Hillsbrae Cemented Gravels
Figure 2b: Cross section of the piedmont, in the valley of the Mitta River at Hillsbrae.
Hardpans also occur in the gravelly high level river alluvium, just below the full supply level of Lake Hume at Hillsbrae (Figure 2b). At these sites, quartzose gravels and sands in a matrix of fine sediments have been cemented (Photo 11) to a condition which has resisted the erosive force of the fluctuating water levels in Lake Hume since the higher levels were reached in the 1950’s. The gravels often overlie weathered gneiss, and at this site (Site No. 9), where the undercut of the high water level has exposed red-brown texture contrast soils, the strongly structured clay subsoil may be formed in the weathered gneiss or piedmont slope sediments.
 Photo 11: Pliocene river gravels at Hillsbrae in the Mitta Mitta River valley west of Tallangatta. These coarse gravels have been in-filled with fine sediments and have become cemented. This site is just below the full supply level of Lake Hume. |
These cemented gravels usually have a well developed polygonal crack pattern which has been emphasised by the greater resistance of the indurated crack-fill material to the wash-erosion of the lake water (Photo 12). The material within the blocks enclosed by the crack pattern, is orange-red and clearly much more ferruginous than the more resistant crack-fill material. This infill material appears to be similar to that of the Tallangatta Hardpan. Examples of this feature may be seen on the Lake Hume foreshore in the Tallangatta Caravan Park (Site No. 4) when the lake levels are low. These materials also appear to be similar to those described by Chartres and Costin (1981) from the Upper Shoalhaven Valley in south-eastern New South Wales, which they interpreted as alluvium subjected to cracking prior to burial by younger sediments.
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