Glaciation of the Chivay source area

These regional data on snowlines are corroborated by evidence of terminal glacial moraines in the Quebrada de los Molinos at 4400 masl and in the adjacent Quebrada Escalera at 4300 masl. On the east side of our study area glacial moraines were observed just west of the Ventanas del Colca feature on the south and east end of a dramatic U-shaped valley at 4350 masl on Quebrada Porhuayo Mayo. The INGEMMET map series (Palacios, et al. 1993) indicates that morainal deposits of silt, sand, and gravel are evident elsewhere in the study area (Figure 4-15), typically at or above 4300 masl, corroborating the evidence from the regional model with a local snowline during the LLGM at 4300 - 4400 masl, or approximately 700 meters lower than conditions evident in 2003.

A team from the University of Maine including professors Daniel Sandweiss and Harold Borns explored the question of the Early Holocene deglaciation of the Chivay Source area in the late 1990s. As shown on Figure 4-15, a¹⁰Be date of circa 10,000¹⁴C BP (9450 cal BCE) was acquired from a quartzite erratic on a moraine at 4650 masl to the east of the Chivay source area (data courtesy of Daniel Sandweiss, 2006). This sample suggests that the terrain surrounding the source was glaciated at this elevation and higher as late as the Early Holocene. Establishing the rate of deglaciation and the exposed areas at a given time period will require further glaciological study.

The evidence of glaciation in the Maymeja area of the Chivay source area is pronounced. Glacial erosion is evident on the south-facing slopes of the northern part of the Maymeja depression, as is expected in the southern hemisphere. The south and south-western slopes of Ancachita peak are steep and unstable, and at the base of this slope is a recessed glacial tarn that appears to retain water during the wet season. A moraine blocks the exit of this tarn feature, but on the slopes below lateral moraines parallel the path of the glacial tongue descending from below Ancachita. In the most deeply eroded part of this northern area contains the only continuous surface flow of obsidian encountered in the entire study area: the Q02-1 source which contained vertical, subparallel fractures and was unsuitable for tool production. Other effects of glaciation on obsidian distributions include the presence of transported obsidian nodules in parallel and terminal moraines in the Quebrada de los Molinos.

In the southern portion of Maymeja a lateral moraine is similarly visible, and the striated and polished benches of lava dramatically attest to the extent of glaciation in the area (see yellow arrow on Figure 4-19). The direction of striation on the these lava flows is consistently south-west or dropping towards the Molinos drainage, and striations persist on high exposed benches suggesting that the glaciers were large as they were striating rocks over twenty meters above the base of the Maymeja area.

During the Middle and Late Holocene glaciers in the south-central Andes, on the whole, have retreated. Glaciological studies conducted in southern Peru and western Bolivia show that retreat is most notable during the time range from circa 10,000 to 3,000 BP (circa 9,000 - 1,000 cal BCE) (Clapperton 1993: 464-466), as well as in last decades of the twentieth century. There is evidence for small advances in glacial extent since 1000 cal BCE, most notably the Little Ice Age circa AD1500-1850. Currently Ancachita is slightly below an altitude permitting glaciation to flow downslope into the Maymeja area, though 5100 masl elevation is glaciated in drainages on peaks like Nevado Sara Sara with large glaciated expanses in the high altitude accumulation zone, and areas of peaks on the eastern side of the Andes like Carabaya (northern Puno) are currently glaciated as low as 4900 masl (Dornbusch 1998). The possible effects of this glaciation on obsidian exposure and weathering in the Chivay area will be discussed in greater detail below.