historically been vulnerable to large landslides, mudslides and avalanches. Huascáran, the highest peak of the Peruvian Andes, rises to an elevation of to the east of the valley. Río Llanguanuco, a tributary of Río Santa, drains a ravine on the northwest flank of Huascáran and cuts a steep southwesterly path to join the larger river, forming an alluvial fan in a wide valley near its mouth. Due to the fertile soils and availability of surface water, the area had been farmed for hundreds of years and was home to many small villages. The original site of Yungay town was an elevated area northeast of the Río Santa and north-west of the Río Llanguanuco; it was outside of the Llanguanuco valley, with a low ridge (Cerro de Aira) separating the two. Debris-flow deposits of Holocene origin make up the Río Llanganuco alluvial fan and underlie the Río Santa bed for depths of hundreds of metres in places, indicating that such flows have occurred here regularly for thousands of years. Geologic evidence indicates that Yungay itself was "probably built on a flow or avalanche deposit." The name of nearby Ranrahirca village means "pile of thrown stones" in Quechua, suggesting that debris avalanches have been observed in the valley since ancient times. These destructive flows of snow, ice, mud and rock are locally termed huaycos. The northwest side of Huascarán has been called "one of the most unstable ice-covered areas in the Cordillera Blanca". Among the many glaciers and permanent icefields on Huascarán is "Glacier511" located below the nearly vertical face of Huascarán's north peak. The glacier fills the head of the very steep Quebrada Armapampa ravine, which eventually flows into Río Llanguanuco. Beneath the snout of Glacier511 the terrain slopes down at an average pitch of 23degrees (in some places exceeding 70degrees), with a total vertical drop of before reaching the more gently sloped valleys below, creating ideal conditions for any large falling mass of rock to move unobstructed at high speed. Yungay before the disaster In the late 1960s, Yungay's permanent population was around 4,000 or 5,000, which swelled greatly on market and church days when thousands of visitors arrived from the surrounding villages. With the notable exception of the massive stone cathedral at the center of town, the city consisted predominantly of two or three-story adobe and wood structures with heavy tile roofs, dating from the Spanish colonial period. This