Mt French is 9 km west of the sign-posted turnoff just south of Boonah. The mountain peak is 579 metres above sea level.
The ancient, volcanic peak of Mt French is recognised not only for its unique shapes and as a favourite bushwalking destination but also as a remnant habitat of key conservation value within south-east Queensland. To help preserve the environment from pests there is a Pathogen Hygiene Station provided for you to wash your boots. Please use it.
Geology:
This distinct peak of Mt French had its origin deep below the belly of a volcano - the ancient Main Range volcano which erupted some 24 million years ago. The eastern flank of this volcano once spread across the Fassifern Valley, probably as far as Mount Maroon, Boonah and even to Ipswich, while on the western side it spread to at least Warwick.
It erupted mainly basalt lavas which may have been as thick as 1000 metres near the volcano's crest. Prolonged erosion over 20 million years has removed mainly the eastern side of the volcano. The relatively steep slopes and greater power of the east-flowing streams rapidly eroded the volcanic lavas to create a retreating escarpment (long, cliff-like ridge), which is now the edge of today's Main Range.
As the volcano and some of the underlying rocks were eroded away, any plugs, dykes or sills composed of resistant rocks - particularly those of trachyte and rhyolite - remained as steep peaks.
Rhyolite is a felsic (silica-rich), extrusive igneous rock that is the volcanic equivalent of granite. The chemical composition of rhyolite is very like that of granite.
The peaks of Mount French consist of multiple gently inclined intrusive sheets (or sills) of white rhyolite. The sills were formed when magma was forced between cracks and weaknesses in horizontal layers of sedimentary rock. These sheets were then eroded over millions of years to leave a flat-topped plateau surrounded on most sides by cliffs. Vertical columns formed from the cooling and contraction of the sub-surface magma.
Today rock climbers value these vertical columns for the numerous climbs of varying difficulty. 400 routes have been established by climbers. Some have initials painted to name them. To the rock climbing world, the northwest cliff of Mount French is known as ‘Frog Buttress’, internationally renowned for its crack climbs (vertical rock crevices). Most of the cracks are smooth sided and parallel. The buttress is small by world climbing standards: 400m wide, average of 50m high in the middle and 20m at its ends.
Getting there:
Access to the mountain top is via sealed road. Mount French has a picnic area with picnic tables, barbecues, toilets and non-potable tap water. Bring your own firewood or use a fuel stove. Firewood must not be collected from the park or roadside - fines apply. Day use is free. Check local websites for overnight campsite booking information as fees apply. Visitors are asked to take their rubbish away with them. Leave the geo-animals at home.
Remember to take only photos and leave no trace of your visit. We hope you enjoy your visit to this fascinating and ancient geological area.
You may log this Earthcache straight away but then please email your answers to the questions to the CO. We will contact you ONLY if your answers need extra work. Logs with no answers sent will be deleted. The answers can be found by observing the landscape, a little research and using a feature of your GPS.
Q1. Are the columns hexagonal, round, square or of irregular shape?
Q2. Which of the following is NOT a form of rhyolite? obsidian, hornblende or pumice
Q3. Look closely at the rhyolite in the cliff face. Is it fine-grained or rough-grained?
Q4. Use your GPS to take an elevation reading at the base of the cliff at S27 58.963 E152 37.296. Record it. Make your way to the top of the cliff and take an elevation reading at S27 58.973 E152 37.302. Record it. What is the height of the cliff at this point?Put all three records in your answer please.