PLATEAU AND MOUNTAINS 419 
nounced swellings first, and then spreading in lobes of 
ice down the broad depressions, we may imagine that 
a very little difference in the contour might determine 
the position of the great outlet glaciers where the ice cap 
drained away to the sea. In other words, the glacier valleys 
do not appear to owe much to pre-giacial topography. 
Let us now survey the marginal mountain range and 
the ice plateau more closely. The plateau seems to rise to 
11,000 feet near the South geographic Pole, and decreases 
gradually to the north, being about 7000 feet at the 
South magnetic Pole. The mountain ranges have peaks, 
such as Markham and Lister, rising to 15,000 and 13,000 
feet respectively, but the average height is perhaps about 
9000 or 10,000 feet, while for considerable stretches near 
Granite Harbour they are only 6000 or 8000 feet high. 
Every 20 or 30 miles this fairly continuous range is broken 
by a huge ' outlet ' glacier. Many of these are now 
well known, such as the Beardmore, which is over 100 
miles long and 30 miles wide, the Ferrar, Mackay, 
David, '&c. They form the only routes from the coast 
to the interior, and were it not for the ice jails where the 
glacier covers some irregularity in its rock floor, or the 
more dangerous crevassed areas^ where it sweeps round 
a corner, or receives the thrust of a large tributary, they 
would not be difficult to traverse with sledges. The 
grade is not very steep, and they are to some extent 
sheltered from the blizzard drift which is the great obstacle 
to Barrier and plateau journeys. Their detailed topo- 
graphy is, however, very different from that of an area 
subjected to ' normal ' erosion. 
