Visitors walking past schist rock at the opening of Flat Top Hill, Flat Top Hill Conservation Area
Landscape and geology
The schist rocks of Central Otago were once sediments, washed from an ancient landmass more than 200 million years ago. The mud and sands settled, compacted and hardened into large areas of sedimentary rock (mudstone and sandstone) on the bottom of prehistoric seas.
Heat and pressure from movement in the earth’s crust, transformed these sedimentary rocks over a period of about 80 million years. The result was a layered metamorphic rock schist, with high quartz content and a tendency to split. The name ‘schist’ derives from the Greek word meaning ‘to split’.
Schist underlies most of Otago, and at Flat Top Hill the rock layers have remained horizontal and uplifted, forming flat topped ‘block’ mountains, or in this case, hills.
Just beyond the reservoir are some paleosols exposed by gold miners' sluicing, dating back 20-25 million years. They display a striking sequence of colours, the brilliant red, yellow and white material contrasting with the dull olive browns of the current soils.
The same types of soils are only found today in tropical climate regions – hot and steamy with lots of rainfall and lush forest. Together with other geological, animal and plant fossil clues, this indicates the type of climate that once existed here.
In contrast, this area is now the driest part of New Zealand, receiving less than 300 mm of rainfall annually due to its location in a rain shadow of the mountain ranges to the west and south. At its lowest point, the edge of the Roxburgh gorge to the east, the land is 140 metres above sea level. It rises to 550 metres on the schist rock plateau before falling again in a series of terraces to about 400 metres at the car park.
Cushion plants on Flat Top Hill
The semi-natural vegetation of Flat Top Hill is important, being a substantial example of native species growing in the driest environment in New Zealand.
Over 182 native plants have been recorded here, more than the average New Zealand forest. Incredibly, thirteen of them are ferns. Features of the vegetation are a set of native spring annuals of the genera Ceratocephalus, Myosotis and Myosurus. The native cottonwood (Ozothamnus leptophylla), native daphne (Pimelea aridula), mountain wineberry (Aristolelia fruiticosa) and porcupine shrub (Melicytus alpinus) are characteristic of the numerous shrubs native to this semi-arid inland area of Otago.
Like the ancient soils, the area has undergone significant changes, most recently as a result of human intervention. Over the past 150 years, fire, the introduction of exotic pasture grasses, grazing sheep and rabbit infestation, have dramatically modified the plant communities of Flat Top Hill. By the 1990’s, Flat Top Hill had become a bare landscape, holding little more in vegetation than native cushion plant (scab weeds), lichens, yellow succulent stonecrop and introduced thyme. Woody weeds such as briar, broom, gorse and wilding pines also became established.
Since being designated a Conservation Area in 1992, sheep and rabbits have been removed from Flat Top Hill. As a result, native grasses have recovered, overgrowing thyme and replacing large areas of cushion plants. Amidst refuges in the rock outcrops and tors, shrub land plants are re-establishing themselves. In time, Flat Top Hill should revegetate to a tussock shrub land system, with a mix of native and introduced flora and fauna.
Each schist rock and tor supports its own ecosystem of lichens, mosses, ferns and shrubs. In turn, these are host to distinctive insects and lizards, such as the Otago gecko (Hoplodactylus sp. Otago).
The semi-natural grassland and cushion fields within and on the crests of Flat Top Hill support many diurnal insects. Wasps, ants, beetles and moths, including the rare Alexandra chafer beetle, (Prondontria modesta) are found here. A special feature on Flat Top Hill are several small saline sites. Weathered from the schist parent material in an environment where the low rainfall allows minerals to accumulate rather than wash away, are small areas of salty soils. These soils host special salt tolerant plants such as Atriplex buchananii, usually found by the sea, and Puccinellia raroflorens, found only in the tidal banks of Stewart Island and some saline areas of Central Otago.