A geological paradox
In the rugged landscapes of Namibia's Kunene Region, certain rocks present a geological puzzle. Large, isolated boulders, some meters in diameter, are embedded within sequences of finely laminated marine mudstone. These fine layers, or varves, are evidence of a calm, deep-water environment where suspended particles slowly settle. Powerful currents that could move large rocks are absent in such places. The presence of these out-of-place boulders—called dropstones—points to only one transport mechanism: icebergs.
These specific dropstones are found within the Ghaub Formation, a unit of the Otavi Group of rocks. They date to the Cryogenian Period, a time in Earth's history between 720 and 635 million years ago. During this period, glaciers growing on land masses would flow into the sea, where massive chunks of ice would break off, or calve, to form icebergs. These icebergs carried rocks and debris scraped from the continent. As they drifted into warmer waters and melted, they released their rocky cargo, which then fell to the deep ocean floor. The impact of the falling rock often deforms the soft sediment layers beneath it, providing clear evidence of its origin. The Namibian dropstones are part of a global phenomenon from this time, with similar glacial deposits found on every continent.
Evidence for a frozen planet
The dropstones of the Otavi Group are records the "Snowball Earth" hypothesis. This theory proposes that during the Cryogenian, the entire planet was covered in ice, from the poles to the equator. Geologists have used paleomagnetism—the study of the Earth's past magnetic field preserved in rocks—to determine that Namibia was located at tropical latitudes, around 12°S, during this time. The existence of glacial deposits like dropstones in these ancient tropical seas is evidence for the Snowball Earth events.
There were two major global glaciations during this period: the Sturtian (approximately 717 to 660 million years ago) and the Marinoan (approximately 650 to 635 million years ago). The Ghaub Formation in Namibia corresponds to the Marinoan glaciation. This deep freeze may have lasted for millions of years. Following the Marinoan glaciation, the geological record shows a sharp transition to a type of rock called a "cap carbonate." These thick layers of dolomite suggest a sudden and dramatic shift to a hothouse climate, caused by the atmospheric buildup of volcanic carbon dioxide over millions of years of ice cover. This rapid warming melted the global ice sheets, raised sea levels, and dramatically altered ocean chemistry, allowing for the subsequent explosion of complex life.