There are many famous rock arches around the world, however, the Hunter has its very own impressive arch tucked away between the rocky peaks of its western ranges
Welcome to Arch Back
The return walk should be easily completed in an hour from the 4x4 carpark, or two hours from the 2wd carpark leaving 20-30 mins to check out the two waypoints. Make sure you bring enclosed shoes and probably long pants as the regrowth from the fire is a bit dense in some sections. Head to Bushwckrs "Yellow Rock' to check out the view.
WP1 The Arch
From here, head along the northern side of the ridge to Reference Point 01 marker. There is a rough path through the regrowth that wanders to the southern side of the ridge, then heads north in a rock clearing to the next Reference Point 02. Head down through the break in the small cliff then to the obvious 'point' overlooking the arch. There is a short awkward step that some may need help with, then along into the mouth of the arch at the listed coordinates:
Take a walk around and you will find that it crosses through a narrow ridge. As you would have seen from driving up here and at the view at Yellow Rock, Broken Back Mountain is fairly flat on top compared with the steep escarpments to the valley. This ridge is particularly narrow with wind erosion shaping the ridge as seen in the diagram below:
Its tricky to tell how long it has taken for the arch to form as there are a few factors, in particular the strength and composition of the rock. Inside the arch, you can see a variety of textures including the softer fine grained honeycombed sandstones up high and the conglomerate sandstone rock elsewhere. Think of sandstone as if it were concrete, made from fine grained cement/sand and larger grained gravel. Too much fine grained material OR too many gravel pieces will make the mix weak and easy to erode. The conglomerate here is a good mix of fine grained material (matrix) and larger pebbles (clasts) which keeps the arch strong.
WP2 Sandstone Cave
Head back the way you came and back to WP2. Simply, the cave here is made from a fine grained sandstone (think of a fine beach sand) stuck together by smaller grains of clay. Because the sand and clay grains are quite small, they are easily eroded and carved by the wind. You will see the delicate honeycombing inside the roomy cavern , a classic wind eroded feature (see above). The hole in the top of the cave that you can enter through is an eroded weak, particularly soft clay (shale) layer:
Take note of the texture and feel of the sandstone, inside the cave and the clay (shale) layer.
To complete the Earthcache, email me an answer to the following question:
Although both Waypoints are carved by wind, the difference comes down to the composition of the sandstone. Describe the difference (of grainsize/texture) between the two waypoints and how it has shaped the appearance/structure of both locations.
Take a photo of your team/self/personal object like a GPS standing inside the arch and upload with your log to complete the Earthcache!
The answer is actually in the text and should be obvious when you visit both locations! You may log a find straight away, but please email me the answer within a couple of weeks to avoid your log being deleted.
Happy Caching!