In the Ice Ages the ice advanced in the form of glaciers from Scotland and Scandinavia and retreated many times over many millennia, the final advance ending at the edge of the Chiltern Hills and where Dunstable is now.
The ice carried with it pieces of rock which had been broken away by frost and by its own advance. The embedded pieces of rock made the ice abrasive, like a scraper and as it travelled it scraped off the surface of the rock.
In the Lake District in England one can still see the grooves scraped on the rocks and this process still continues in many parts of the world.
When the ice thawed it formed great rivers of melt water and it gradually let fall bits of rock from as far away as Scotland and even Scandinavia, all the rock debris that it had carried so far and for so long.
Do you think it is possible that this is where the River Thames began?
Yes, the river that we would know as the Thames flowed due east and joined the River Rhine. Eventually it broke through the hills to escape south through the Chilterns at the Goring gap in the Chilterns and then east to the sea.