Geography Reference
In-Depth Information
57
In the lower reaches of Ferrar Glacier the deep walls were composed of granite and
gneiss, but midway up the glacier, in the vicinity of Cathedral Peaks, a dark horizontal
layer began to appear overlying the granite high up the slopes (see Fig. 2.11). Ferrar pre-
sumed that this layer was the dolerite of the moraines, but he had not yet seen it up close
in outcrop. When the party reached Knobhead, it had found dolerite right down to the
base of the mountain, and no granite in sight. Partway up Knobhead were horizontal lay-
ers of sandstone, then at the top another thick sheet of dolerite.
Two possible scenarios presented themselves to Ferrar as he ascended the glacier.
The horizontal contact between the granite and the dolerite suggested that the top of the
granite was an old erosion surface (an unconformity). But had the dolerite been erupted
as a layer of volcanic magma over this surface, with subsequent deposition of the sand-
stone, and then more volcanism over that? Or, alternatively, had the sandstone been de-
posited over the granite, followed by intrusions of the dolerite along the erosion surface
and between sandstone layers, a type of tabular pluton called a sill?
That question had been answered as the combined parties passed Finger Mountain
a week earlier (Fig. 2.12). There, magnificently displayed on its vertical face, was an out-
crop where the dolerite cut across layers of sandstone stepping from a lower horizon to a
higher one. Blocks of sandstone hung in the once liquid dolerite, dislodged as the magma
intruded though it. This showed clearly that the dolerite was an intrusion formed later
than the sandstone.
As soon as the western party broke camp, the geological party pulled about three
miles down into the windless, sunny lee of Depot Nunatak (see Fig. 2.9). This was a place
to dry out all the sleeping bags and clothes, so while Kennar and Weller spread them out
across the moraine, Ferrar hiked up to the nearest outcrop. The rock was all dolerite with
weathered surfaces a rich, chestnut brown, belying the dark gray coloration of the fresh
rock. At this locality the dolerite formed exceptionally well-developed columnar joint-
ing. Caused by the cooling of the sill, which contracted and fractured into a honeycomb
of smooth, flat-sided columns, the outcrop was reminiscent of the Giant's Causeway in
Ireland or Devil's Postpile in the Sierra Nevada. Ferrar found no sandstone in the out-
crop there, but back down on the moraine were sandstone boulders with intriguing black
flecks in them, possibly graphite, possibly of organic origin. The day had been a good
start at exploring the geology.
The next day the wind had dropped oV completely and the sun was shining in a
cloudless sky. The men pulled across the blue-ice surface to the cliVs on the south side
of Finger Mountain (see Fig. 2.12). The rock face loomed, perhaps three hundred feet of
sandstone overlain by five hundred feet of dolerite. The cliV was fairly sheer, but a steep
spur gave a path for climbing up through the section. Ferrar donned his tools of the
trade, a rock hammer, compass, and sample bags, and started climbing.
As with any sedimentary sequence, the first thing that struck him was the layering,
straight and horizontal, but within individual beds were the wispy festoons of cross-
bedding, telling him that water currents had worked these sands. The grains were essen-
tially pure quartz, coarse-grained and even granular at places. Here and there were thin
layers or lenses with quartz pebbles, in rare cases as big as hen's eggs. It would have taken
some pretty strong streams to move cobbles that large, currents that had swept away any
