"While the bigger of the two cairns has been there for some time, over the last six months it really has increased in size," writes David Osmond of Dickson. He adds: "Now there's a smaller one next to it!"

Osmond isn't the only Canberran to contact this column in recent weeks perplexed as to the origins of the two cairns. John Jones of Aranda also wonders about the mystery cairns, made from thousands of rocks loosely piled on top of each other. "Perhaps they've been erected by disoriented aliens, a bit like those who supposedly make crop circles," muses Jones.

Adding to the intrigue of the curious cairns is that they aren't located at the actual summit of Gossan Hill, but rather in a clearing near an old fence line.

Never one to shy away from solving a good suburban mystery, this week your akubra-clad columnist attempted to uncover their origins.

First stop was the ACT Office of the Surveyor General, where, after an extensive search of historic mapping and plans and the database for survey marks, a spokesperson concluded that "it's an odd-shaped cairn but isn't any official trig point".

Next up was ACT Heritage, where I wanted to check my theory that perhaps there'd been a cairn there for many years and bushwalkers have just added to it. While the good folk in the heritage unit couldn't advise how long the cairns had been in situ, a spokesperson did direct this column to an extensive assessment report for the heritage registration of Gossan Hill, prepared in 2015. However close examination of this report failed to reveal any mention of the cairn(s).

Finally, I turned to ACT Parks who manage the reserve. While they couldn't shed any light as to the origin of the cairns, they did express concern at their size.

"That's a monumental effort over a long period of time, but it's disappointing as clearing so many rocks from the nature reserve destroys the habitats for many critters which live under rocks" laments Andrew Halley, ranger in charge with the ACT Parks and Conservation Service. Halley, who "regularly dismantles 'rock towers' built by bushwalkers throughout other parts of Canberra Nature Park", also points out "in building these structures, not only are people destroying habitat for creatures, but they are also potentially moving indigenous artefacts."

Great: three government agencies, three dead ends. However, not wanting to leave any stone unturned (yes, pun intended), as a last resort I grabbed my magnifying glass and along with Emily, my 7 year-old-daughter, beat a path to Gossan Hill.

The cairns, located at the intersection of two tracks behind Crisp Circuit in Bruce, are relatively easy to find and surprisingly not on the hill's true summit. Rather, they are in a clearing which commands a lovely view to the Brindabellas near an old fence line (watch-out for rusting barbed wire). Maybe the stones were part of a hut that once stood here, however, pertinently there's no mention of any such structure in the aforementioned heritage report.

Most remarkable is the size of the big cairn – it's huge. "It's a bit like the summit of Mt Kosciuszko," says Emily, who is amazed by the cairn's size.

It's also obvious that the cairns have been carefully constructed with stones from surrounding bush for, unlike the rest of Gossan Hill, which is littered with similar sized stones, for a significant area around the cairns the ground is devoid of them. In a way it's similar to how a farmer would clear a patch of land to increase pasture productivity, but of course there hasn't been grazing here for decades.

Interestingly, while exploring the nature reserve for several hours Emily and I uncover many other random (and much smaller) stacks of rocks. Perhaps someone with a rock piling obsession is wandering Gossan Hill by day and night? Someone must know.

Mystery cairns aside, Gossan Hill is home so several natural treasures, some millions of years older than these man-made (ok, or alien made!) contemporary cairns. Here are my top 3:

1.It's all in the name: A gossan is a Cornish word meaning 'iron hat', and is used to describe a geological feature where there is an outcropping of rocks on the surface indicating an ore-forming process underneath. Gossan Hill is one of only two true gossans in the ACT. The other one is at Paddys River which has been heavily disturbed by mining activity. Studies indicate that the reddish stones on Gossan Hill have links to the molten crust of the earth extruded an incredible 430-415 million years ago.

2.Ochre Pit: Near the College Street end of the reserve in a clearing near Radford College is an ochre pit. According to a nearby 'Canberra Tracks' sign, "apparently the Ngunnawal people held large gatherings here and used the coloured ochres as a ceremonial pigment.". Further, ACT Heritage advises that early European settlers in the area, Frederick Campbell and Samuel Shumack, both noted that the area was used for corroborees and stone artefacts in the reserve and the surrounding area also reflect this past use. Who'd have thought such a significant indigenous site was smack bang in the middle of suburban Belconnen?

3.Beauty and birds: According to ranger Andrew Halley, "the reserve has dry forest - woodland vegetation with high plant diversity, including some amazing orchids (best spotted in spring). The reserve also provides important habitat for threatened and declining woodland birds including the varied sittella (Daphoenositta chrysoptera) and the ground-dwelling speckled warbler (Chthonicola sagittata) which breeds in the reserve," he says.

