Most readers were upbeat about the occurrence of the native Ferny Azolla (Azolla pinnata) in some of our suburban ponds. "It really brightens up the walk," states Steven Brown of Kambah who, after reading about it in this column, stepped out on the track at Cooleman Trig Nature Trail to check it out for himself.

Some correspondents like Janis Norman of Melba reported it growing in other locations. "On our way from Strathnairn to the Link, we came across another 'pink pond'," she reveals.

Meanwhile, some readers who live on farms in nearby NSW saw red. Literally.

"It sucks all the oxygen out of the ponds/dams and is surely a sign of a system out of balance," grumbled one farmer.

Dr Fiona Dyer, a water science professor at the Centre for Applied Water Science and the Institute for Applied Ecology at University of Canberra, said while a mat of azolla on top of the water could "definitely prevent light and oxygen from being incorporated into the water column", she doesn't believe it indicates an unbalanced system.

"A mat of azolla can affect other organisms such as frogs and fish, but this can be a part of natural cycles rather than necessarily being a system out of balance," she says.

"While our human-centric world doesn't like to think of fish and frogs dying, when it is part of natural cycles and they can re-establish, then it isn't a problem.

"Of course, if you are trying to pump from the water body in which you have it, then azolla can be a real nuisance."

Further, Jonathan Davis of Duffy raised questions about its "sudden appearance" in Canberra. "As a lifelong Canberran, I have only been aware of it in the region in the last 10-15 years," he says.

On the contrary, Dr Dyer explains "there are reports of azolla being quite prevalent in the Canberra region, at least back into the 1970s".

Your akubra-clad columnist suspects this perception that azolla is new to suburban Canberra may be due to our increased awareness of the colourful fern due to the recent explosion in social media posts about it. Dr Dyer thinks this perception may also be due to the seemingly random distribution of the fern.

MORE YOWIE MAN:

"Given it is spread by spores, we'd expect it to be almost everywhere, but it isn't," she says.

"I've noticed it for many years in ponds at the back of the AIS - which for some reason are currently free of azolla.

"Talking with a colleague from CSIRO in Albury, they have examples of a pair of ponds where it will be present in one, but not the other and they aren't sure why. It will come and go."

Dr Dyer says more research needs to be undertaken into azolla, including its use as a biofertiliser and distribution in suburban fringes. "I'd love to have a map of where your readers have spotted it," she says. "I think there is a great student project to head out and sample those locations to see if we can build a better understanding of it in our urban areas."

Please contact me at the address at the end of this column if you've spotted azolla in Canberra's suburban ponds.

Did You Know? Changes in light levels and/or a reduction in phosphorus levels causes the usually green fern to turn pink/red.

Who'd have thought an early 1960s photo of two men driving past an old service station in Captains Flat would attract so much attention?

And no, it wasn't the antique bowsers (although, they'd be worth a quid these days) or the discontinued Atlantic brand that resulted in most comments, rather what the two men - the late Claude Ryan (driver) and the late Keith McDonald - were eating (or should that be drinking?).

In response to my assumption that Claude and Keith were tucking into ice-creams, those who knew the men claimed they "wouldn't be spotted dead" eating ice-cream when beer was on offer just 200 metres up the road at the Captains Flat Hotel.

"Knowing those two blokes, they would not be ice-creams at all, I'd suggest they were cans of beer," quipped Bruce McIntyre whose father ran the service station.

"They definitely don't look like the type of blokes that would eat ice-creams" added Barry Snelson of Calwell.

However, after zooming in on the driver and his passenger, several readers, including Phil Tarrant, have unanimously concluded they were, in fact, feasting on ice-cream.

"They are definitely ice-cream cones," asserts Phil.

"It's very disappointing to burst the bubble on these 'hard' Captains Flat men but beer doesn't have a head on it like that," adds Steve Jones of Kambah, who also concludes "they are 100 per cent ice-cream cones".

