23 
Cretaceous Stratigraphy of Lower Murchison River Area^, 
Western Australia. 
this track the country is impassable lor motor vehicles of the ordinary 
type. 
JL GENERAL ({EOLO(iY. 
The Cretaceous rocks fall readily into two divisions; a loiver of 
sandstonoj mostly reddish and cross-bedded, and an upper of softer, more 
easily eroded rocks, such as loosely cemented saiulstones, shales, and 
chalk. As already noted the Miireliison River enters a deep sandstone 
gorge at the point whei’e it leaves the Ere-Cambrian rocks north of 
Ajanu. Some glimpses of this gorge can be obtained from the road leading 
from Ajana to Murchison I louse Station, particularly in the vicinity of 
Pine Thicket rain-shed, about Id miles XAV. of Ajana and about the 
same distance S.E. of dleaiiarra Hill. The valley here is shallow and 
wide and the river o(^cuj>ies an over-dee[)ened gorge of which only the 
u[>per part is visible from the rain-fihed. This gorge apparently continues 
to a point south of Aft. Curious, hut farther west the valley widens and 
erosion has (-ut more dee[)ly into the lower sandstones all the way from 
Itottie Crossing to the sea shor'^. 
On the north-western side of the rivei', in this last ]>art of its course, 
the strata di]> to the X.W. Occasional dips of as much as o"’ have 
been measured, l)ut these are exceptional. X(t dips were imaisured on 
the south-eastern side of the river. 
South-east of the river in this part lh(‘ belt of ('Xj)osed sandstone 
is not moie than a mile wide, and in many places it is less, whereas 
north-west of tlu* river it is i-arely narrower than two miles, and, in many 
places is as much a.s four miles wide. This suggests a regional north- 
westerly dip. 
One of the most noticeable features of the sandstone l)elt on both sides 
of the river is the jointing. AVe noticed the presence of many parallel vertical 
joints but did not have time to study this fe.a.ture more closely. The iullueiice 
of jointing on the topogi'ai)hy becomes very clear on inspection of the aerial 
j)hotogra])hs. 
Text Fig. 1. 
Geological section the Murchison valley from Meanarra Hill 
to the vicinity of kSecond Gully (for location see Plato II.). 
The strike of the joints i.s between and 140®. Over wide areas 
differential erosion has taken place along them and cr)use(pienlly the out- 
cro]'>s are channelled by iuniiimerahlc paralh*] furrows. On the ])hot(jgraphs 
it can 1)0 seen that they are fi’om about 200 In 400 feel apart. Often they 
can be followed from decjily erod^ul into less (U'oded comitry, where the^^ 
may only ho indicated as sti-ips of .slightly denser vegelalion. On the gTound 
we observed the presence of numerous open joints with their walls ns much 
as four feet apart. Tu many places it seemed that they were due to tensional 
movements rather than to erosion. 
