100' high Shaggy Cliffs (East Otago) Traditional Cache
Hidden : 6/2/2014
Difficulty:
2 out of 5
Terrain:
3.5 out of 5

Size: Size: small (small)

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Geocache Description:


At the southern end of Shag Beach, there's a grassy knoll with a lovely view of the beach and spectacular hundred-foot-high cliffs. Sit and rest and reach behind you to find this one litre snaplock.

The cache is just under an hour's walk from the Andersons Lagoon carpark, and about 35 minutes from Shag River mouth. See 3-hour Shag Beach Saunter and ANDERSONS LAGOON to SHAG RIVER WALK MAP for the route description and waypoints.

The Shags

Ngai Tahu appropriately named this river Waihemo, dwindling water, since the flow drops to a trickle in summertime. But early sealers and whalers sailing up the coast renamed it the Shag after the spectacular breeding colony of Spotted Shags ( Stictocarbo punctatus, parekareka) on the coastal cliffs. If you visit during spring or summer, those ledges of harder sandstone in the cliff face are very shaggy indeed!

This is one of four main Spotted Shag breeding areas in Otago (Nugget Point, Taiaroa Head and Heyward Point are the others). There are perhaps 5,000 to 10,000 breeding pairs in NZ, a third of these in Otago. Spotted shags feed in deep water up to 16 km offshore, diving for small fish and squid. At the Shag they breed during spring and early summer, incubating 3-4 eggs for 31 days and feeding chicks another 28 days to fledging. Outside the breeding season they form large feeding and roosting flocks of up to 2000 birds.




The Cliffs

When we go caching in the Silverpeaks or Central Otago the rocks we see are Haast schist, the basement rock of Otago. Here in East Otago, the schist is still down there underneath everything, but 700 metres of various sedimentary rocks overlie it. The top layer is these impressive hundred foot high cliffs along Stony Beach and Shag Beach, formed from Goodwood Limestone. Can you spot any fossils?

Goodwood Limestone is part of the same Otakou Group calcareous sandstone as our local Caversham sandstone. See this mapped in orange on GNS Science New Zealand Geology Webmap visit link. Otakou Group rocks show up all along the Otago coast from Caversham and Tunnel Beach up to All Day Bay, Bushy Beach at Cape Wanbrow and even a bit north of the Waitaki. They also form the seafloor a long way out to sea. How did this rock end up as the top layer?

Around 85 million years ago the micro-continent Zealandia began to move away from Gondwana as the Tasman Sea opened. The schist basement rock weathered down to form a nearly flat peneplain.

From 75 – 65 mya meandering swampy river systems created gravel, sand and peat deposits which became the Taratu Formation (named after Taratu Rd in Kaitangata where the lignite is mined). But, Zealandia had a sinking feeling… as the new landmass cooled, it sank and the sea gradually covered it. The Taratu sandstone and coal seams became buried under layers of sand, mud and shells.

As Zealandia continued sinking, overlying marine sedimentary layers kept accumulating for the next 40 million years. They are often named after the Dunedin suburbs where they are most easily seen – eg Abbotsford Mudstone (200 + m thick), Green Island Sand (45 m), Burnside Mudstone (55 m).

By 25 million years ago New Zealand was just a chain of islands, about a fifth of today’s land area, and was still sinking. We would be caching underwater now - but luckily a new tectonic plate boundary was developing through New Zealand. This gave us an uplifting experience, which accelerated once the Alpine Fault formed 15 million years ago. (Even so, only 5% of Zealandia is above water today.)

As land reappeared above the sea it began to erode. Between 25 to 15 million years ago thick layers of sand were deposited in the quiet seas off East Otago, together with shells of tiny marine organisms. This formed 230 metres of calcerous sandstone – our Caversham Sandstone and, along the coast north of Waikouaiti, impure Goodwood Limestone. Offshore, deposition of similar sediments continued until about 2 mya. From here inland to the base of Puketapu, the limestone has been lifted above sea level, and the sea has eroded it to form these wonderful cliffs. Apart from the layers of harder sandstone which have formed those very convenient shag nesting ledges!

There is one final layer to the geology – if you look inland to Puketapu or south to Bobby’s Head, they are formed of basalt from the Waipiata Volcanic Field (now included as part of the Dunedin Volcanic Group) between 21 to 10 million years ago.


Information:

• GNS Science New Zealand Geology Webmap visit link Otakou Group rocks (Caversham sandstone and Goodwood Limestone) are shown in light orange. Click on the map to see further information.
• GNS video "Pushing New Zealand's boundaries: the tectonic evolution of New Zealand over the last 65 million years" visit link
• Otago Rock and Mineral Club visit link & Bushy beach field trip visit link
• Geology of the Waitaki Area visit link
• “The Dunedin Volcano” Jane Forsyth

Additional Hints (Decrypt)

Haqre byq srapr cbfg. Ernpu haqre bireunatvat cvar oenapurf.

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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