A Tentative Classification of Sand Dunes Its Application to Dune History in the Southern High Plains

Abstract

Part I. Existing sand dunes fall naturally into two groups-complex and simple, the former being much the more common. Variable factors, such as wind direction, depth of sand, irregularity of vegetal growth, etc., are responsible for the rarity of the simple dunes. It is only by first studying simple forms, however, that one can discern the origin and history of surfaces of complex form. Following is a basic classification of simple dunes which is at the same time genetic and naturalistic. It is based on the assumption that the sand-moving wind blows with unvarying direction. Simple Dunes A. Bare Surfaces or Loose Sand I. The barcan dune. An isolated bare-sand hill on a nonsandy base II. The transverse dune series. Formed on bare, loose sand of "unlimited" surface area and "unlimited" depth III. The isolated transverse dune ridge. Formed (frequently with human aid) from bare, loose sand of unlimited surface area, but shallow depth IV. Lee dunes 1. Wind-shadow dunes ("umbracer" dunes). Longitudinal dunes. Formed from a continuing (unlimited) sand supply in the lee of a bedrock obstacle 2. Shrub-coppice dunes (see p. 124) 3. Source-bordering lee dunes ("umbrafon" dunes, lee-source dunes). Formed from a continuing (unlimited) sand supply leeward from a source of sand of limited area (e.g., stream flood plain) B. Formed by Wind in Conflict with Vegetation I. Shrub-coppice dune cluster. Formed in and to leeward from bunch or clump vegetation, such as mesquite bush, on an unlimited and smooth surface of very shallow sand II. (a) Blowout or parabolic dunes; (b) elongate-blowout dunes; and (c) windrift dunes. Formed by gentle, moderate, and strong winds, respectively, on deep sand with shrub- or grass-covered surface Under the heading "Complex Dune Forms" is considered the ideal simple case of two equally effective sand-moving winds blowing at right angles. It is pointed out that even this degree of complexity is much simpler than that of many of the well-known sand-dune areas. PART II. A study of dunes in the southern High Plains by means of vertical aerial photographs reveals: 1. Up to the decade from 1920 to 1930, active sand dunes were insignificant in number and total area, compared to the anchored dunes of Recent or post-Pleistocene age. 2. There has been a systematic shift in the direction of the prevailing sand-moving wind during the last 15,000 years (estimate), such that winds which formerly blew toward S. 70° E. have been replaced gradually by modern winds blowing N. 20° E. in most of the region. For convenience in description these dunes are divided into three series: I, II, and III, from youngest to oldest. 3. An unusual series of large parallel windrift dunes, indicating strong persistent winds, was formed around 15,000 years ago. Sand dunes of this type (Series III) are apparently not forming at present to any noteworthy degree, either in the interior of North America or along the coasts.