Sastrugi breaking equipment—ripples and waves are often undercut and the surface is hard and unforgiving, with constant minor topographic changes between ridge and trough. The words "sastrugi" and "zastrugi" are Russian-language plurals; the singular is "sastruga" or "zastruga". The form "sastruga" started as the German-language transliteration of the Russian word заструга (plural: заструги). A Latin-type analogica singular "sastrugus" is used in various writings on exploration of the South Pole, including Robert Falcon Scott's expedition's diaries and Ernest Shackleton's "The Heart of the Antarctic". Under the action of steady wind, free snow particles accumulate and drift like the sand grains in barchan
Surface states be satisfied for each formula_24 separately and for each formula_24 a single, but generally different energy level for the surface state is obtained. A surface state is described by the energy formula_26 and its wave vector formula_24 parallel to the surface, while a bulk state is characterized by both formula_28 and formula_29 wave numbers. In the two-dimensional Brillouin zone of the surface, for each value of formula_28 therefore a rod of formula_29 is extending into the three-dimensional Brillouin zone of the Bulk. Bulk energy bands that are being cut by these rods allow states that penetrate deep into the crystal.