Upland swamps (dells) on the Woronora Plateau, N.S.W.

The surface of the Woronora Plateau, in the southern Sydney Basin, coincides with the top of the Triassic quartzose Hawkesbury Sandstone. It is etched by broad, shallow valleys which are the headwaters of deep gorges. These upland valleys are sediment-choked, swampy and treeless. They resemble the 'dellen' (dells) described by early German geomorphologists, and the 'dambos' of southern Africa. On the Woronora Plateau dells occur at 450-550 m elevation, in an area with 1300-1600 mm annual average rainfall, mean annual temperature of 17 degrees C and an annual average excess of rainfall over evaporation of about 850 mm. They occupy between 2% and 9% of the catchments of streams dissecting the plateau, being most extensive and most numerous where the Hawkesbury Sandstone is least deeply dissected. Erosion of the plateau supplies a sandy detritus to the dells. Because the streams in the dells have small catchment areas and gentle gradients, they do not flush all the sand into the lower valleys and gorges; thus, sediment accumulates. Sediment washed into the dells is differentiated during overland transport. On the sideslopes, the sediments are characteristically coarse-medium sands with low organic content. In the valley axes, the organic content rises very abruptly (to more than 10% organic carbon) and the sediments are fine-grained. The waters of the dells are acidic and are low in dissolved oxygen, organic carbon and silica. The vegetation is dominantly sedgeland with some heath. It is differentiated in patterns which reflect sedimentary and hydrological variations within the dells. Some of the dells display linear patterned ground which resembles that found in fens in the Boreal zone. The patterns are reinforced by biotic factors (particularly vegetational differentiation and crayfish activities) but appear to be initiated by slow, near-surface flow of saturated sediment. Basal dates of sediments in the dells span at least 17,000 years. As they are not clustered, sedimentation in the upland valleys has not been triggered by any single, regionally-effective environmental change. Indeed the old sediments closely resemble t.hose which are presently accumulating, which indicates that environmental change since the late PleIstocene has not been dramatic. It is suggested that the sediments now infillinq the dells are the most recent accumulations in vaIleys which are episodically flushed when intrinsic geomorphic thresholds are reached. Fires followed by severe storms probably trigger these erosional events.