Mima Mounds

Coordinates: 46°53′55″N 123°03′02″W / 46.8985602°N 123.0504799°W / 46.8985602; -123.0504799 Mima mounds ( /ˈmaɪmə/) is a term used for low, flattened, circular to oval, domelike, natural mounds found in the northwestern United States, Idaho, Oregon, and Washington, that are composed of loose, unstratified, often gravelly sediment that is an overthickened A Horizon. These mounds range in diameter from 3 to more than 50 m; in height 30 cm to greater than 2 m; and in density from several to greater than 50 mounds per hectare. Within the northwestern United States, they are typically part of what is commonly known as hog-wallow landscape. This type of landscape typically has a shallow basement layer such as bedrock, hardpan, claypan, or densely bedded gravel. In the northwestern United States, Mima mounds also occur within landscapes where a permanent water table impedes drainage, creating waterlogged soil conditions for prolonged periods. Mima mounds are named after the Mima Prairie in Thurston County, Washington.

Mima mounds occur outside the northwestern United States in three major regions west of the Mississippi River as illustrated by Cox and Washburn. First, they are found in a strip consisting of northwest Baja California, western and northcentral California, and southcentral Oregon where they are typically known as "hogwallow mounds". Within this strip and the area of San Diego, they are often an integral part of the local vernal pool landscape. Second, they are found in a roughly north-south strip of the Great Plains containing parts of northcentral New Mexico, and central Colorado, and central Wyoming where they are typically called "prairie mounds". Third, they are found in an area containing parts of East Texas, western Louisiana, southeast Oklahoma, and southern Missouri where they are commonly known as either "pimple mounds" or "prairie mounds". Finally, isolated patches of Mima mounds also are found in Iowa, eastern North Dakota, and northwest Minnesota.

Within Louisiana, Missouri, Oklahoma, and Texas, pimple mounds, also called locally "prairie mounds" and "natural mounds", consist of low, flattened, circular to oval, domelike, mounds composed of loose, sandy loam or loamy sand. Typically, these mounds consist entirely of a thickened loamy and sandy A and E horizons lying either on a more or less flat or slightly, but noticeably depressed, clayey B horizon. Pimple mounds range in diameter from 6 m to more than 45 m; in height 30 cm to greater than 1.2 m; and in density from several to greater than 425 mounds per hectare. Unlike the Mima mounds of Oregon and Washington, pimple mounds are not limited to the relatively flat and poorly drained surfaces, i.e. late Pleistocene coastal and fluvial terraces. They also occur in abundance of the slopes, summits and crests of hills created by the deep erosion and dissection of unconsolidated and unlithified early Pleistocene and middle Pleistocene, Pliocene, and older coastal plain sediments. Rarely, pimple mounds that occur on these hillslopes are elongated in an upslope-downslope direction.