Mudstone and memory.
Penny Dunstan 
Did you know that rock has memory? In its form and shape, rock remembers long-distant times. This is a story about a rock type known as mudstone, which interleaves between coal measures in the open-cut coalmines in the Hunter Valley.
Stone is made from even older stone. Mudstone is made from ancient pasts, weathering, washing and dribbling down Permian mountain sides to lay on ancient forest floors, where it formed soils to grow the giant forests that made the coal seams we now mine. Mud from ancient soils was carried by heavy rain into salty lake bottoms between bands of flattened forest, where, with aeons of resting, drying, squashing and squeezing, it was pushed into layers and transformed by deep time into rock. That rock rested as deep as 500metresbelow the surface for around 260 million years, until its slumber was interrupted by humans looking for coal. Great seams of rock from the Permian age were broken up as the coal was retrieved. The layered mudstone was blasted, dug and transported to the non-working side of the open-cut void to form Anthropocene mountains of mine overburden.
But does rock remain rock once the pressure of layers of time are removed, once the stopwatch of creation has been set back to zero? Free of the weight of history, the stone is now jumbled boulders with air spaces and water voids. Mudstone remembers it was once soil, full of fungi and bacteria, plant roots and humus. It remembers that it was once living and breathing, generating food for countless flora and fauna. The stone again shifts form, disintegrating in a geological blink of an eye(between three months and two years), forming a substrate of small particles, a 260-million-year-old approximation of soil.
But it is not soil.
Decomposed mudstone is both highly saline and highly alkaline and this makes it unable to host fungi and bacteria required for plant growth: it is also chemically hostile to most plant roots. Even when topsoils rescued from the working side of the pit are spread, carpet-like, over the degraded mudstone, most plants cannot use mudstone-derived ‘subsoil’ for nutrients or even anchor points.
Yet, mudstone remembers what it was to be a soil and, given time, will become fertile again. The catch is that time for mudstone is measured in Earth time, rather than human time. The Earth will heal itself, but perhaps not at the pace that humans would like. To record the start of the transition of mudstone into soil, I dug pits one metre deep in three locations on one mine site near Singleton, NSW, and collected a vertical soil section from each pit.

Vertical soil sections are known as soil profilesand they show the transitions of soil types, soil colours and how plants roots interact with soil changes to depth. Soil profiles are used to determine soil composition and soil health. From these metre-deep holes in the earth, I made three soil profiles: the first was collected from a natural, undisturbed soil: the second and third came from rehabilitated mine land. The unmined natural soil profile, collected from an open forested area, shows a bright orange sandy clay loam topsoil transitioning into a brown sandy clay subsoil.  There are lots of healthy plant roots of many species throughout the soil. The soil has a strong structure that lets water and air into the root zone. The strong and healthy roots throughout the profile show that the soil is functioning well.  The two post-mining profiles show a transported topsoil layer over mudstone overburden. The second soil profile is only two years old and shows a defined layer between the orange topsoil and the grey degrading mudstone that substitutes for the subsoil. Plant roots are common in the topsoil but almost non-existent in the mudstone subsoil and those roots are twisted, shorten and gnarled from the excessive alkalinity released from the mudstone as it breaks down. The soil structure in the topsoil is weak (it has been dug up, transported and bulldozed, and has not yet recovered). The mudstone ‘subsoil’ still looks like rock lumps rather than soil peds. The 22-year-old profile shows a less obvious layer between an orange-brown topsoil (more organic matter) and mudstone ‘subsoil’, and, encouragingly, there are Rhodes grass (Chloris gayana) roots in the mudstone-derived part of the soil profile. Rhodes grass is a very tough plant and, along with Acacia saligna, seems happy to tolerate extreme rootzone conditions most plants cannot. But can the mudstone section of the soil profile be classified as a soil?  My soil science colleagues say no, since there is no formation of soil peds. In the Hunter Valley, there will be tens of thousands of hectares of rehabilitated post-mining land when the coal industry is done. But that is not the end of the story; the land will continue to change over many hundreds, perhaps thousands, of years. Mudstone boulders brought to the surface by mining will be breaking down into component parts not just on the surface, where plant roots can reach, but also in the depths of the hills. Whole landscapes made of mudstone may drop in height. Concrete structures that now direct water flow down the hills will fail as the mountain sides move and settle. Obvious gully erosion that exists in this present day around post-mining overburden hills will become vast if left untreated. Erosion makes land unsafe and unproductive, and runoff often affects surrounding lands. Care and maintenance of post-mining land is a problem that we leave future generations to solve. The human-made mountains are imperceptibly on the move, weathering, washing and dribbling down engineered hillsides to lay on agricultural lands where we used to grow our food, changing the water quality of our aquifers and rivers on their way. Bringing Permian rock to the surface gives rise to many environmental issues. We are familiar with the relationship between mining coal and climate change, but there will be other penalties associated with mining that we impose on future generations. The loss of functional subsoils restricts soil water storage, making the land more drought-prone; it restricts nutrient availability for plants, making the plants smaller, weaker and more prone to disease; it challenges the stability of large trees; and it changes the flow patterns of water and air. In the short term, degraded mudstone cannot act as soil no matter how much we might wish it, but, with certain levels of ongoing care, mudstone will do its best to remember how to act as a soil. With geological time on its side, mudstone derived soils will provide shelter for fungi and bacteria, plant roots and micro-organisms. In Earth time, mudstone will remember that it once knew how to grow forests.