46
PLEISTOCENE AND RECENT

Considerably more than half the surface of the map-area is covered
by unconsolidated Pleistocene and Recent deposits. In general these are
thinnest above timber-line (5,000 feet) and become progressively thicker
and more widespread toward the lower valley surfaces. The Pleistocene
deposits are conveniently classified as: (1) mainly non-stratified deposits,
herein represented by scattered erratics and ground moraine; and (2) mainly
stratified deposits represented by glacio-fluvial and glacio-lacustrine
accumulations laid down beyond the ice fronts and by kame terraces,
kames, and eskers deposited in contact with the ice. Recent accumulations
include stream deposits, deltas, alluvial fans, talus, soil, and spring deposits.

The non-stratified Pleistocene deposits found well above timber-line
are mainly scattered ice-transported boulders, cobbles, and pebbles. These
lie on nearly all surfaces except those too steep to retain them. Near
timber-line the erratics are accompanied by unsorted sandy and gravelly
detritus and form a thin till mantling large areas of bedrock. Presumably,
a similar but thicker till underlies most of the timber-covered lower slopes,
although it is there mainly obscured by younger deposits of soil or other
material. These tills are interpreted as ground moraine, material generally
considered to have accumulated largely beneath ice. No clay-rich till
(boulder clay) was noted.

The observed stratified Pleistocene deposits were formed during the
gradual melting and thinning of the ice. Great torrents of waters were
- released into partly ice-filled valleys. Some of this water, flowing along
the margins of thick valley glaciers, formed gravel and sand deposits that
now appear as discontinuous or short benches (kame terraces) at many
places high up on the valley walls. Tributary streams, dammed by glaciers
occupying the trunk valleys, formed occasional marginal lakes well above
the valley floors; and in these lakes were deposited sands, silts, and finely
bedded silty clays. As the fronts of the valley glaciers receded, turbulent
streams issuing from them continually deposited great quantities of coarse
detritus on the valley floors beyond the ice front. Thus, with the final
disappearance of the ice, except for alpine glaciers, the bottoms of all the
major valleys were deeply buried by outwashed sand and bouldery gravel
varied here and there by eskers, kames, and kettles. During this process
the present McConnell Creek Valley was occupied for a time by a lake,
perhaps ice-dammed, within which were deposited the silts that now underlie
the gold-bearing gravels.

In Recent time, the major streams, lacking the former abundant supply
of glacial water, shrank to more nearly their present volumes, and have
been engaged mainly in terracing and trenching the Pleistocene outwash
deposits. Some of the resultant load has been deposited in conspicuous
accumulations such as the delta of Niven River, but much has been widely
dispersed or carried beyond the map-area. Other clearly Recent deposits
are the numerous alluvial fans laid down where tributary streams debouch
onto the wide floors of trunk valleys.

Talus clothes the lower slopes of many of the peaks and ridges, but
is particularly characteristic of areas underlain by the Omineca intrusions
or by the meta-andesites and associated rocks of the Takla group. The