The Disappearance of Gertrude Rock

In the 2006 version of an Antarctic chart LINZ erased an islet called Gertrude Rock that had vanished off the radar.

Then in May 2007, the New Zealand Geographic Board Ngā Pou Taunaha o Aotearoa (NZGB) officially removed its place name, a final testament to a geographic feature now relegated to history. But the disappearance of the islet, which was first identified in the 19th Century, remains shrouded in mystery.

The 2003 version of nautical chart NZ14906 (left) showing The Sisters as they were, and the 2006 version following the survey completed in 2004.

In March 2004 a survey launch headed for the coordinates where Gertrude Rock had last been seen. The islet, about 200 metres wide, had been one of The Sisters - two stacks, or pillar rocks, lying close to the northern tip of Cape Adare in Antarctica. Further out to sea, the crew of the Research Vessel Tangaroa watched and waited as the ship rolled in the heavy swells.

The RV Tangaroa and its purpose-built hydrographic launch Pelorus were surveying the Cape Adare area of the Ross Dependency. LINZ had contracted the two vessels, which are owned and operated by the National Institute of Water and Atmospheric Research (NIWA), to capture hydrographic information for an update of the area's nautical charts.

The LINZ Hydrographic Services team is responsible for surveying New Zealand waters and producing nautical charts. LINZ Hydrographic Data Analyst Carol Kohl was involved in producing the new chart.

"Up until then we had been using information about Cape Adare from other nations' hydrographic charts and our record of The Sisters was based on this."

When the crew of the Pelorus reached the chart coordinates, they discovered that the entire mass of Gertrude Rock had disappeared. In the place where Gertrude Rock had once been, all the crew could report was the blue depths of the sea.

"A 17-metre depth was found at the chartered position of 'Gertrude'," the survey report stated.

At that point the mystery of Gertrude Rock took hold, more than 100 years after the British Antarctic Expedition had first charted and named The Sisters. Led by the Norwegian commander Carsten Borchgrevink, the expedition was travelling across the pack ice near Robertson Bay when they spotted the two rocks on 14 July 1899. Borchgrevink wrote about the day in his book First on the Antarctic Continent.

"Near land, about five miles from the two rocks at Cape Adare, which I named The Sisters, we observed a great many large loose basaltic rocks on the ice."

Then later, during the 1910-1913 British Antarctic Expedition, Victor Campbell (Northern Party Leader) named the individual rocks Gertrude and Rose, who were two sisters mentioned in a favourite comic song of the time. For nearly 100 years Gertrude Rock and her sister Rose Rock remained on Antarctica's charts.

The new version of NZ14906, the chart showing the Ross Sea area around Cape Adare and Cape Hallett, was published in 2006, revealing the disappearance of Gertrude Rock. Although Rose Rock avoided the fate of her sister, she is now much smaller than she used to be, perhaps providing a clue to the fate of her sister.

One idea was that Gertrude Rock had fallen victim to the awesome force of Iceberg B-15A. Once the world's largest free-floating object, the 160-kilometre long B-15A broke off from the Ross Ice Shelf in 2000 and drifted toward Cape Adare. However, a detailed examination of B-15A's progress revealed that Gertrude Rock had disappeared before the iceberg reached Cape Adare.

After analysing the recent survey data, LINZ Hydrographic Services passed on information about The Sisters to the New Zealand Geographic Board.

"It's highly unusual for a geographic point to disappear off the chart," Carol said. "When I noticed it, I knew it would be an issue for the Board."

An independent statutory body responsible to the Minister for Land Information, the NZGB is responsible for official place naming in New Zealand and the Ross Sea region of Antarctica. This means it investigates any proposed alteration of a place name or any proposed new name. LINZ provides administrative support, research assistance and advice to the NZGB.

So what happens when not only a geographical feature disappears off the chart, but also the name that goes with it?

At its April 2007 meeting the NZGB decided to remove the geographic place names for Gertrude Rock and The Sisters (given that it now describes a single rock), and to retain Rose Rock as the remaining (though smaller) feature.

As for Gertrude, the iceberg theory is still alive. Another iceberg, perhaps one relatively small and unnoticed, may have destroyed Gertrude Rock. Or perhaps the forces of the sea had taken effect over the span of time. The truth is that we may never really know. The only witnesses are nature's great nautical navigators, the penguins and seals living off Cape Adare. And they're not revealing much.