Professional ski instructor Aaron McLean-Foreman, 24, said the discovery on Corstorphine Hill in Edinburgh left him in shock and disbelief.
It was some time before he could compose himself enough to go to police and report the find, a murder trial heard.
A jury at the High Court in Edinburgh has heard the remains on Corstorphine Hill were later identified as Philomena Dunleavy, 66, from Marino, Dublin.
Her son went on trial yesterday accused of beheading his mother then burying her dismembered body. James Dunleavy - also known as Seamus Dunleavy -denies battering her to death between April 30 and May 7 last year.
He also denies attempting to defeat the ends of justice by trying to cover up the alleged murder and destroy evidence.
At the time, Mr Dunleavy, 40, was living in a flat in Edinburgh's Balgreen Road, close to where the body was unearthed.
Mr McLean-Foreman, who discovered the remains on June 6 last year, said at the time he was planning a trip to New Zealand to further his sports career and had been in Edinburgh city centre.
Mr McLean-Foreman borrowed his father's bike to cycle to his home in the East Craigs area of the city. The Corstorphine Hill nature reserve was slightly out of his way, he said. "I did not mind the detour and enjoying the sunny weather," he said.
The day was "as good as it gets in Scotland" as he pushed his bike up the hill.
"I decided to have a break in the sunshine and relax and enjoy the fine weather," Mr McLean-Foreman told the trial.
Because of nettles and other vegetation he looked for a clear place to sit and noticed a likely clearing ahead. "Almost immediately the first thing I noticed was very white teeth," he told advocate Depute Alex Prentice, QC, prosecuting. "Teeth and a skull."
Mr McLean-Foreman said there were marks in the earth he thought had been made by a shovel. "It appeared to me, after coming to terms with what I was looking at that the soil had been altered."
A photo of the site was shown in court, after Mr Prentice warned it might be "disturbing".
Mr McLean-Foreman continued: "I have seen a fair share of sheep skulls and deer skulls while walking in the hills.
"I wanted to believe that was what I was looking at - but it was fairly clear that was not what I was looking at."
Mr McLean-Foreman took a photo so that he could lead police back to the spot and left.
Later that afternoon he went to his local police station, wondering if they would believe what he had seen.
The murder charge alleges that in Mr Dunleavy's Balgreen Road flat he inflicted "blunt force trauma" by means unknown, compressed his mother's throat and cut off her head and legs with a blade and something like a saw.
The trial continues.