In 1987, the Caltech biomagnetist and paleomagnetist Joe Kirschvink gave undergraduate Dawn Sumner a rock sample to study for her senior thesis. The sample, collected by UCLA paleontologist Bruce Runnegar, was a reddish, uncompacted, rhythmically laminated siltstone from the Elatina Formation, a late Neoproterozoic, glacial and periglacial unit widely exposed in the Flinders Ranges and elsewhere in South Australia (Preiss, 1987; Lemon and Gostin, 1990). The rhythmic laminations are interpreted to be lunar tidal bundles (Williams, 2000), implying a shallow marine depositional environment. Glacigenic deposits (diamictites and ice-rafted dropstones) occur in most sections of the Elatina Formation (Lemon and Gostin, 1990), but previous paleomagnetic studies suggested that the siltstone was deposited close to the equator on the basis of unusually stable remnant magnetization carried by detrital hematite (Embleton and Williams, 1986). Kirschvink was skeptical that glaciers would ever reach sea level in the tropics, so he instructed Sumner to perform a fold test (McElhinny and McFadden, 2000) on soft-sediment folds (Williams, 1996) in Runnegar's sample. A positive fold test would prove that the natural remnant magnetization (NRM) is primary; a negative result would show that it is secondary, posing no constraint on the paleolatitude of the Elatina glaciation. To Kirschvink's surprise, the fold test was positive (Sumner et al., 1987), as others subsequently confirmed (Schmidt et al., 1991; Schmidt and Williams, 1995; Sohl et al., 1999). A stratigraphically consistent, polarity reversal test (Sohl et al., 1999; see also Schmidt and Williams, 1995) confirmed the primary component of NRM in the Elatina Formation, while the existence of multiple reversals (Sohl et al., 1999) suggests that the Elatina glacial epoch lasted for several 105 to a few 106 years.

The Elatina results refocused attention on Neoproterozoic glaciations. A critical review of the stratigraphic, geochronological, and paleomagnetic constraints on virtually all late Neoproterozoic glacial deposits (LNGD) world-wide was recently published (Evans, 2000). A total of 16 regional-scale units, known to have formed near sea level, possess primary or near-primary NRM components giving at least "somewhat reliable" paleolatitudes (Evans, 2000). Many were apparently deposited within 10 degrees of the equator, and none was laid down at a paleolatitude greater than 60 degrees (Fig. 1). Increased non-dipole components in the Proterozoic geomagnetic field (Kent and Smethurst, 1998; Bloxham, 2000) would not greatly affect those conclusions (Evans, 2000) (Fig. 1). The observations are surprising and they argue that the Elatina result is no fluke.

While LNGD (Fig. 2) closely resemble Phanerozoic glacial deposits lithologically, their distribution and mode of occurrence have never fitted comfortably into Phanerozoic stereotypes (Harland, 1964; Schermerhorn, 1974; Deynoux, 1985; Crowley and North, 1991; Eyles, 1993; Crowell, 1999). LNGD (Fig. 1) are widely distributed on all continents (Mawson, 1949a; Cahen, 1963; Harland, 1964; Hambrey and Harland; 1981; Evans, 2000) and they are sharply interposed in normal marine carbonate successions (Fig. 2c) in several regions (Harland and Wilson, 1956; Schermerhorn and Stanton, 1963; Martin, 1965; Roberts, 1976; Preiss, 1985). Distinctive "cap" dolostone (and rarely limestone) layers sharply overlie most LNGD without significant hiatus (Fig. 2f), implying a sudden switch back to a warmer climate (Norin, 1937; Mawson, 1949b; Deynoux, and Trompette, 1976; Kröner, 1977; Williams, 1979). Cap carbonates (Fig. 3-5) have unusual sedimentological, geochemical and isotopic characteristics not found in other Neoproterozoic or Phanerozoic carbonates (Aitken, 1991; Fairchild, 1993; Grotzinger and Knoll, 1995; Kennedy, 1996; James et al., 2001), and they occur even in successions otherwise lacking carbonate (Spencer, 1971; Deynoux, 1970; Plumb, 1981; Myrow and Kaufman, 1999). Not surprisingly, they have long served as time markers in regional and even inter-regional correlation (Dunn et al., 1971; Kennedy et al., 1998; Walter et al., 2000).

The glacial origin of the Elatina Formation was first recognized by Sir Douglas Mawson—the older, thicker and more localized, Surtian LNGD in the same region were recognized much earlier by Howchin (1908), following the first described LNGD in northern Norway (Reusch, 1891)—and he discovered its cap dolostone (Mawson, 1949b). Although conservative by nature (Sprigg, 1990), Mawson was the first (to our knowledge) to argue that late Neoproterozoic glaciation was global, with large ice sheets in the tropics (Mawson, 1949a). He went on to suggest that climatic amelioration paved the way for the first metazoa (Mawson, 1949a), which had been discovered by a Mawson protegé, Reg Sprigg, in the Ediacara Hills west of the Flinders Ranges (Sprigg, 1947). Ironically (for an Adelaide resident), Mawson was opposed to continental drift, and his argument for tropical ice sheets depended critically on the occurrence of LNGD in tropical Africa today (Mawson, 1949a). [To his credit, Mawson urged students to read his contemporary's book (Wegener, 1922) and Sprigg, for one, identified the Adelaidean succession as a pre-Mesozoic rifted continental margin (Sprigg, 1952).] Mawson's argument, however, collapsed in the plate tectonic revolution, with most subsequent workers attributing the extent of LNGD (Fig. 1) to rapid drift of different continents through polar regions at different times (Crawford and Daily, 1971; McElhinny et al., 1974; Crowell, 1983, 1999; Eyles, 1993).

Most, but not all. Brian Harland of Cambridge University cut his teeth in the Arctic archipelago of Svalbard, sentinel of the Barents Sea shelf, and host to a pair of LNGD (Harland et al., 1993; Harland, 1997). Harland was no fixist, but he independently reiterated Mawson's arguments (Harland, 1964; Harland and Rudwick, 1964) and reinforced them with paleomagnetic measurements. Low-inclination NRM in LNGD and associated strata in the North Atlantic region and elsewhere seemed initially to prove that ice sheets had indeed extended to low paleolatitudes (Harland and Bidgood, 1959; Bidgood and Harland, 1961; Chumakov and Elston, 1989). Later, however, with the recognition of widespread, low-temperature remagnetization (McCabe and Elmore, 1989), field tests and demagnetization procedures that constrain the age and subsequent history of magnetization became the gold-standard for acceptance of paleomagnetic data (McElhinny and McFadden, 2000). The Elatina Formation was the first LNGD for which there were multiple field, rock-magnetic, and petrographic tests indicative of a primary, low-inclination NRM, contemporaneous with sedimentation (Embleton et al., 1986; Sumner et al., 1987; Schmidt et al., 1991; Schmidt and Williams, 1995; Sohl et al., 1999).