A Brief Guide to Parent Material and Landforms Developed for the New Mexico Envirothon Logan Peterson, NRCS Introduction When soil scientists make maps of soil, we search above the ground for clues before we dig holes. As explained in “From the Surface Down,” a soil gets its unique properties from the interaction of five major factors: climate, living organisms, landscape position, parent material, and time. Once we learn how to read a landscape, we can identify differences in each of these factors and, thus, predict differences in soil properties. A steep north-facing slope will be cooler and support different vegetation than a steep south- facing slope of the same mountain. Because these two slopes will differ in their landscape position, microclimate, and living organisms, we can expect that they will have different soil properties. As another example, let’s compare two landforms: a mountain slope and a floodplain along a stream. The mountain slope is made up of bedrock which is several million years old, while the floodplain is made up of sediments which were recently deposited by water. The mountain slope is relatively steep, and water readily runs off of it, while the floodplain is flat and often flooded. Lastly, the hillslope hasn’t changed much in shape for several thousand years, while the floodplain was deposited during a heavy rainstorm just thirty years ago. We can see that the soils on these two landforms differ in their parent material, landscape position, and in the amount of time they have had to form. Because of its landscape position, the floodplain soil receives much more water, so it will grow a very different plant community (organisms) than the mountain slope soil. For all of these reasons, we can expect that these two soils will have very different properties. Case in point, the best way to predict where different soil types will appear is often to look for differences in landforms. In the table below, I have summarized the differences between these two soils—in terms of soil forming factors and soil properties. Mountain Soil Floodplain Soil Parent Material Decomposed bedrock (residuum) Sediment (alluvium) Age of Soil (Time) 20,000 years 30 years Effective Moisture 16 inches per year 50 inches per year Plant Community Ponderosa pine forest Willows and grasses Soil Horizons O, A, B, C, R A, C Depth to Bedrock (R) 85 cm 400 cm Depth to aquifer 400 feet 80 cm Textures (top-down) fibrous organic matter, loam, clay sandy loam, loamy sand loam, sandy loam, bedrock 1 Parent Material In simple terms, parent material is the stuff that soils develop out of. Most soils form in decomposing mineral deposits. Because various life forms such as plants and earthworms live and die in and on the soil, soils also contain different types and amounts of decomposing organic matter. In this sense, parent material can be divided into two basic categories: mineral and organic. Parent material is also categorized by how it ended up where it is. There are four kinds of parent material which are defined based on their mechanism of transport. Alluvium is a kind of parent material which was deposited by water (washed into place). Rocks which have been bounced along the bottoms of waterways for miles become rounded, so rounded rocks in a soil profile are good indicators of an alluvial soil. As a rule, faster-moving water can carry larger particles. As water slows down (picture rapids flowing into a deep pool), it deposits the larger particles it carries. The speed of water flowing over a given spot tends to fluctuate for a number of reasons. The result is that alluvial deposits tend to be stratified: divided into horizontally-stacked layers which are dominated by different particle sizes. For example, a gravelly layer may have been deposited by fast- moving flood waters; it sits directly on top of a silty layer that was laid down when the river was low and calm. Over the years, the fluctuation of water speeds can lead to a repeating pattern of gravelly-silty- gravelly-silty-… The translocation of clay particles through a soil profile can cause differences in texture to occur between soil horizons in non-alluvial soils, but the stratification of different sizes of rounded rocks is a telltale sign of an alluvial deposit. Colluvium is parent material which was deposited by gravity rather than flowing water. A familiar example of colluvium would be a pile of rocks at the base of a cliff. As the cliff face is weathered, pieces of rock break off and pile up at the base. Soils that develop in colluvium often contain rocks of various sizes. Because these rocks weren’t carried and deposited by water, they are generally not rounded, nor are they stratified. Residuum is parent material composed