Gossan Hill Nature Reserve: This 47 hectare reserve, part of Canberra Nature Park, is surrounded by the Belconnen suburb of Bruce. The mystery cairns are located in the western section of the nature reserve at the intersection of walking tracks, 'Gossan Six' and 'Gossan Seven', which is about 250 metres east of 95 Crisp Circuit, Bruce.

Tim's Tip: Allow plenty of time to wander Gossan Hill as it's criss-crossed with a maze of fire trails and bush tracks and easy to become lost.

Did You Know? Following World War One, Gossan Hill was leased by the Commonwealth to solider-settler Robert Butt who died in 1926, aged just 31. While using gelignite to help split timber for fencing, he took several plugs of gelignite to the nearby Molonglo River, intending to use the explosives with his cousins to obtain a feed of fish. According to the 'Canberra Tracks' sign on Gossan Hill, "unfortunately, some gelignite exploded when he placed it in a pickle bottle attached to a fuse, blowing fingers off both his hands, lacerating his face and causing many other injuries. Although admitted to Queanbeyan Hospital that evening, he died from shock and haemorrhage." What a terrible way to go.

Tree Tales

While the origins of Gossan Hill's stone cairns is unsolved, Mike Crisp has solved the mystery of the Gossan gum with unusual markings (Belco Beauty, January 6)

Crisp, an Emeritus Professor from the Research School of Biology at the Australian National University, and who often walks on Gossan Hill believes the "striking, almost gothic" appearance of the tree, "likely a E. mannifera or E. rossii" is the result of "cankers which are caused by a pathogen that infects the wood, usually a fungus."

If you want to see the tree while it's alive you might want to head there soon for Crisp advises there is another, nearby tree "with the same lesions on the trunk, and it's dying".

According to Adriana Vanden Heuvel who first brought the tree to this column's attention, "it is located on the 'Gossan 2' track, about 40 metres south of the intersection of the Radford fire trail". Watch out for snakes.

Earlier this week, while trekking through the eastern slopes of the NSW Snowy Mountains, Eric Rojahn of Calwell was surprised to stumble upon of all things… a pyramid. "The really amazing thing was the little pyramid sitting on top," reports Rojahn, who is reluctant to publicise the exact location of his find. "I'm not going to be more specific regarding its location as there could be un-looted riches inside," he muses.

The rock reminds me of another high country pyramid – the quirky Hide 'N' Rest Pyramids near Adaminaby where you can bunk down in one of two pyramids carefully-crafted to the same dimensions as the Great Pyramid of Cheops.

Hide 'N' Rest Pyramids are located at Rocky Corner, near Adaminaby, about a 2-hour drive south from Canberra. Ph: 6454 2536 for more details.

Clue: Tourist Park absolutely nowhere near the Hume Highway.

Degree of difficulty: Hard

Last week: Congratulations to Terence Sheales of Melba, who was the first reader to correctly identify last week's photo, sent in by Gary Poile of Collector as a photo of the last wooden punt travellers used to cross the Clyde River at Nelligen prior to the opening of the village's bridge in the mid-1960s.

Lily O'Brien, former Canberran and now of Tuross Head recalls "the punt had a capacity of about eight cars and the much bigger punt at Batemans Bay carried about 38 cars," adding "the trip from Canberra to Batemans Bay in the 1950's was mainly on dirt roads and at peak holiday periods the journey could take as long as five and a half hours but the highlight for us kids was when we could get out of the car and explore the workings of the punts."

For many readers the photo brought back memories of not only the punt but also the nearby Steampacket Hotel. David Nott who recalls "in the hot summers in the 1940's and 50's, sometimes a very long queue would form and a cheerful lady from the pub would hand out scones and tea to the very hot people waiting to board the punt."

Meanwhile, Sharon Loiterton recalls things a little differently "once the their cars were in line often the husbands would walk down to the pub leaving their wives to move the car forward as each group of cars were loaded on."

The name of the pub written in huge letters on its roof was the subject of conversation for many. "Because the way the roof was built the name looked a bit like 'ST EAMPACKET'," recalls David Nott, adding "as a result there was always discussion in our car as to whether the Saint came from Nelligen or was it his hotel."

These days the punt is sitting at the bottom of the Clyde River "having sunk while being towed above Nelligen sometime after the opening of the bridge" reports Stephen Dunn, whose grandfather Harry Dunne, along with Nes Christensen built the punt in 1925.

So if you are travelling across the Nelligen Bridge in busy holiday traffic this weekend, drive safely and if there is congestion, thank your lucky stars that don't have to cross the river just eight cars at a time in an old wooden punt. How times have changed.

How to enter: Email your guess along with your name and address to timtheyowieman@bigpond.com. The first email sent after 10am, Saturday 27 January, 2018 will win a double pass to Dendy.