3

more than 7,500 feet in elevation, but the general summit level is about
6,500 feet. The elevation of Dease lake is 2,440 feet, so the maximum
relief is about 5,000 feet.

The northern part of the area is drained by Dease river, which flows
north via Liard and Mackenzie rivers to the Arctic ocean, and the southern
part by Tanzilla and McBride rivers, which flow south to Stikine river.
The divide between the Arctic and Pacific waters is 1 mile south of Dease
lake and only 150 feet higher.

Most of the larger streams occupy broad U-shaped valleys. The
valley bottoms are drift filled and in some places contain numerous shallow
depressions called kettle-holes. These depressions were caused by melting
of blocks of ice that had been covered with stream wash or glacial debris.
Long, marrow ridges of gravel, called eskers, are another characteristic
glacial feature of some of the valley bottoms. These gravel ridges are
believed to have been formed by streams flowing under glaciers.

Some of the tributary streams occupy hanging valleys graded through-
out the greater part of their length and precipitous near their junction with
the main stream valleys. These valleys lie transverse to the direction of
movement of the Pleistocene ice-sheet and do not appear to have been
glaciated. Tanzilla and McBride rivers and most of their tributaries head
in glacial cirques, some of which contain small glaciers.

Stream-cut terraces constitute one of the interesting topographic
features of the area. The Tanzilla valley, south of Dease lake, is terraced
to an elevation of 3,000 feet, that is, 400 feet above the present divide
between Dease lake and Tanzilla river. Gravel terraces are also present
along McDame creek and on the banks of Dease river below Cottonwood
river. Stream terraces also occur on a flat 2 miles wide immediately below
Cottonwood river and were formed by this stream, which formerly entered
Dease river several miles below its present outlet.

GLACIATION

Johnston ‘and Kerr say the Pleistocene ice-sheet moved south across
Dease Lake area. In the northern part of Eagle-McDame area numerous
granite boulders occur as glacial erratics up to elevations of 6,000 feet
north of the Cassiar batholith and show that there the ice moved to the
north. The distribution of erratics along the Muddy, Tanzilla, and
McBride valleys indicates that the last movement of the ice in the southern
part of the area was also to the north. These valley glaciers were at least
2,000 feet thick, for granitic and volcanic boulders from Hotailuh moun-
tains are found on all but the higher peaks bordering Tanzilla and Muddy
River valleys. In the northern part of the area the direction of movement
of valley glaciers, during at least the waning stages of the Pleistocene, is
apparent. Ice moved down Cottonwood river to join the ice stream along
the Dease. Part of the Cottonwood Valley ice moved north through a
broad pass to the junction of Quartz and Trout creeks and then down
McDame Creek valley. Part of the McDame Valley ice stream moved
down the valley to Dease river and part moved north through a broad
pass to join the Dease Valley ice stream 20 to 30 miles below McDame
creek.