Every Stone Tells a Tale

In preparation for the arrival of our grandkids (and their mum and dad) I have prepared a walking track down to the river where they will be able to build a dam, bounce stones or (if they’re lucky) splash me.

I sat down on the raised stony beach and donned my geology hat – not that I’m in any way an expert on geology, but the hat fits.

The stones here are greywacke, originating in the Kakanui Range and swept down by the surges of the Waianakarua River.

Few have any interest in stones, rocks or geology other than to know that stones and sand are the main component in concrete, or our roads sit on a bed of them and oh yes, ‘build your house on rock not sand’.

The beach-stones, pebbles, sand and silt actually do not originate in the Kakanui Range, because if you look at each stone, pebble or fragment, you notice that they are made up of sand and rock particles.

I’m pretty much peeing against the wind here because the theory of continental drift does not sit well with my left-handed logic. On a map, it all fits nice and neatly but with the forces of erosion, the shapes would not even closely match after a long geological period.

Ever driven a car with a wheel that’s out of balance? If all the landmass was concentrated in one lump, the spinning globe would be similarly out of balance.

The movement of tectonic plates however is obvious, one plate forced beneath another, causes quakes and a remodelling of the landmass, uplifting on the upper plate and perhaps melting or fracturing of the downward heading plate.

The greywacke pebbles I held in my hand were originally layered sandstone sitting under pressure at the bottom of the sea.

Hang on though, if it is layered sandstone, where did the sand come from?  Well there must have been a landmass - scientists call it Gondwana - that eroded to form the greywacke sand. I suppose it is possible for rocks underwater to erode and that could have happened, but what’s the possibility of Gondwana being uplifted from some other sandstone originating in a previous geological time?

All of this is difficult for me to get my head around, the most difficult being time. How can the passage of a million years be imagined? I know 13.5 kg of Radiata Pine seed equates to one million seeds, but time is unfathomable to me – and what can happen during that time.

How long does it take for a piece of bedrock to be reduced to silt? What sea pressure is required to force sea mud/sand to bond and become a hard rock?   

The highest point on the Kakanui Range is Mount Pisgah at 1643 metres, but the peaks must have eroded since they were uplifted from the bottom of the sea – that was surely a violent and turbulent time!

We sit on the top river terrace (under water at some time), some 60 metres above the river, but the alluvial terrace on the south bank is barely 10 metres high extending out to sea forming the farming flats known as the Waianakarua Settlement.

All this alluvial material has been created either by the widening the catchment or the eroding of rock outcrops.

The peaks of the Kakanui Range are not bare rock, so erosion (at least nowadays) has a battle with vegetation, which binds the rock together.

It is a slow process. Bits of rock are broken off when water collected in little cracks freezes [expands]. Cracks may be formed when heat from the sun cause the rock to expand and contract. Wind and rain can work their magic – uprooted trees exposing the rock or landslips doing the same.

However it happens, the riverbed fills with rocks that have been broken off with sharp edges, and by the time they reach my part of the river, they are worn smooth – or mainly smooth.

Few of the rocks are bigger than a loaf of bread and all are different. There are the dark, almost black ones that are made of fine silt and are quite hard – still, looking carefully the layers are there.

There are the lighter grey variety and in them there may be minute black flecks – maybe of the dark kind, second-time-around. Nearly all have a quartz band, often running through at right angles, or obtuse to the layering – how does that happen?

There are colours; green and orange, some with square (not river-worn) bits in the conglomerate mix.

Regularly, but not frequently there are purple rocks, seemingly the same material, only purple – or pink, depending on your recognition of colour.

The pebbles on the beach, are all different– if only they could talk.