https://phys.org/news/2018-06-reveals-british-isles-ice-sheets.html

At the same time, an even larger ice sheet covered much of Scandinavia. Back then the North Sea, was narrower and deeper than it is today, like a large 'fjord'. Periodically, the ice sheets from the British Isles and Scandinavia advanced into water depths of around 250 m and generated icebergs nearly 300 m high.

By 1.9 million years ago the two ice sheets repeatedly merged in the centre of the North Sea, filling the 'fjord', as they advanced and retreated in response to climate changes controlled by the Earth's orbit.