Lavaka

Lavaka, the Malagasy word for hole, is a type of steep sided erosional gully formed by groundwater sapping, most common on hillsides across the Central Highlands of Madagascar and also documented in South Africa, the Democratic Republic of the Congo, Brazil, Eswatini, the Great Plains and South Carolina. They develop on slopes of 35 to 55 degrees where a layer of hard laterite roughly a meter thick overlies thick saprolite, and form when water seeping through cracks in the laterite chemically leaches minerals from the saprolite below until the weakened material collapses and erodes away, typically leaving a teardrop shape with a steep rounded headwall narrowing downhill into a shallow outlet channel. Although human activity such as deforestation and overgrazing can accelerate their formation, lavakas also occur naturally, and radiocarbon dating shows some are as old as 20,000 years, predating human presence on Madagascar. They are most common in tropical regions between the Tropic of Cancer and the Tropic of Capricorn.