116 THE AUSTRALIAN NATURALIST. 
scale. This jointing is quite different from faulting, as there 
is no displacement along the joint plane, and as it is generally 
a result of intense pressure there is no separation even in the 
case of a master joint. Below the vertical cliffs is a gently 
sloping hillside well covered with trees. It is really. a scree 
heap which is renewed more rapidly than the fragments can be 
carried away by stream action, and is a testimonial to the in- 
stability of the massive cliff formation. The cause of this in- 
stability will appear later, when we examine the few thin beds 
of soft shaly character at the bottom of the Canyon. 
The track down to the Canyon follows the direction of a 
small tributary stream, after an initial steep drop, whereby 
we descend. through a very hard rock stratum, taking advantage 
of the widening of a master joint by atmospheric denudation. 
The walls of the gap show complex patterns standing out in bold 
relief, indicating highly mineralised sandstone from which the 
portions with softer cementing material have been fretted away 
by wind and rain. Excellent examples of rock sculpture on a 
grander scale can be found below this level in a little to the 
left of the track. A further drop brings us to the lower reaches 
of the creek, and as the volume of water is now restricted on 
account of a greatly reduced catchment area, the valley has 
enough soil of a clayey character to secure an excellent growth 
of ferns and tree-ferns. That erosion is still in progress, how- 
ever, is evident from the exposure of great root masses. 
In a short time we reach the Canyon itself. A track down- 
stream leading to the Beauchamp Falls is well worth following, 
as the Canyon becomes very narrow and the perpendicular walls 
tower upwards to a maximum height. Near the Falls we can 
read the secret of these vertical walls. In spite of their thou- 
sand feet of hard sandstone they have “feet of clay” (or shale), 
and this thin bed of Narrabeen shale is at the same level as 
the top of the scree-heap mentioned before. 
However, most of our story will be best unfolded by a walk 
upstream. A little above the junction the water appears to flow 
from under a great mass of solid rock; but closer examination 
shows that this rock is a fragment fallen from above, and that 
the stream is slowly but successfully removing an obstacle, 
which has blocked its flow for many generations. Further up- 
stream we shall find many of these massive fragments, some as, 
large as a cottage, which have fallen from the very top of the 
cliffs and form more or less “temporary stoppings.” The epi- 
