Geology Reference
In-Depth Information
Table 7.1.
Summary of a ground-ice classifi cation system
according to Pihlainen and Johnston (1963).
A. ICE NOT VISIBLE
Group Symbol
Subgroup
Description
Symbol
N
Poorly bonded or friable
Nf
No excess ice
Nbn
Well-bonded
Nb
Excess ice
Nbe
B. VISIBLE ICE - LESS THAN 1 INCH THICK
Group Symbol
Subgroup
V
Description
Symbol
Individual ice crystals
Vx
or inclusions
Ice coatings on particles
Vc
Random or irregularly-
Vr
oriented ice formations
Stratifi ed or distinctly-
Vs
oriented ice formations
C. VISIBLE ICE - GREATER THAN 1 INCH THICK
Group Symbol
Subgroup
Description
Symbol
ICE
Ice with soil inclusions
ICE
+
soil type
Ice without soil inclusions
ICE
Note: The impression received by the unaided eye is that none of
the frozen water soils in the N group occupies space in excess of
the original voids in the soil. The opposite is true of frozen soils
in the V group.
While easy to use, this descriptive system provides no indication of genesis; predictions
as to the extent and possible occurrence of ground ice are diffi cult. Today, a more com-
monly used classifi cation is based upon (a) the source of water immediately prior to freez-
ing, and (b) the principal transfer process which moves water to the freezing plane
(Johnston, 1981; Mackay, 1972a). The result is seven mutually-exclusive ground-ice types
(Figure 7.1). It must be noted that the various types of buried ice are excluded. The advan-
tage of this classifi cation, in addition to its clarity, is that it emphasizes the variety and
complexity of the transfer process, recognizing sublimation, gravity transfer, and soil-
water expulsion, together with the effects of soil-water pressure and soil-water gradient
as mechanisms. Although the principles behind these transfer processes are beyond the
scope of this topic, the classifi cation has the added advantage that it focuses attention
upon four broad types of ground ice: (1) vein (wedge) ice, (2) segregated ice, (3) intrusive
ice, and (4) pore ice. The fi rst three types are important from a geomorphological