Geography of the Botanic Gardens

A wide range of aspects occur on the garden's site - allowing for a variety of planting conditions. The altitude of the site ranges from 10 metres at the lowest point of Deep Creek to about 60 metres in the south-west corner adjacent to the Princes Highway.

The topography of the site is gently undulating with slopes overall of about 1:15. Slopes are locally steeper, especially along the creek.

Flora

Trees species include Eucalyptus elata, Eucalyptus longifolia, Eucalyptus piperita, Eucalyptus globoidea, Angophora floribunda, Corymbia maculata, and Backhousia myrtifolia. There are substantial areas of Casuarina spp. and Acacia spp. understorey.

Rainforest shrubs and small trees such as Backhousia spp., Prostanthera spp., Pittosporum spp. and Elaeocarpus spp. form the understorey along Deep Creek, with ferns of many species (including some tree ferns) sometimes in dense patches.

Geology and Soils

A number of soil groups exist at the Gardens as a result of the sedimentary nature of the rocks in the area, notably siltstone, claystone, sandstone, quartzite and chert. The best rock exposures within the gardens are in the steep slopes of the west side of Deep Creek, in the northern quarter of the Gardens. These exposures include the full range of sediments typical of the Adaminaby Group and demonstrate the interbedded nature of the different rock types. They also include a vein of quartz generated during burial and compression of the sediments.

Exposures within the gardens show no significant rock types. There is therefore, no evidence the vegetation differences within the Gardens are directly related to the geology. Rather they reflect slope, aspect or soil differences, particularly where soils have been derived from weathering of the underlying rocks, or from material that has crept down-slope during the current erosional cycle.

Over much of the Garden site, soil is generally skeletal grey loam, 1 cm to 5 cm deep over gravelly clay. Nutrient poor soil derived from sandstone is apparent in some areas. On steeper slopes in the south-west of the Gardens, areas of stripped soil and gravel patches are common. Immediately adjacent to the major creek lines, deeper loam soil supports a rich flora dominated by large trees. Prior to the development of each section of the Display Gardens, a detailed analysis is undertaken of the soil to assist in the development of that Display Garden.

The Past

It is difficult to realise in the present quiet of the Gardens that more than 400 million years ago this was where two plates of the earth's crust were colliding. It would have been an earthquake zone as active as any in the world today.

To the west was a continental plate - the earliest Australia - to the east an oceanic plate. As the plates pushed together over many tens of millions of years, the lighter continental plate was forced to ride up over the denser oceanic plate, crumpling and faulting the rocks as this progressed. This resulted in the formation of a big mountain range, comparable with the present European Alps or even the Himalayas.

Parts of the present Great Dividing Range are the last sad remnants, the roots of the mountain range. Where the oceanic plate was diving down a deep submarine trough formed. Such currents are commonly triggered by the collapse during earth tremors of unstable, unconsolidated deposits on active continental margins. The presence of turbidites is evidence of the rising mountain range and the adjacent submarine trough in the earlier stages of the collision, one providing the source of so much detritus through erosion, the other providing space for so much sediment.

While the deep trough was developing on the continental margin, much further out in the deep ocean, different very fine-grained sediments were accumulating, together with basalt of the sort extruded at mid-ocean ridges.

The rocks within the gardens area are part of a widespread sequence of sandstone, siltstone and shale, possibly many thousands of metres thick, and called by geologists the Adaminaby Group. They extend as the name suggests, from the coast to the Snowy Mountains area. Deposition of the Adaminaby Group sediments took place over a period of perhaps 30 million years. Starting some 500 million years ago. The Adaminabv group deposits are called "turbidites" because they show the characteristics of having been deposited from turbidity currents. These are sea bottom currents, carrying large amounts of sediment in suspension and flowing turbulently down submarine slopes.

It is an indication of the extent of plate movement that some of these rocks now occur, thrust against the Adaminaby Group rocks, in an area from Batemans Bay, south east to Burrewarra Point, immediately east of the Gardens site. They are well exposed in the coastal cliffs. Like the Adaminaby Group sediments, these are folded and faulted, having been caught up in the later phases of the plate collision.

Kilometres underground, as the plate collision advanced, tremendous pressures and high temperatures were changing the rocks, melting some, resulting in deep granite intrusions and intense volcanic activity where heated rocks escaped through fractures to the surface.

The granite around Moruya, used for the construction of the pylons for the Sydney Harbour Bridge is one such intrusion.

Climate

The Eurobodalla region is characterised by a temperate climate, with cool winters and warm summers. Because the Garden site is located 5 kilometres from the coast and situated in a valley, it is protected from immediate coastal influences.

Temperature

The average temperature at the Gardens is colder in winter and warmer in summer, compared to adjacent areas on the coast. The Eurobodalla region is influenced by slow moving high pressure systems during winter, resulting in fine clear weather with sunny days. Although heavy rain can occur in any month, on average the period from May to September has relatively low rainfall. During summer low pressure influences, sometimes with severe thunderstorms, can bring heavy rains.

Frosts

There are on average 30 days with frost per annum at the Gardens, with 1 day in both May and September, 6 days in June, 9 days in August and 13 days in July.

Humidity

The humidity levels at Batemans Bay are relatively high reflecting the coastal influence. These humidity levels are the closest indicator of likely levels obtained at the Gardens, Batemans Bay being less than 5 kilometres north of the Gardens site.