Mima Mounds
Mima Mounds: Still A Mystery
Scott Piazza
Mima mounds are mounds of soil that occur in random places all over the world. At UCSC there are two places on campus where mima mounds are located: across from the Arboretum, sometimes referred to as Mima Meadow, and West Marshall Field.
Mima mounds are of soil between three and four feet tall and 15 feet wide. These mounds are located where the water table is relatively high. The question is how did they form? Their origin is still unknown, although “geologists tend to favor explanations involving physical processes such as stabilization of ancient sand dunes. . . while biologists favor biotic process such as the wallowing . . . of buffalo and prehistoric elephants or the mound-building of giant gophers.” Others have suggested they are Indian burial mounds.
Whatever their origin, the conditions of Mima mounds can allow vernal pools to form between the mounds, but as Rick Stanley explains in the “Rocks and Landforms” chapter of The Natural History of UC Santa Cruz, the "hog wallows" between the mounds don't form vernal pools but very temporary ones too shallow to be considered true vernal pools. However, the depressions between the Mima mounds do collect more moisture than the mounds, and they stay wetter longer in the spring. They therefore have a different sort of vegetation than on top of the mounds—more plants that are adapted to wetter conditions. This is most common during the rainy season and sometimes into spring. As one source explains, “Most of the curious mounds in the northwestern corner of campus are natural geomorphic features. A number of ecologically interesting plants and animals are specially adapted to life in and on the edges of the frequently flooded "hog wallows" between the mounds.”
[Aerial view of the Mima mounds across from the West entrance to UCSC.]
When approaching the Mima mounds, the first thing I noticed was the variety of plant life around them. These mounds located in West Marshall Field house reportedly have played host to a plant called a halophyte, a salt-tolerant plant, though Tonya Haff, of the UCSC Environmental Studies Department, points out that they “are super rare, I’ve never seen them.” These plants require very wet conditions, usually during winter and spring. One problem is weed invasions. Efforts by UCSC in West Marshall Field attempt to protect these native species have met with mixed success.
Other native species around the mounds include the Nutall’s Quillwort, only found in the hog wallows between the mounds, one of the only vernal-pool adapted species on campus,” Haff explains. It “is the only true halophyte found in the Mima mounds and has not been documented in many years, unfortunately,” and the large flowered star tulip, uncommon but not as rare, require these wet conditions that the mounds and vernal pools provide. The coast trefoil, although not rare, can be found between the Mima mounds, but like the star tulip, is not confined to them. While UCSC’s goal for West Marshall Field is to protect and enhance these native species, Haff believes the programs have not been effective: “The vegetation in and around the mounds, particularly around Marshal Field, is far from safe. It is being invaded by non-native grasses, and the habitat (and the Ohlone Tiger Beetles that mate there) therefore is threatened.”
Like many of the natural features of the UCSC campus, most of us overlook them in our busy life rushing to classes and opting for a shuttle or bus to get there. Meanwhile, the mystery of the origin of the mima mounds remains.
