It was a land of ice indeed.  Forty million years ago, a giant ice sheet, likely over a mile thick, covered nearly two-thirds of Westeros, and extended as far south as 40° north latitude, just shy of King’s Landing.  This ocean of ice would have dwarfed the great Wall (similar to the ice sheet that covered much of North America during the last ice ages).  To understand this land of ice, it is important to recognize the distinction between “greenhouse v. icehouse” planetary conditions and “glacial v. interglacial” conditions.  The former describes conditions that vary on time scales of millions of years or longer (like snowball Earth, nearly 650 Mya), while the latter generally describes climate variability on time scales of millions of years or less (like the last ice age, during the Pleistocene).  While the data is sparse, we propose that 40 Mya, this planet experienced icehouse conditions that created the massive ice sheet.  While it is currently unknown, the reanimation of hominoid biota currently north of the Wall may be a local indicator that appears concurrently with glacial cycles.  This area of bio-geological research deserves further investigation.

The physical evidence of this large-scale glaciation appears in a number of locations, most notably the sizable gap in the Mountains of the Moon, southeast of The Twins.  Only large-scale glaciation could provide the erosive power necessary to carve this region relatively flat.  In the north, west-northwest of Winterfell, the northern mountains are more subtly eroded, allowing for pines to grow below the tree line.  While not specifically mentioned in texts or personal accounts, large depressions left by the retreating glaciers are typically found on Earth.  And so we have interpreted two such basins: one extending southeast from Winterfell, and the other extending southeast from the Twins.  Each of these depressions, on the order of 100-1000 mi², would later be filled with younger sediments deposited by rivers flowing from the nearby mountains.  While it is difficult to say with certainty, it is possible that the retreating glacier carved out the God’s Eye Lake, south of Harrenhal (similar to the Great Lakes of North America).  This geologic evidence strongly suggests that forty million years ago, winter appears to have come for the entire planet.