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Steep Island The Kimberley

3 March 2019 | Part of the “A Pioneering Cruise Line” Anthology of Stories

Learning about the Kimberley Coast with Peter Sartori

By Sandy Scott

Dr Sandy Scott joined the Coral Expeditions crew as a Guest Lecturer in 1999 on the Kimberley coast and later added the Cape York and Arnhem Land cruise itineraries to his repertoire. Sandy has averaged three annual Darwin-Broome-Darwin cruises since starting and although he researched plant ecology for his higher degrees, he has taken a great interest in geomorphology interpreting for our guests the role of geology and process of weathering and erosion in the formation of the fascinating Kimberley coastal landforms.

Sandy’s first of two articles introduces Peter Sartori and his role in assisting the establishment of the company on the Kimberley coast. Many of the ideas presented in his second article about this mainly rocky coastline are expanded upon in his book ‘A Traveller’s Guide to the Kimberley Coast: bays, basins, islands and estuaries’.

Left: Hathaway’s Hideaway, Kimberley. Next page: Landing at Cathedral Cove, Bigge Island, Western Australia.

Peter Sartori was a charter boat operator out of Broome in the 1970s and early 1980s and was the first master of the Coral Princess I, which first began adventure cruising between Darwin and Broome in 1996.

Before taking up Coral Princess I duties, Peter facilitated fishing trips for his guests, one of which included the southern reaches of Montague Sound. On these cruises, he was accompanied by his partner Gail Hathaway who assisted him during charters. On one occasion Gail asked to be dropped off on a beach to explore while Peter took his clients fishing. During her time on the beach, Gail found an interesting cave system supported inside by many large sandstone columns. The cave is damp as its base is washed during spring high tides and it provides a habitat for several dark-loving creatures including spiders, lizards, small frogs and bats. Peter incorporated a visit to the newfound site now known as Hathaway’s Hideaway on the Coral Princess I itinerary, a practice continued by Coral Expeditions.

Peter’s expertise and understanding of the Kimberley coast was also called upon when the new ship Oceanic Princess was launched. During the early cruises, he accompanied the original masters, acting in a pilot’s role to familiarise them with safe passages in what were often uncharted waters, and safe anchorages in waters that experience huge tidal ranges and are home to underwater hazards.

The Kimberley Coastline

Old and Interesting

Extremely old. It is sobering to think that the six main types of rock exposed on the Kimberley coast are all more than 1.7 billion years old. This contrasts with the Sydney Basin rock formations that were laid down between 290 and 200 million years ago and those around Melbourne, where bedrock dates from 441 million years ago and much of the nearby basalt, or bluestone as it is locally known, was formed just 30 to 55 million years ago. However, we must remember most of the rocky Kimberley coast landforms we see today have developed over the last 20 million years.

A range of rock types. The rocks of the Kimberley were formed as layers of sediments and were deposited in a vast shallow basin with one intermediate period where volcanic activity spread a layer of mainly basalt lava over much of its surface. After all the subsequent sedimentary strata had been laid down, there was a break of some 40 million years at which time magma under great pressure pushed upward. Where this occurred, the molten material intruded the joints and in between the layers, particularly of the lower strata of the basin. It did not reach the surface but cooled to form dolerite.

Different rates of weathering and erosion. Travelling by sea along the Kimberley coast the strata of sandstone, siltstone and volcanic material (referred to as formations by geologists) may be clearly distinguished as the building material of cliffs and promontories facing the sea, around bays or into estuaries. In many places, the cliffs are very stable made up of quartz sandstone which is unreactive and therefore a slow weathering material. However, if these cliffs are underlain by less resistant, easily weathered material like the volcanic rock (e.g. basalt), dolerite or a soft siltstone formation, then landform changes are likely. Most often the tough sandstone cliffs will fall as their support structure weathers and erodes away beneath them. Eventually, this results in the development of receding coastline cliffs, the formation of wider valleys or the opening up of spaces that have become today’s bays, basins and inlets. The resulting coastline is highly irregular with deeply incised rivers and streams with many rocky cliffs, points, promontories and wide gulfs and bays. It is surprising to learn that most of the landforms associated with the Kimberley coast have been formed by weathering and erosion occurring in the open air, above sea level on land that was crossed by streams and rivers, rather than by the influence of the maritime conditions.

