|Examples of some rocks that make up the Armidale beds|
The big sample at the front has been partially turned into silcrete.
The Armidale beds are comprised of fluvial (river) and lacustrine (lake) deposited sediments. These are principally conglomerates, siltstones, sandstones and shale. Interestingly, the shales are laminated possibly as a result of seasonal deposition in a lake. They also contain abundant plant fossils. The material that makes up the sediments is particularly obvious in the conglomerate. The conglomerate clasts are derived from the older underlying geology, for example clasts of jasper, quartzite and granites.
The Armidale beds occur in small remnant areas (the balance of the beds having been eroded away). These remnants occur throughout the Armidale area but almost as far away as Wollomombi to the west, near Dangars Falls to the south-east and Kellys Plains to the south-west. Voisey 1942 named this area Armidale Lake which is a palaeo-lake that only exists now in the sediments that it left. The formation of Armidale Lake occurred either before or during the volcanism that ended up covering a majority of the Armidale region in blankets of basalt lava (lavas of the Central Volcanic Province). In fact, the Armidale beds were preserved by this blanket of basalt both directly and through metamorphism beds in the area of contact. This metamorphism of the Armidale beds created a layer of hard silcrete (once known as greybilly) which itself was resistant to erosion and helped preserve the underlying un-silcretized sediment from being washed away.
The picture above is an example of the Armidale Beds that occur near the Armidale garbage disposal centre. A very accessible example is located on Madgwick Drive on the way to the University. It is actually one of the best remaining exposures of the unit and has been used for years by local schools and the university. Indeed, Banaghan & Packham 2000 have the road cutting as a stop on their Armidale-Yarowych geological tour.