of weathering bedrock. This material can range in age from just a few hundred years (as in fresh lava flows that are just beginning to decompose) to a few billion years (as in Precambrian metamorphic rock). Soils that formed in residuum are often relatively shallow to hard bedrock. Residuum is a dominant parent material on upper mountain slopes, and on the tops of plateaus and mesas. Eolian Deposits are parent materials that were moved by wind; sand dunes are a familiar example. Only the most powerful winds can carry mineral particles larger than sands, so rock fragments larger than 2 mm in a soil horizon are a pretty good sign that the parent material in this layer is not eolian. Many of New Mexico’s soils formed in thin eolian deposits on top of other materials. For example, many of the soils on top of mesas in Northeastern NM formed in eolian deposits which were deposited on top of residuum. Because silts and very fine sands are easily eroded and carried by wind, they tend to dominate eolian deposits. 2 Alluvium Colluvium Residuum Figure 1. Three soil profiles derived from three kinds of mineral parent material. Illustration by Logan Peterson. 3 Figure 2. Two soils with alluvial parent material. Note that the rocks in both are rounded, and that the alluvium on the left is stratified. Photos by Logan Peterson. Figure 3. On the left is a pile of fresh colluvium beneath cliffs in Montana. On the right is a soil formed in this type of parent material. Photos by Logan Peterson. 4 Figure 4. A soil formed in residuum. Tape is in inches. Photo by Charles Hibner. Figure 5. Eolian deposits (sand dunes) near the southeastern border of NM. Photo by Dr. Lynn Loomis. 5 Common Landforms of New Mexico If you pay attention to the Earth’s surface as you drive across New Mexico, you’ll notice that it is made up of a number of distinct shapes. Many parts of the state, such as the plains around Roswell, are quite flat. Other areas are dominated by rounded shapes of foothills or steep shapes of mountains. Geologists have come up with a number of names for such shapes, or landforms. We generally define a landform based on its shape, as well as the way that it formed. For example, mesas and floodplains are both flat surfaces which often have steep slopes along their edges. However, a floodplain was deposited by water, while a mesa is made up of bedrock. Below are some examples of landforms you are likely to encounter at the New Mexico Envirothon competition. Landforms can be divided into two categories, based on how they formed. Landforms which got their shapes by being carved away by erosion are referred to as erosional. Landforms which were deposited by moving water are referred to as depositional. Floodplains, stream terraces, and alluvial fans are depositional landforms; mountains and mesas are erosional. Floodplains are relatively new alluvial deposits which lie along the channel of a stream, river, or arroyo. They are relatively flat, and their slopes usually have the same gradient (steepness) and aspect (direction) as the channel below. Floodplains often have steep walls (or “cut-banks”) along the channel. The flat upper surface of a floodplain is low enough that it floods often (at least once per 100 years). Because floodplain soils are made up of alluvium, they are often stratified and frequently contain rounded rocks. Stream Terraces are similar to floodplains, but they are older and rarely flooded. In fact, all stream terraces were floodplains in earlier times. However, as adjacent channels gradually cut deeper into the Earth, it took progressively larger flows to flood these landforms. A stream terrace floods less than once per 100 years. Stream terraces are shaped like floodplains, with flat tops that have similar slopes to the channels below. The soils of stream terraces are quite similar to those of floodplains, but are older, so they usually have horizons which are more distinct. Alluvial Fans form as water rushing off of steep slopes deposits parent material onto the valley floor below. As torrents of floodwaters come rushing down steep slopes, they erode materials and carry them off. These floodwaters converge in steep canyons and arroyos, which direct them to flat valleys at the bases of the slopes. When these floodwaters hit level ground, they slow down, depositing the largest fragments in their sediment loads first. During the next flood event, waters will be directed around the piles of sediments left by the last flood. In this way, temporary channels meander back and forth across the growing alluvial fan, depositing sediment as they move. Alluvial fans are shaped roughly like hand-held fans; they are steepest and contain the largest fragments at their highest points.