Sunday, November 22, 2009

Return to Orchid Bay

Today the kayak tribe returned to Orchid Bay
on the Beecroft Peninsula.

Tuesday, November 17, 2009

Change is the only constant.

Paddling out into the Tasman Sea between the North and South headlands it is curious to imagine that about 10,000 years ago it was not possible to do this from the southern side of Port Jackson.

According to C.F Laseron (1954), in the last post-glacial period South Head was contiguous with Middle Head, forcing the Parramatta River to empty out via Bondi, over what is now the low-lieing sand flats of Rose Bay.

Water draining from Middle Harbour still escaped at this time via the Heads.
And there was a minor channel running straight out through what is now the Manly Corso.

It wasn't until the landbridge between Middle Head and South Head was eroded away, along with the closure of the Bondi passage by alluvial deposition, that the single harbour entrance that we know recognise as Port Jackson was formed.

Sow & Pigs reef is the submerged echo of the Mosman / Vaucluse landbridge.

Diagram sourced from C.F Laseron (1954).

North Head is predominantly composed of Hawkesbury sandstone, however it also contains lenses of shale and volcanic intrusions - as can be seen forming the leading edge to the rock platform on the lower left side of the image above.

Paddling into Salmon Cove, there are two very distinctive geological features.
Firstly there is the vertical chasm left by the erosion of an igneous intrusion. Secondly there is the striking block-like nature of the sedimentary strata, mid-way up the cliff face.
The headlands of the Sydney coastline typically have rock platforms at their bases. These platforms are destructive barriers to most vessels yet they provide convenient landing stages for nimble kayaks - when the swell is low.

Tony landing his fibre-glass hull on the barnacle-encrusted rock platform of Salmon Cove.
Ideally our kayaks would have a re-inforced ridge line on their hulls to withstand the abrasive action of this local condition. Note also the igneous remnants of the Salmon Cove dyke beyond Tony.
Although the sandstone cliffs seem permanent and enduring they are in a constant state of slow motion erosion with the occasional dramatic release of massive blocks. It is this constant mutation over thousands of years that leads to the circumstance described previously where the harbour's entrances have opened and closed.

A twenty five tonne iron-infused block of sandstone that has fallen out of the cliff's face and neatly fractured along a single bedding plane.

More recent examples abound.
A headcrusher that looks as though it fell as recently as last night.

The broad rock platform on the north side of Salmon Cove displays the distinctive north-east parallel fracturing which is so characteristic of the sedimentary rock around Sydney Harbour.

A lense of shale.

Meanwhile, Derek was paddling southwards from Collaroy to rendezvous at Salmon Cove.
Derek appearing from the horizon ...

Snorkelling amongst the boulders one hopes that another section of cliff is not about to release itself from its 200 million year bond.

Sea garden.

Patterns of weathering.

Some of the sediment contains the fossil remains of Glossopteris.

Striking lichens.

Temporary visitors.

Northern track = Derek's passage from Collaroy.

Southern track = Tony & Luke from Point Piper.

Sunday, November 8, 2009

King prawn

Capt Pete writes:

After meeting up at South head, Derek, Tony and Peter decided against an open ocean adventure as an easterly swell and wave action created a messy sea outside the Heads. Despite that we ventured around to the Oasis and then turned back around South Head to Chowder Bay with the thought to undertake some rolling practice and search for sea horses.

Not much luck on the seahorse front unfortunately, however we almost succeeded in getting Derek rolling. With chilled torsos and little UV on offer, Tony kindly shouted us all a coffee and Derek thoughtfully brought a stash of chocolate croissants. Eventually when Peter's fingers returned some colour, we headed back north.

Tony grabbed this prawn off the water which seemed to have surfaced like a miniature submarine. It seemed to be skipping along the surface in some sort of predator luring act of self sacrifice! Placed back on the water it continued its bizarre behavior!

King T with his latest sea catch.