Next to granite in abundance is an association of layers or beds of fine-grained rather soft, laminated rocks which may be called shale. slate, or schist and coarser sandy rocks called siltstone, sandstone. or quartzite (if extremely hard). These rocks were originally mud, silt and sand and they have been changed into hard strong rocks by intense pressure and heat. Originally laid down in the sea as horizontal layers or beds they were crumpled and folded and now their bedding is often nearly vertical. They occur in broad bands, often a kilometre or more in width, usually trending in a north-north-easterly direction.

One such band crosses the Summit road near Rawson Pass where it is partly exposed in a large gravel pit. It extends many kilometres in both directions, and is well exposed in another road-gravel pit north of Schlink Pass. In a few favourable locations, where the rocks have not been too much altered, fossils – graptolites – have been found which establish the marine origin and Upper Ordovician age of these rocks.

There is a much greater variety of rock types in the northern part of the mountains than in the south.

There are two large areas of cavernous limestone, at Yarrangobilly and at Cooleman, north of Tantangara Reservoir. Several very fine caves in the southern end of the Yarrangobilly limestone have been opened up for inspection, and are well worth a visit.

The limestone plateau further north in the same belt is also honeycombed with caves. Many have partially collapsed forming depressions in the ground surface called sink holes. Many streams flowing towards the Yarrangobilly River disappear underground when they reach the limestone, and reappear in the river gorge.

The Cooleman Caves are less well known and probably less extensive. Fossil corals and shells, usually not well preserved, can be found in places in these limestones which indicate that the rocks were formed during the Silurian Period.

The area extending along the Tumut River from Ravine through Lobs Hole to Talbingo, and across to the Snowy Mountains Highway, north of the village of Yarrangobilly, is of particular interest for the variety of rock types which occur there. There are grey shales (such as immediately west of Yarrangobilly village), intensely coloured purplish red shales, sandstone, grey and purplish red conglomerates composed of well water-worn gravel, and a variety of volcanic lavas, ranging from rhyolite to andesite.

Talbingo Mountain itself is probably one of the centres of eruption of these lavas; being thus the remnants of a very ancient volcano. On its western side lavas form very prominent cliffs. In the Ravine area there is as well a thick bed of marine limestone which, towards the top, contains abundant, well preserved shell-type fossils and corals, which establish the age of the limestone as Middle Devonian, considerably younger than, and quite distinct from, the nearby Yarrangobilly limestone.

The closely associated shales, red beds and lavas are probably Lower Devonian in age. Broadly speaking, these rocks are in the form of a shallow basin. They are in beds, which are usually only gently tilted, dipping inwards towards the centre of the area, forming the basin-like structure.