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26 Oct 2022

Mount Tomah's underground secrets

The beautiful floral displays at the Blue Mountains Botanic Garden are not just due to the talents of our horticultural team – what lies beneath also plays an important role.

As you wind your way down the spiral walkway of the Blue Mountains Botanic Garden Mount Tomah, admiring the stunning displays of blooming proteas, waratahs and grevilleas as tiny honeyeaters flit through the foliage, you may notice the decorative grey feature walls, artfully constructed in a dry-stone manner. These hexagonal-shaped boulders are made from basalt, hewn onsite – a wonderful repurposing of natural materials.

Volcanic origins of a lovely landscape

While most of the Blue Mountains region is made from sandstone – think of the stunning escarpment at Katoomba, where the Three Sisters is the most prominent landmark – basalt is a feature in some of the higher altitudes in the western Blue Mountains, including Mount Wilson, Mount Irvine, Mount Banks and Mount Tomah.
An igneous rock, the basalt was formed in the middle Miocene period, between 17 million and 14 million years ago, and is a remnant of a volcanic lava flow – hence the dark grey coloration.

But as attractive as they are, these basalt boulders are not just decorative – they are actually responsible for the rich fertile soil of Mount Tomah, and the reason why the eucalyptus forest, rainforest and cool climate shrubs and trees of the Blue Mountains Botanic Garden grow so well.

Soil matters

The basalt cap soils – clay loam coloured deep red by a high iron-oxide concentration – are rich in phosphorus and have a higher level of moisture retention than sandstone soils, while the joints in the basalt rocks trap water that trickles out in springs. Combined with the cooler air from the altitude and higher rainfall due to the topography, it creates the perfect conditions for lush English-style gardens.
It's fitting, then, that the concentric circle of the Rock Garden is such a focal point in the overall design of the Botanic Garden, as it is a reminder of the connection between the natural and human-constructed elements of the garden, as well as representing the life force of the site itself.

“The intended outcome of the reinterpreted, revitalised Rock Garden is to provide visitors with a memorable destination experience as well as an appreciation of what’s involved in designing and sustainably managing a garden with rocks as the primary landscape feature,” Geoffrey Britton, the Blue Mountains Botanic Garden’s original landscape architect, said during an interview with the ABC.
So next time you’re meandering down the pathway, soaking up the views and admiring the floral displays, think about what lies beneath, and its power of creation.

Learn more about the Garden's rich and unique history on a Guided Walking Tour, every Sunday and the first and third Wednesday of each month.