c. 18- to 35.5-cm horizon.
ice grains with the c-axes normal and nearly nor-
mal to the natural ice surface. The horizontal sec-
tion shows the grain diameter and degree of
extinction and the vertical section shows the co-
lumnar nature of the ice down to approximately
40 cm (16 in.). At the 5-cm (2-in.) depth, a hori-
zontal discontinuity is apparent with crystals less
than 0.5-mm diameter in this zone. As the vertical
grain boundaries are discontinuous at this gap,
probably the top 3 cm (1 in.) experienced melting
and recrystallization a second time.
The number of inclusions decreases with depth.
Between 18- and 35-cm (7- and 14-in.) depth, in-
d. 35.5- to 49.5-cm horizon.
ternal damage is apparent with 2- to 7-mm (0.8-
to 3-in.) radial cracks. Grain boundaries are not
Figure 55 (cont'd).
well defined and the grains are more elliptical
with jagged boundaries in the horizontal plane.
At the 40-cm (16-in.) depth, a distinct boundary
approximately 45 cm (18 in.). Bubbles increase in
occurs, delineating the meltwater ice to glacial ice
size with depth until the bottom of the core at 68.5
transition. From 40 to 68.5 cm (16 to 27 in.) the ice
cm (27 in.).
is characterized by 0.5- to 5-mm (0.02- to 0.2-in.)
At 10,000 ft along the runway (south end), the
diameter, randomly oriented grains with some
ice at the upper horizon (011.5 cm or 04.5 in.) is
intrusion of melt at the boundary to a depth of
randomly oriented and grain sizes range from
53
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