Geology of The Falkland Islands - Glaciation and Weathering

Glaciation and Weathering

The effects of the ice age erosion that occurred in the Pleistocene glaciation between 25,000 and 15,000 years ago are visible in the islands. The tops of the hills have been exposed to most of the effects of freezing and thawing. Also, the strong winds that are characteristic of the region carry sand grains up to heights of a metre above ground level; the resulting sand-blasting causes rocks and pillar-shaped rock structures to show the most erosion in their lowest exposed levels, at one metre and below. This unique pattern of erosion is evident in the higher elevations of West Falkland where quartzites of the Port Stephens Formation are exposed at the surface. During the last glaciation, snow that accumulated year after year formed glaciers in some elevated leeward zones. These and previous glaciers modified the landscape on the eastern mountain slopes that were protected from the westerly winds. The reason for glacier accumulation on the eastern slopes could be that the climate could have been very dry, or that the prevailing wind prevented glaciers from forming in places other than eastern slopes.

Another effect of glaciation can be seen on East Falkland in the basins called glacial cirques that were created on Mount Usborne. On West Falkland there are glacial cirques on Mount Adam and on the Hornby mountains. Several of these hold tarns, or small cirque ponds.

Rock flows or moraines are another characteristic left by glaciation. In the Falklands, these generally occur only at the mouth of the cirques, although in one valley they extend some 3 km south of the drainage divide that was the likely ice source; north of this same divide, a closer-in moraine loop dams in a small lake. These moraines are morphologically quite different from the very widespread stone runs, which are the result of periglacial processes.

All the boulders in the stone runs are remnants of quartzites that were repeatedly ground down from repeated weathering, perhaps by a combination of frost action and thermal fatigue. They are found principally in the Port Stanley Beds and to a lesser degree in the Port Stephens Formation. Excavations show that the colour of the upper parts of the rocks is different from the lower part. This is due to rainwater erosion that has whitened the stones and left them a light grey colour. Below ground level, where the rocks have been protected from erosion, they acquire an orange colour from iron oxide.