Mt Walsh Granite EarthCache
Hidden : 3/25/2014
Difficulty:
2.5 out of 5
Terrain:
4.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:


Mt Walsh Escarpment.


The Rocky Bluff of Mount Walsh looms over the town of Biggenden, a few kilometers to the north. It is formed by a single body of granite. The granite and some of the volcanic sequence are within the Mount Walsh National Park, and more volcanics are included in the adjacent forest reserves to the west.

The first rocks erupted were thick layers of pyroclastics and various volcanic derived sediments. At some stage there was a truely gigantic explosion, with unimaginable volumes of hot, fine lava particles and gas ejected into the air. This huge fluid cloud eventually collapsed and consolidated on the ground to form a layer 200m thick, with the material so hot that it welded together again to form a hard, tough rock.

The blasting upwards of such a huge volume of material caused the former underground magma chamber and the rocks above it to collapse downwards; to form a huge oval depression surrounded by a ring fracture. This depression was about 40km long and 25km wide, extending from near the present Mount Walsh to well west of the Ban Ban Springs. Most of the welded tuff accumulated within this depression.

Despite such a massive eruption, further magma rose to the surface, with domes of rhyolite erupted and intruded within this depression. (eg. Coongara Rock and adjacent similar hills).

There was then a major surge of magma from below where the whole ground surface bulged upwards. Much of this magma remained below ground level to slowly cool and consolidate to various types of granite. Mount Walsh is one example of a very fine grained granite called aplite or granophyre formed at this time.

Since those distant times, erosion has removed much of the volcanic sequence in the depression, re-exposing some of the older basement rocks beneath. The granites that were once below the surface have been revealed, and being more resistant than other rocks, have remained as prominent peaks, such as Mount Walsh.

As erosion gradually removes the weight of the rock above the granite, stresses are released, allowing the granite to expand upwards and crack along fractures roughly parallel to the surface. Vertical joints crack the horizontal slabs, and weathering and decomposition proceeds down them. Where the fractures are sparse, large dome like masses of rock remain as resistant monoliths, slabs or pinnacles above the rest of the landscape.

Weathering down the fractures concentrates on the sharp corners and edges. The process eventually converts the horizontal slabs into accumulations of rounded boulders. The phenomenon results from water and air penetrating minute fractures in the rock and initiating chemical decomposition, assisted by repeated expansion and contraction of the surface skin from daily heating and cooling.


The Questions to answer.

For you to claim this earth cache you will need to go to the published co-ords and three other Way Points answer the questions below.

At the published co-ords you will find an information sign
Q1. How long ago should you have been here?
Q2. What is the giant depression called?

At 'The Big Boulder' way point.
Q3. What colours can you see on the rock?

At 'The Slabs' way point.
Q4. What colours can you see on the rock?
Q5. Describe the surface texture of the different colours.
Q6. Why do you think there are different textures?

At ' The Summit' way point.
Q7. When you look to the South and South-West, the mountain has many more cracks visible and has more 'broken' terrain. What process has caused this?
Q8. Why has this process happen up here and not lower on the mountain?

Please include the GC code and name of this cache when you email your answers. You are more than welcome to include photo’s of yourself at various locations on the Mountain but please do show photo's at the waypoints which will give answers to the questions.

The Information Sign.
You will find an information sign at the published co-ordinates. There is a lot of extra information about Mount Walsh National Park, it's history, how various people view the park, a hiking warning and what general behavior is expected of visitors. Please read this before proceeding on the hike to the summit.

The Hike.

The hike to the summit will take between 1.5 - 2 hours for most people. Then you will spend a little time up there and then the return trip down. Allow 3-4 hours minimum for this hike.

Do not forget to take a torch / headlamp, water, sun protection, first aide kit and anything else you will need on a serious hike such as this.

If you are struggling by the time you reach this sign then do not continue as the track only gets steeps and more challenging the higher you get ... and there there is the scramble up the slabs and the gully climb at the top.

Due to the rugged nature of the park and lack of reliable water, extended walking and access are limited.

The park is without developed walking tracks and other major facilities in order to maintain this wilderness.

Open fires are not permitted
Do not feed the wildlife
Take all rubbish and scraps with you. ( remember to CITO )
Leave your pets at home



Information Source: "Rocks and Landscapes of the National Parks of Southern Queensland" by Warwick Willmott. 2004

Additional Hints (No hints available.)



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