GEOLOGICAL HISTORY
Today’s Waitakere Ranges are the extensively eroded eastern flank of the Waitakere Volcano dating back to the early Micocene period between 22 and 15 million years ago. In that period, the Australia and Pacific tectonic plates collided and a massive volcano, called the Waitakere Volcano was pushed up out of the sea.
The Waitakere volcano was 60 kilometres across and 40 kilometres from north to south, making it the second largest volcano that has ever erupted in New Zealand. Two lines of volcanic vents began erupting on the uplifted eastern flanks, one of these along the west coast. Striking features of the west coast formed by the volcanic vents include the cliffs at O’Neills Bay.
In some places magma flowed out onto the slopes of the volcano, producing flows under the ocean that created pillow lava, an example of which can be found at the south end of Bethells Beach.
About 20,000 years ago when sea level was about 130m lower, the Waitakere River and its tributaries flowed seaward through a deep valley now buried beneath the beach. When sea level rose to its present level about 7000 years ago, the lower Waitakere Valley would have been flooded by the sea, forming a small harbour with arms extending up the Wainamu and Mokoroa valleys. Subsequently, long shore drift has dumped sand across the harbour entrance and some has blown inland and dammed a number of valleys to form lakes including Wainamu and Kawaupaku.
LAKE WAINAMU
Lake Wainamu is a classic example of a dune lake. It stretches about one kilometre in a westerly direction but is less than 200m wide.
Lake Wainamu formed during past 6500 years, when the Wainamu Stream Valley was dammed by huge dunes of dark iron sand that were slowly blown inland by the prevailing wind.
The existing dunes are still moving slowly into the lake but the dune stabilization programmes along the coast have stopped the supply of more sand.
Three streams provide most of the water flow into the lake. One of thesepours over the Waitohi Falls at the head of the lake. Another stream flows out of the lake at the north-western end but tends to dry up in summer. The valley here is filled by a huge amount of loose dark sand.
The surrounding steep hills are mostly covered by native forest. There is a small amount of pastureland on the northern side of the lake.
Pied shag, little shag, black swan, and duck can often be seen on the lake. Australasian bittern and North Island fernbird also live around the lake. Native freshwater fish in the lake include banded kokopu, inanga, common smelt, common bully, grey mullet, longfinned and shortfinned eel.
Exotic fish recorded at the lake include catfish, perch, rudd, tench, and goldfish.
Lake Wainamu lies within the Auckland Centennial Park and is a popular recreation spot. Dogs are not permitted at the lake.
THIS EARTHCACHE
To claim a find on this earthcache you will need to visit the published coordinates and the surrounding sand dunes and answer some questions. Your answers must be sent to the cache owner before you log your find online. Don't visit at night as you won't be able to see anything and therefore you won't learn anything!
1. Measure the height of the face of the sand dunes at the end of the lake.
2. Walk down to the water's edge at the base of the dunes and using your GPS unit, measure the elevation of the lake, and take a reading of your coordinates at the edge of the water. These coordinates will be used to track movement of the dunes relative to the lake.
3. Explore the top of the dunes and find what you believe to be the highest point. Take a reading of the elevation and the coordinates.
4. (Optional) Please take a photo of the lake from the sand dunes, and also a photo from the highest point on the dunes, including your GPS unit, if possible. These photos should be uploaded with your online log.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
Happy Earthcaching!!!
Greenblat