The polar regions are mostly composed of continental glaciers, also known as ice sheets.

Here's why:

* Size and Extent: Continental glaciers are vast, covering large portions of continents like Antarctica and Greenland. They're significantly larger than valley glaciers, which are found in mountainous regions.

* Thickness: Continental glaciers are incredibly thick, often thousands of meters deep.

* Flow: While valley glaciers flow downhill, continental glaciers flow outward from a central point due to their immense weight and the force of gravity.

So, the vast ice sheets covering Antarctica and Greenland are the dominant type of glacier in the polar regions.