However, I'll leave the last word to Jill McDonald, Keith's widow, who now lives in Holt, and who was "surprised but delighted to see the photo in the paper".

"It would be right to assume it was beer, but on this occasion, it definitely looks like ice-cream, which he also loved," she reports, adding she is somewhat "amused" by the attention the 60-year-old photo has received.

Several readers, including Sam Patterson of Melba, remain "gobsmacked" at reports that late Edgar Penzig would fire a percussion pistol during public presentations during the 1970s.

The bushranger afficionado, who penned more than 20 books on Australian bushrangers and colonial history, especially the gangs in southern NSW, apparently used it as a technique to keep his audiences on their toes, or at least to check they were listening.

As a teenager, Binalong performance poet Robyn Sykes witnessed it first-hand, not once, but twice.

"He came to talk to Rotary Exchange students so we'd be informed of bushranging history before we left overseas," she recalls, "and in both presentations ... he woke us up with a shot from a pistol.

"Both times it came as surprise, it took your breath away and most definitely woke people up," she recalls. "I can guarantee everyone paid attention for the rest of the talk." Gee, I bet they did.

Note: I have it on good authority that no one was in any danger as Edgar always ensured there was no powder of charge in the barrel - it just sounded loud.

Meanwhile, debate continues to rage (at least in my inbox) as to whether John Gilbert is actually buried in the hillside grave on the outskirts of Binalong.

The last to join the chorus is Paul Briggs of Yass who believes he was actually buried in a nearby paddock.

"A Binalong lad, Father Brian Maher, knew of my personal interest and family history with bushranging [and] told me some 20 years ago that Gilbert's grave had been moved from the police compound graveyard (some 300 metres away) to its current location. This was thought to have occurred around 1906 as the owner did not wish to create a shrine to a criminal, nor have strangers on his property."

Rating: Easy-Medium

Clue: If you went to school in Canberra, you'd know this was one of Canberra's oldest buildings

How to enter: Email your guess along with your name and address to tym@iinet.net.au. The first correct email sent after 10am, Saturday June 10 wins a double pass to Dendy, the Home of Quality Cinema.

Last week: Congratulations to first-time winner Lauren Allen of Scullin who was first to identify last week's photo as the former Mount Pleasant observatory. Built in the 1930s, the observatory was primarily the domain of Professor Gilchrist (or "Gilly" to his students), a mathematics professor at the adjoining the Duntroon Military College.

According to Royal Military College of Australia 1911-2011: Celebrating 100 Years at Duntroon by Jason Hedges (Royal Military College Duntroon, 2012) "the exterior of the observatory resembled an outdoor toilet and appeared to be in a state of total disrepair". Apparently "the junior cadets were often asked to check that the light in 'Gilly's Shouse' [as it was cheekily nicknamed] was out ... and to sign a book in the building to prove that they had been there". The joke was that the light was never actually out as "the only light was a red blinking navigational light on the roof to warn approaching aircraft".

The observatory was near the current lookout atop Mount Pleasant and was demolished in 1954. The road to the lookout is open from dawn to dusk and offers great views of Lake Burley Griffin and beyond.

After this column's recent feature on Canberra's "lost" limestone, National Rock Garden (NRG) chair Brad Pillans reports, "We have a captive piece of Canberra limestone at the garden - might be the best place for people to see it." Prior to its current home beside the bike track at the NRG off Barrenjoey Drive near the National Arboretum, the large chunk of limestone "originally came from a building excavation on the corner of Northbourne Ave and Macarthur Ave, before being used as a security barrier in a Russell carpark".

Meanwhile, a reader who would like to remain anonymous reports "I know a bloke who gave his wife on their wedding anniversary, a small piece of limestone, with a pink ribbon around it, and a note attached that read "My love is like Canberra limestone, not always obvious, but always there." Thanks Russell.

CONTACT TIM: Email: tym@iinet.net.au or Twitter: @TimYowie or write c/- The Canberra Times, GPO Box 606, Civic, ACT, 2601

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