Early formation of gnammas (weathering pits) in a recently glaciated area of Torres del Paine, southern Patagonia (Chile)

Incipient gnammas (weathering pits) were identified and measured in the Francés Valley of Torres del Paine, southern Patagonia. The gnammas were located on the granite blocks of a lateral moraine deposited during a Late Holocene neoglacial advance. The gnammas were measured for maximum and minimum depth, length, and width in addition to other observations. Based on these measurements, the surface area and the volume of the gnammas were calculated. Most of the gnammas have < 1 L of rock volume eroded, and the maximum depth is always < 10 cm. The gnammas do not develop erosional spillways. A high correlation exists between maximum and minimum depths, indicating these values are not completely independent. Thus, the depth ratio has been used to characterize the gnammas for discrete locations. The gnammas from Francés Valley belong to a unique normal population and a δ-value based on the depth ratio was defined to represent this population. The δ of Francés Valley is 1.33 ± 0.07, comparable with the lowest values of other stations around the globe. I suggest that the δ-value could be used as an indicator of gnamma evolution.