Horizontal and folded rock formations. While cruising along the coastline, it is also easy to see that most of the rock formations are formed along fairly horizontal lines, a view that is especially true when sailing from the north. However, once we reach the Yampi Peninsula in the south-west, we find that these horizontal lines change to become folded and tilted landforms. Whereas earth movements may have at times horizontally lifted the formations as a block over most of the Kimberley, on this part of the coast, the rock was squeezed up as if in a vice to form ridges and valleys. Given the long time spans involved since they formed, many of the ridges have been weathered down to stumps, and many of the valleys are now flooded with sea water. In other locations, intriguing features have formed, like the Horizontal Falls complex. Many folded shapes have emerged and, combined with the various rock colours, provide fascinating photographic highlights for travellers.

After the last Ice Age. Following the long series of glacial-interglacial cycles the Ice Age came to a peak with the Last Glacial Maximum (LGM) about 20,000 years ago after which ice caps and glaciers began melting and so causing sea levels to rise. This rise continued until about 6000 years ago when the sea level stabilised. By this time the coastal valleys had become estuaries flooded with seawater producing habitats, such as mangroves, suitable for organisms including, shellfish, turtles, dugong, Indo Pacific crocodiles and many bird species. Elsewhere in Australia, coastal valleys like the Hawkesbury and Brisbane Rivers also became estuaries around this time and some small basins close to the coast like Port Phillip Bay and Sydney Harbour filled with seawater.

A wide continental shelf. During the LGM sea levels were some 120 to 130 metres lower than today, and most of the Kimberley was fringed by an extensive coastal plain over the continental margin that was more than 200 kilometres wide. This contrasts dramatically with the southeastern coast of Australia where the coastal plain was often only 10 to 25 kilometres wide during this period.

Coral Expeditions Kimberley Coast Cruise

Horizontal Falls. The black line of cyanobacteria marks the high tide in the rocks. Talbot Bay, Kimberley, Western Australia.

Interesting effects of the tides. Today the Kimberley has a rock-dominated coast where the coastal plains and inland reaching valleys have been flooded, and the original hills on the coastal plain have been surrounded by sea to form over 2400 islands. This flooding by seawater and the formation of estuaries, bays and islands is a relatively recent event, occurring while ancestors of the Aboriginal people were occupying what must have been an ever-changing seascape.

The gravitational effect of the moon and the sun is the driving force influencing tides in all parts of the world. Of all the other hundreds of variables that affect the tidal range at any one location, the most significant on the Kimberley coastline is its extensive continental margin. As the tidal wave passes from the deep ocean to the coast, it progresses through the shallowing waters of the continental shelf. The shallowing conditions actually slow down the tide and, without going into the physics involved, this causes an increase in wave height. It is hard to imagine, but a deep ocean tidal wave of less than half a metre in height may be amplified to eight metres by the time it reaches this coastline. In Australia, only the Kimberley has conditions that create extensive tidal ranges over such a long expanse of coastline.

Tides reach their maximum range twice each lunar month, at a full moon and about 14 days later when there is a new moon. These tidal peaks are called the spring tides or ‘springs’. When the moon is at its first or last quarter, the tidal ranges are reduced to their minimum and these periods are known as the neap tides or ‘neaps’.

Another intriguing feature of this coastline is the range in height between high and low tides. Near Collier Bay, waves can be mega tidal with over an 11-metre range, the highest in Australia. Many other parts of this coast are macrotidal having ranges between four metres and eight metres, and on just one small part of the coast, on the northeastern corner, is it in the microtidal range, generally measuring less than two metres. Along with manyparts of the coast the black line of cyanobacteriamarks the mean high tide mark in the rocks and on beaches this point is marked by lines of stranded debris near where the beach meets the sand plain ordune vegetation. As already indicated, we only expect these very high tide levels to be reached during the spring.

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