What soil can tell us about the Pecos, NM area

What is soil? To a farmer, it is loose material that can nourish and sustain vegetation. To an ecologist or geologist, it can be any material at the earth’s surface that supports an ecosystem. Whether they rest above bedrock or sediment, soils can take a long time to develop, and in hilly areas are easily eroded. Some of the types of vegetation in the upper Pecos River valley are quite dependent on specific soil types, while others are tolerant, adaptable and widespread. It’s easy to see that the hills surrounding this valley have a lot of exposed rock outcrops, where soil is generally thin and erosion is moving materials into the valley, where they can be transported and deposited as sediment.

What is under the soil? In some places in the valley floor, it is sediment carried by the Pecos River, Glorieta Creek, and their tributaries. Sediment consists of mud, sand and gravel; it is created by weathering of rocks. The valley has seen many cycles of deposition and erosion of sediment in the past million years, as the river’s flow has increased and decreased during the Ice Ages (geologically called the Pleistocene Epoch). Underneath this Pleistocene sediment is a surface that would have been the valley floor in earlier times when glacial meltwater enlarged the rivers. As the last glacial cycle ended only about 10,000 years ago, it may have overlapped with the first human inhabitants in this area.

And what is underneath the older valley floor? Bedrock – which geologically means whatever is extensive and continuous below the loose material of soils and sediment. The cliff bands on the face of Glorieta Mesa are part of a bedrock succession of sedimentary rocks over a thousand feet thick. These strata can be divided into formations, which are rock units that are mappable over large areas. Imagine that, prior to erosion of the upper Pecos River valley, the formations exposed in the mesa edge once extended across to the northeast in continuous sheets. These strata are slightly inclined from horizontal, about 10 degrees toward the southwest. Therefore, they dip deeper underground to the southwest, and used to rise over the mountains to the northeast before being eroded.

And beneath the sedimentary formations? The rocky slopes of Pecos Baldy, Lake Peak, and Santa Fe Baldy, which are visible from many parts of the Park, rise to over 12,000 feet above sea level, a mile above the valley floor. These peaks are made of crystalline rocks such as granite, gneiss, and amphibolite, which form many miles deep underground. These “basement” rocks are very old (Precambrian, about 1.7 billion years) and widespread across the Southwestern US, but are covered by the younger sedimentary rock layers in most places away from the mountains.

Last updated: July 18, 2018