Magnetic profiles collected by ships and planes north of Iceland yield sea‐floor spreading rates of 1.0 cm/yr normal to the ridge crests between Iceland and the Jan Mayen fracture zone and on Mohns ridge. The trans‐Arctic extension of the ridge is characterized by anomalously great water depths and spreading rates probably 1 cm/yr or less. The magnetic anomaly signatures reveal exceptionally low amplitudes compared to other ridges, even when these factors are taken into consideration. The Iceland‐Jan Mayen ridge may be as young as 4 m.y., with previous spreading having occurred along an axis now identified only by a region of seamounts at the floor of the Norwegian basin.

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ABSTRACT. A summary of the techniques used and results obtained from three oversnow traverses in Marie Byrd Land and the Ellsworth Highland between January 1957 and January 1959 is presented. Seismic reflection shooting at 30 nautical mile (55' 5 kIn.) intervals was combined with gravity, magnetic and altimetric measurements to determine the glacial and subglacial topography. It was found that a vast portion of West Antarctica has an ice-rock interface well below sea-level. A major connecting channel with a maximum depth of more than 2,500 m. below sea-level exists between the Ross and BellingshausenAmundsen Seas, whereas there is no major topographic connection between the Ross and Weddell Seas. This channel divides West Antarctica into two provinces with granite and rocks of sedimentary origin to the east and south, and a volcanic region to the north-west. Present ice flow is outward from two high areas, centred over mountainous regions on either side of the channel. It is concluded that the present ice sheet has grown from the convergence of the two smaller ice sheets which formed in the mountainous areas and joined across the intervening open water. RESUME. Les THISpaper presents a summary of the techniques used for determining glacial and subglacial topography and the results obtained from three traverses in West Antarctica between January 1957 and January 1959. In Figure I are shown the routes covered during the two-year period. Owing to the unavoidable delay in getting equipment to Antarctica, the first, or Little America to Byrd Station traverse, did not get under way until 28 January 1957, and the scientific program was necessarily abbreviated. The traverse route followed the Army-Navy trail to Byrd Station which had been established during the previous three months. Altimeter, gravity and magnetic observations were made throughout this traverse every 5 statute miles (8 km.). No seismic soundings were made in the first 200 miles (322 km.), which were on the Ross Ice Shelf, as this was scheduled to be done at a more opportune time by the Little America traverse party. From Mile 200 (322 km.) to Mile 500 (80S km.) seismic reflections were obtained every 50 miles (80' 5 km.), thence every 20 miles (32 km.) to Byrd Station (Mile 675; km. 1,086). Byrd Station was reached on 27 February after just one month on the trail.The second (Sentinel) ; then, following a route which looped a bit to the north to skirt the Horlick Mountains, to lat. 84'3°S., long. g2°W.; thence back to Byrd Station, covering this time 916 nautical miles (1,6g8 km.). On both of these latter traverses altimetric, magnetic and gravity observations were made every 3 nautical miles (5 .5 km.) and seismic soundings every 30 nautical miles (55' 5 km.). EQUIPMENTThe seismograph used was a 24 trace Texas Instruments 7000B Portable Seismograph System with a basic frequency range from 5 to 500 c./sec. and with a selection of gain, filter, JOURNAL OF GLACIOLOGY mixing, and automatic gain control settings providing a large number of operating char...

Three methods have been employed at Little America V to determine seismic velocity as a function of depth in the Ross ice shelf: (1) refraction‐shooting interpreted by means of the Herglotz‐Bateman‐Wiechert integral, (2) seismic logging of a deep borehole, (3) Robin’s formula relating P wave velocity to the density and temperature measurements made in the borehole. Results of the three methods are compared, discrepancies are discussed, and the preferred P and S velocity‐depth profiles are obtained. From the profiles and the measurements of density in the borehole, a complete determination of the elastic properties of the ice shelf is possible. Small velocity differences are obtained at depth by the methods of refraction‐shooting and borehole‐logging. The effects of crystal orientation on seismic velocity is discussed and found to be sufficient to account for the differences. A new determination of the variation of seismic velocity in ice as a function of temperature is made on the basis of recent experimental work.