heard.


The view from the platform over the valley shows the chaotic mass of boulders, marching like a prehistoric army down towards the Hokianga Harbour.  The boulders are of an average 
diameter of 3 to 5 m at the bottom end of the valley,  but can reach a maximum dimension of about 30 m in the middle and towards the top of the “heap of rocks”.

 


Picture 4:  View across the Wairere Boulders, direction north-north-east.
Picture 4, on the left shows clearly the remnant of the Horeke flow creating the horizontal plateau on located on the right. The boulder on the left (the “Magik Rock”, see Picture 1 and 4) shows the level and thickness of the flow.  Huge boulders gravitated 
towards the centre of the valley,  where they seem to hold their “geological conference” in close contact with each other.  Their further movement is blocked,  unless constant stream erosion lowers the floor of the valley further down in vertical direction and allow the boulders to follow.
  
The Surface Texture of the Boulders: Lapiez, Solution Pits, Karst, Karren, Clints or Runnels

  
Already in 1948 J.A. Bartrum and A.P. Mason (1) have discovered the interesting surface erosion on basalt boulders in the Hokianga, which they classed as “Lapiez” and “Solution Pits”.  In 1952 A.P. Mason (2) gives a description of the Horeke flow (page 363) and of its petrography (page 365).  The form of erosion met at the Wairere stream is rather uncommon and the geologists seem to take over the terminology used for limestone to describe the phenomenon.


 
Picture 5:  Fluting on basalt boulders sitting in paddocks.  

The boulders shown in the paper (1) look similar to the ones on the left.  The paper indicates a distance between the runnels of around 120 to 150 mm

and a depth reaching an average of about 50 to 75 mm.  There are many rocks showing erosion of such type which gives the valley a rather “wild” and outstanding look, see picture 5.  

 
 
Picture 6:  Very accentuated fluting reaching a distance of 400 mm between runnels and depth of up to 750 mm.  

More recent discove- ries have revealed that fluting in the Wairere Boulders can reach a far greater magnitude.  On this well formed boulder, distances between runnels are around 400 mm and the depth can reach up to 750 mm (see picture 6, on left).  Visitors to the valley

have spoken about depth of up to 1200 mm, but so far we could not yet find such a place. The variety of different shapes and forms is virtually endless and an amazes viewers and admirers.  


The Cause for the Surface Erosion



Picture 7:  Fascinating surface erosion on some of the basalt boulders.  The runnels can have any direction possible.

The only indication of a possible cause for these lapiez and solution pits on the Horeke Basalt boulders can be found in the paper written by J.A. Bartrum and A.P. Mason in 1948(1). They ascribe the lapiez-like forms,  which developed on broad steep faces of 
blocks of rock, to down flowing consequent streamlets fed by rain-drip from overhanging forest trees. Dissolved organic acids have caused solution of the basalt along the courses of the streamlets. 
  

Picture 8:  Process still continuing, the runoff flows over the ground in paddocks and bush, before discharging over the edge of the boulder.
  
The above discussed kind of erosion can be found in places where rocks are eroded on all sides.  They often display an acentuated ridge formation.  Unfortunately no studies on this form of erosions was done since 1948, or recent years, despite new technology which may have been a help to further explain this uncommon occurrence.

Sometimes rocks are leached just on one side.  Therefore, further assumptions are that in certain places the acidic nature of the soil has 

contributed to the leaching of the surfaces. It is also uncertain in how far acidic gases from volcanoes generating acid rain could have contributed to the erosion. 


Picture 9:  Vertical rock face shows different leaching than the undersurface.
  

Sometimes rocks show different kinds of erosion on different surfaces.  This is probably due to the non homogenous structure of basalt rock.  The picture on the left shows clearly quite accentuated and deep runnels along the vertical face and a different form on the underside, looking like standing waves in suspended animation. Also 
here the process is still continuing as -during heavy rain- the water pours down the runnels.  At the end of these,  the water runs along the underside to find a protruding spur,  where it can break the adhesive force holding it to the wet surface and drip down to the ground.


Picture 10:  Boulder turned during gravitational transport to valley floor.

Often boulders are tossed and turned around on their way down towards the valley floor.  This can be seen, looking down from the platform (picture 3). 

It is quite obvious that the narrow horizontal runnels (in picture 10) had once a vertical position and that the left, almost vertical face,  was probably the horizontal top of the boulder.  The now newly, horizontally positioned runnels are decreasing in size towards the right,  which might be an indication that the flow lost its acidic nature during the spill over the side. Unfortunately there is little known 

about the ph level of today's rain that dissolves organic acids while dripping and running through the canopies of Kauri forests. Nor do we have any indication of the ph levels of the soils on top of the Horeke Basalts or of the underlay of the flow.  All these data would been a great help to figure out what nature has produced in the last 2.7 mio years.
There are many more questions about the formation of the valley and the cause of the surface erosion unanswered.  Like for instance:
  1. Why can a fluted basalt rocks sit right besid unfluted ones.  
  2. Why are boulders in higher locations (altitude) not fluted?
  3. Do all basalts under a Kauri forest cover show the same features, and if not, in what way does the composition (or whatever) differ?
  4. Is there thin section evidence of solution taking place (a slice through the outer few millimetres of a flute base should show it quite nicely)? 
  5. Do only Kauris have such an acidic "aura"?
  6. How porous are the basalts?

The list can be continued.  Some questions might be answered but are unknown to me.  

It was suggested to find:
  • someone who could do a B.Sc honours project on the subject (proposal by Dave Smale)     or
  • someone who would study the weathering as a fourth year project for a graduate student. (proposal by Mike Isaac)
  
Since October 2000 the Wairere Boulders have been "transformed" into a Nature Park,  which is now open to the public.  We, the owners (Rita & Felix Schaad) have during 3 years built 22 wooden structures, and dug diverse tracks in the valley which are mostly metalled.  
     Scientists and nature lovers can now enjoy this soul-rocking encounter with boulders on a leasurely and easy walk,  which leads over, around, between and under the boulders.  There is a small loop walk for which you need about 1 hour, but if you want to walk all tracks you need around 3 hours. 
For more information about the Nature Park see www.wairereboulders.co.nz 
Picture 11:  Going over boulders.
  
Picture 12:  Young geologist, ducking under boulder.
  
Picture 13:  Dragon's Cave shows the underside of a huge boulder.
 
Picture 14:  Close encounter of the 5th kind.  Experience the Wairere Boulders.
 
Picture 15:  Bridges lead from boulder top to boulder top Picture 16:  Lod bridge spans the Wairere stream, sitting on a boulder on each side.

(1) back to text  J.A. Bartrum & A.P. Mason, Lapiez and Solution Pits at Hokianga, New Zealand, The N.Z. Journal of Science and Technology, Vol. 30, Sec. B, No 30,  1948

(2) back to text A.P. Mason,  The Geology of the Central Portion of Hokianga County, North Auckland, Transactions of the Royal Society of NZ Vol 81, Part 3,  Pages 349 – 374, Dec 1953

 

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