A hummocky landscape is an uneven or undulating terrain characterized by numerous small, rounded hills or mounds.
Defining Hummocky Landscapes
This terrain is an irregular, undulating surface, distinguished by many small, rounded mounds or hillocks interspersed with depressions or hollows. These mounds can vary significantly in size, from just a few centimeters to several meters in both height and diameter. The term applies across different scales, encompassing micro-topography found in a marsh or larger features spanning a vast landscape. Such unevenness often results in a chaotic pattern of elevated areas and low-lying zones, which may collect water.
How Hummocks Form
Hummocks originate from a variety of geological and ecological processes. Glacial activity is a common source, where retreating glaciers deposit irregular mounds of till, forming hummocky moraines. This can occur when ice melts beneath a layer of debris that has flowed into position, or where sediment has been stacked near the ice margin. Some hummocky moraines also form at active glacier margins through the thrusting of polythermal ice under compression.
Permafrost dynamics also contribute to hummock formation through repeated freezing and thawing cycles. These cycles can create features like palsas, which are peat mounds with a permanently frozen core. Palsas typically have steep slopes and can reach up to 12 meters in height and 150 meters in diameter, with their formation linked to ice segregation and differential heat transfer in peat. The dry peat on the palsa surface insulates the frozen core, preventing it from thawing during summer, allowing them to persist in climates with mean annual temperatures just below freezing.
Peatland and wetland development also lead to hummocky terrain. In bogs and fens, differential plant growth, like Sphagnum mosses, combined with varying water levels, creates raised hummocks and lower hollows. These hummocks are drier elevated areas, while hollows are shallow depressions often inundated with water. This micro-topography is a result of moss species interacting with the physical environment and varying rates of peat accumulation and decomposition.
Mass wasting events, such as landslides, can also generate hummocky topography. Chaotic deposits from these movements result in an undulating and uneven surface, with hummocks forming from extensional faulting as the landslide begins to move. Volcanic activity can also produce hummocky landscapes, particularly through large landslide avalanches that originate from volcanic collapses. These debris avalanche deposits are characterized by rounded or conical mounds, often containing large, intact blocks of rock from the volcano’s flanks.
Common Hummocky Environments
Hummocky terrain is found in several distinct environments across the globe. Glaciated regions, such as parts of Canada, Scandinavia, and the northern United States, commonly exhibit hummocky moraines.
Arctic and subarctic permafrost zones, including tundras and boreal forests, are also characterized by hummocky landscapes. Here, cryogenic hummocks like palsas and lithalsas are prevalent. Palsas are found in subarctic regions of northern Canada, Alaska, Siberia, northern Fennoscandia, and Iceland, typically in areas with long winters and thin snow cover.
Wetlands and peatlands, including bogs, fens, and marshes worldwide, frequently display hummock-and-hollow micro-topography. These patterned peatlands are a result of specific plant compositions and hydrological conditions. The Sphagnum mosses, which are key species in many northern bog peatlands, contribute to the distinct hummock-lawn-hollow patterns.
Regions affected by significant mass wasting events, such as post-landslide areas, often feature hummocky terrain. For instance, the “Hummocks” trail system in Yellowstone National Park is an example of such topography resulting from landslide debris. Certain volcanic landscapes also exhibit hummocky characteristics, formed by large debris avalanches from volcanic flank collapses. Mount St. Helens in Washington State, for example, has large hummocks on its 1980 landslide deposit, which are remnants of the volcano’s summit.