Environmental Engineering Reference
In-Depth Information
Figure 3.54.
Broad valleys downstream from glaciers in New Zealand, underlain by great depths of
glaciofluvial gravels and sands. Photo courtesy of Mr. Lloyd Homer, DSIR, New Zealand.
The braided streams and lakes produce very complex, lenticular deposits of sands,
gravels, silts and clays. Terzaghi and Leps (1958) describe the design, construction and per-
formance of Vermilion Dam, a 39 m high zoned earthfill structure built on such complex
glaciofluvial deposits, with maximum depth of 82 m.
In some deposits there are lenticular beds of “open-work” gravels or boulders which
are uniformly sized materials with large voids and extremely high permeability. Cary
(1950) describes the widespread occurrence of these materials in glaciofluvial deposits in
northwestern USA.
3.12.4
Periglacial features
Periglacial conditions are defined here as those under which frost is the predominant
weathering agent. They are often, but not always, associated with glaciers. Permafrost con-
ditions are commonly present but are not essential. Permafrost occurs where winter tem-
peratures are rarely above freezing point and summer temperatures are only high enough
to thaw the upper metre or so of the ground.
Figure 3.55
, modified slightly from a diagram of Eyles and Paul (1985), shows features
which may be developed under periglacial conditions.
1. Deep-seated creep of weak sedimentary rocks into the valley. Competent beds
develop widely gaping or infilled extension joints (gulls) and move downslope on the
weak materials as complex slides or rafts;
2. Weak rocks contorted and bulged upwards, overlain by terraced gravels;
3. Outcrops showing evidence of toppling and cambering, with scree and rockfall
deposits downslope;
4. Outcrops of very strong crystalline bedrock surrounded by blockfield of frost-heaved
bedrock. Terraces cut by nivation - free-thaw and slopewash at the margins of snow
patches;
