The British Ice Sheet
20,000 years ago, Scotland and the rest of the world was several degrees colder than they are today. Snow that fell in wintertime no longer melted over the summer. This snow accumulated into small glaciers up in the mountains, which then flowed together to form a massive Ice Sheet – which is the name we give to very large glaciers. Ice from the Scottish highlands flowed across much of the UK – including over Ireland to the west, into the North Sea to the east, and down across much of England to the South.
This ice sheet was around 1000 metres thick, or about 0.6 miles. The ice would have been much thickness in some deep valleys, closer to 2000 metres thick (or thicker than Ben Nevis is tall). Over the past 100 years, many geologists and glaciologists have worked together to understand how large this glacier was, and when exactly it melted. A very nice map is available to scroll through this evidence at the following link: https://shefuni.maps.arcgis.com/apps/webappviewer/index.html?id=fd78b03a74bb477c906c5d4e0ba9abaf