Geology of the Ugly Islands.

The Ugly Islands are mainly made of igneous rocks , but there are also sedimentary rocks . As far as anybody can tell, the Ugly Islands have been there in the same place (more or less) for about thirty million years. The islands are built on and from the remains of old volcanoes, filled in with coral sand and limestones that have formed from coral fragments.

The whole area lies on an undersea ridge that lies between two small basins in the Pacific Ocean. The two main islands are mostly made of volcanic rock. Usually, volcanic islands are quickly eroded away, but here, there was a continual series of small gentle volcanic eruptions, enough to keep the islands above the waves, but not enough to ever wipe out all of the life on the islands. The coral reefs around the outside have also helped to protect the islands from erosion.

The last volcanic eruption was probably 400 years ago on Little Ugly, and about 300 years ago on Big Ugly, each being a small and gentle lava flow. There are thermal springs in many places on Little Ugly, and Finnegans Lake is a thermal lake, 4 km long and 800 metres wide, fed mainly by groundwater from somewhere on Cloudmaker.

There has been plenty of time for sedimentary rock to form in some places close to the sea. Mostly, these sedimentary rocks are wind-blown mixtures of shell-grit, coral fragments, sand and volcanic mud. Some interesting fossils have been found in the area. Unfortunately, there are very few continuous "records", so the evidence is still a bit patchy.

The igneous rocks: hardrock | pipestone | mudstone | glass-stone | bellstone

The igneous rocks of the islands are of several distinct kinds, most of which would be recognised straight away by anybody with any geological training. The first settlers on the Ugly islands lacked any real knowledge of geology, so they named the rocks they found, according to how the rocks appeared or were used.


"Hardrock" is a particularly tough form of basalt, an ordinary, hard volcanic rock, often with dykes of olivine basalt running through the hardrock. Most of the hardrock layers are more or less horizontal, but in some places, close to the volcanic vent they came from, they slope as much as 30 degrees to the horizontal.

Some hardrock is used in road building, and dry stone walls made from pieces of hardrock are used in some places to keep cattle in their paddocks. The dry stone walls are made in a standard fashion, just as these walls are made in other parts of the world, but curiously, there are some giant walls made in the same way, mainly out of pipestone columns with a rubble fill of hardock, out on some of the reefs around the islands.

Most of the hardrock formed as sheets, with layers of ash or other soft rock in between -- all of these are grouped together as mudstone . When the softer rocks weather out, the hardrock is left behind, ready to be collected and used.


Most of the cliffs around Big Ugly and Little Ugly look like giant pipe organs. The "pipes" are huge columns of basalt, formed by the slow cooling of the molten lava. You can see the same formations in many parts of the world -- the Giants' Causeway in Ireland is probably the most famous. The "pipes" are usually long hexagonal columns of stone, although a few of the columns have seven or eight sides.

There is one place off the shores of Big Ugly where a large number of these pipes have been carried out into the lagoon, and piled up on the outer reef. there is a lot of argument about whether these piles of stone are human-made or not, and there is some active research going on about this at the moment.

At a number of points around Big Ugly, the fallen pipes at the bottoms of cliffs have produced a rich system of caves and crevices which are popular with the more adventurous islanders. There are also some small caves in the calcarenite, but most of these are only enough to shelter a few bombats .


This is mostly an igneous rock, the sort that the geologists call a tuff, although any soft rock goes by this name, including some breccias, which are just like tuffs, but with larger rock fragments. Because the rock is mostly made of small pieces of volcanic ash, it looks a lot more like a sedimentary rock, and even forms in layers.

Mudstone is very important on the islands, as it weathers easily to give a rich deep soil, in which plants grow very easily. There is no other use for mudstone, which usually occurs between layers of hardrock .


This is obsidian, a glassy volcanic rock, formed when the molten lava cooled very quickly. In many parts of the world, obsidian is the preferred material for making arrow heads and cutting edges.

Glass-stone is fairly unusual on the islands, although there is a large outcrop of it on three of the larger Uglets. Some of the islanders who fish around the Uglets prefer glass-stone knives because they are free, and they have a very sharp edge.


Bellstone is a form of basalt which rings like metal when it is struck. Bellstone formed as flat sheets when dykes were forced through the surrounding volcanic basalt or tuff, and it is much more resistant to weathering than tuff. Because it was forced into gaps in cold rock, the basalt cooled very fast, producing a stone which rings like a bell when struck with a large pebble.

When the surrounding stone weathers away, the bellstone is left sticking out of the ground. This gives the islanders the chance to collect large slabs of bellstone, which they make into massive tables. These tables, placed in the shade of a suitable tree, somewhere near the house, are used as places for social gatherings. As a group begins to form around a bellstone table, the people pound out a rhythm on the table which rings out, letting others know that a bellstone party is starting.

Bellstone pieces often wash up on the shores of the south coast of Big Ugly, where they are collected and made into wind chimes.

The sedimentary rocks: airstone | calcarenite | dripstone


This is what geologists call a coquina , a rock made of shell fragments that have become bound together. This rock can be cut with an ordinary saw, and large blocks are used to make many of the islanders' houses. The problem with this is that you can only get airstone on some of the Uglets, and the quarrying causes a lot of environmental damage, and the Council is still arguing about what should be done about the issue.


The islanders call this rock limestone, but it is a mixture of wind-blown sand and lime. Much of the shore-line of the main islands is formed of this rock, which often leaves off mushroom-shaped pieces sticking up out of shallow water.

Some of the calcarenite deposits have been scoured out by wave and water action, making small cave systems , but these have been little explored, so far, as the islanders believe that the caves are unsafe. This is probably a wise view, since the caves often drop pieces of roof after bad weather.


This is a porous sandstone, made of wind-blown sand grains. Many islands homes have a dripstone filter in the kitchen to clean the well water or tank water that most islanders have to rely on. The sandstone was originally cemented together by tiny crystals of calcium carbonate. These crystals later dissolved out, leaving a stone that has tiny crevices and tunnels for water to pour through, but which traps the larger life forms and solids in the water.


The caves on the Ugly Islands come in three forms. There are lava tubes in parts of Big Ugly, some of them running for considerable distances, small cave systems in the local limestone which is formed from coral sands and fragments, and the labyrinth caves which are often found at the base of pipestone cliffs.

The smaller lava tubes formed when roofs formed over the tops of lava flows, but some of the deeper lava tubes on Big Ugly have been followed for several kilometres between openings, and these seem to have formed when liquid lava drained away. The lava that they formed in has since been eroded somewhat, and there are about forty known openings in a branching system that covers much of the area to the west of the Big Ugly Range.

Some of these tunnels are ten metres high, and smaller branches have been explored, down to about a metre in diameter. So far, nobody has found any sign of any lava tubes on Little Ugly or the Uglets, although there are many small "caves" in the volcanic remnants around the Uglets. These appear to be just the remains of old gas holes that have been eroded away.

There is also a smaller set of lava tubes in the Gorgeland area, and some of the islanders believe that the two systems are joined together, although professional geologists who have been there doubt it.

The labyrinth caves are really just crevices between blocks of rock, and only a few of the smaller sections are well-known. The cliffs to the west of Finnegans Lake provide large open structures, where it is almost impossible to get lost, but deep enough to let people find almost complete darkness. The only problem is that the caves often provide shelter for Shouting Spiders , which sometimes "shout" at people, causing them to jump with fright and bang their heads.

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Last revised March 6, 2007.

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