It was his favourite place to sledge when he was a young boy, hurtling down the slopes which stretch all the way from the conservation village on the edge of East Renfrewshire to Fenwick in Ayrshire.
Now, the towering turbines of Whitelee Windfarm cover the moorland as far as the eye can see. "How do you feel about this place?" he asks between photographs. "Are you for or against? I think it's great."
Stewart – friendly geologist, entertaining TV presenter, university professor, expert commentator, husband and father – is back in Scotland briefly to record commentary for his latest television series and to visit his parents in East Kilbride. The 47-year-old grew up in the town, a 10-minute drive from the moors, with his two younger brothers Graeme and Frazer.
"I like East Kilbride, it was a good place to grow up in," he smiles. "There was always lots to do: tennis, bowling, sporty stuff, shops. I also liked the fact you could get out of it fast too."
Now living in Plymouth, where he is Professor of Geoscience Communication at the city's university, Stewart relishes any chance to come back to Scotland. He has his own family with him and is looking forward to spending some time with his wife Paola, whom he met on a geology field trip at university, and their daughters Cara, 14, and Lauren, 10. "Inevitably, when you're busy travelling and filming and doing two jobs, it's family that takes the hit," he says. "It takes a bit of juggling."
Stewart's love of performing started with a childhood role in the BBC adaptation of John Buchan's Huntingtower and developed during a spell in amateur dramatics with East Kilbride Rep. He is amused to learn a photo of him appeared in the group's recent History of Panto exhibition, alongside another famous East Kilbride face.
"Me and my friend Norrie played coolies in Aladdin – it must have been in the early 1980s – and a fortnight before the show opened, Norrie announced he was leaving to go down south with his family," he recalls. "I remember the director saying he was going to try out this other guy, who was an electrician and desperate to get into acting.
"I was a bit put out because I thought I was something special, having done a BBC show, but it was obvious from the start that this bloke was really good. Within a couple of hours, the director was saying, 'Yeah, Iain, if you could just copy John, that would be great."
Despite his brush with future Hollywood star John Hannah, Stewart chose academia over acting and went to Strathclyde University to study geography.
"I had great geography teachers at Claremont High: Mrs Begg, Mr Elder and Mr Cassidy," he says. "I was a middling student, never really at the top of the class, nor at the bottom. Which I think is good, in a way. When you're out there at the top, it can be quite isolating."
He discovered geology at university and after graduating, completed his PhD at Bristol University on research into earthquakes in Greece and Turkey. He went into teaching, he says, "to do something useful".
It was during a spell teaching geology at the West London Institute of Higher Education that he got his first taste of presenting. "Sky's main office was just across the road from us, and one day they needed an expert to talk about earthquakes, so I did it," he says.
He was an expert academic on the 2002 BBC Horizon film The Real Atlantis, about the destruction of the Greek city of Helike by earthquake and tidal wave in 373 BCE and this, he says, gave him "a hunger to get more geology on telly".
In this he has succeeded and a host of TV and radio projects followed, including the award-winning Journeys from the Centre of the Earth, six one-hour films charting how geology has shaped the history of the Mediterranean; Earth: The Power of the Planet, about the forces that have shaped the world; Making Scotland's Landscape, in 2010; and last year's Men Of Rock, about scientists working in Scotland who pioneered geological study and understanding.
His latest BBC series, due to be shown in spring, is How to Grow a Planet, for which he visited Africa, Vietnam and America and, famously, spent 48 hours in a sealed box to see if the oxygen from plants could keep him alive.
"It was fun," he says, of the experience which almost made him ill. "The point was to show how important plants are to us. It was very hot. But the worst thing about it was waking up to find people staring down at you. It was strange, to say the least."
Later this year he will travel the world again, to make a four-part BBC series with the working title The Story of the Continents.
"It's probably one of the last of these big, epic productions," he says. "Budgets are getting cut, and there probably isn't much appetite from viewers for watching people like me and Brian Cox standing on mountain tops with helicopters swirling about us. Most people probably think, what a waste of money."
Like physicist Cox and anatomist Alice Roberts, Stewart is part of a new generation of enthusiastic science presenters trusted to front big series in place of celebrities or better known names.
"It was quite a brave move to do that, but it reflects what people want to see, I think they'd rather have someone who is an expert in the field and can communicate than a celebrity who doesn't know that much about the subject," he says.
"Someone like Steve Backshall, for example, who does Deadly 60 on CBB. He is so enthusiastic and passionate about his subject, that engages viewers. But with that comes a bit of responsibility – what if one of us was to come out with something really daft, or just wrong? You have to be careful who you listen to."
With "getting more geology on the telly" comes exposure, of course, but Stewart is not getting carried away with all the attention. The man once described as "the James Bond of geology", thanks to daring exploits like abseiling into active volcanoes, has a YouTube video dedicated to "his awesomeness" (set to a track called I Want to Make Violent Love) and his fair share of admirers.
"It's flattering, but I don't get recognised that much, I'm not in Brian Cox's league," he shrugs. "The only time I've been stopped in the street was in the St Enoch Centre in Glasgow, by a young bloke who kept shouting, 'I do know you, where do I know you from? I've definitely met you before,' as I politely said, 'Eh, no, probably not, I don't think so.' Eventually he gave up and walked on, only to come racing back a few minutes later shouting, 'You're that volcano bloke off the telly!'"
He adds: "I realise I have a shelf life – it's fantastic making programmes about different aspects of geology, but I know sooner or later, something else, and someone else will come along. A while back it was all about archaeology, thanks to programmes like Time Team, then it was forensics with the CSI stuff, now it's our turn in the spotlight. And if we can't make things like volcanoes and diamonds and dinosaurs exciting, then we must be doing something wrong."
There are still places Stewart would like to visit – Patagonia, for one, and Papua New Guinea – but he maintains the most important part of his life, outside his family, is his job as a lecturer.
"I'm lucky that I have two jobs I love and when one of them, the TV one, stops I can still do my main job," he says, adding with a laugh: "I suppose it is a bit like performing, with a captive audience. They can't switch me off or walk away."
Prof Iain Stewart, geologist and TV presenter
Favourite film: Young Frankenstein.
Last book read: William Boyd's Any Human Heart.
Favourite music: Alabama 3 or King Creosote.
Favourite food: Thai curry.
My highest point in life: Late 2002, six months after chucking my lecturing job in London when the savings were draining way and before the first television work finally came good.
My lowest point in life: In 2004 when I filmed in the Dead Sea, Jordan
Ideal dinner guests: Ricky Gervais, KT Tunstall, pictured, Ellen McArthur, Bruce Springsteen.
Best trait: Positivity – always thinking the glass is half full.
Worst trait: Saying yes too much, and then struggling to fit everything in, especially alongside family life.
Biggest influence: Mike Russell, geology professor at Strathclyde University. I still steal most of his best lines.