Evidence
Subglacial
Sed Def

  1. Direct obvs Boulton & Hindmarsh 1987

Displacement of deformable rods in upper till

Rods moved sideways over time

Skepticism

No photographic evidence

Breidamurjokull, Iceland

What is clear

Structures seen within tills in deglaciated envmts

evince their ability to deform

  1. Borehole & Seismics
    Siple Coast

Why AIS ice streams flow so quickly ?

up to 600m/yr (similar to some GrIS outlets)

Ice Stream B (Whillans)

1. Borehole investigation (Alley 1987)

Borehole inclinometer showed only a few m/yr internal def

Drilled borehole ~ 300m upstream from g.l.

∴ Excess surface velocity must result from basal motion (basal sliding or soft sed def?)

Velocities ~ 50 - 850 m / yr ! (up / downstream respect)

  1. Seismic study tills
    (Blankenship 1987)

Seismics showed ~ 7m thick till layer

Probs responsible for 17-30% Whillans motion

  1. Sampled till (Kamb 1991)

Till only support VERY low basal stresses

Surface velocity there ~440 m/a

What?

Explosives set off @ Z in snowpack

Array of geophones receive signal

Nature of returns informs bed material properties

Recovered till samples from base of boreholes

∴ Subglacial sediments deform readily (much > overlying ice)

Borehole evidence elsewhere

Subglacial sed def

Trapridge Glacier, Yukon

Storglaciaren Sweden

Borehole photog (Engelhardt 1987)

Blue Glacier, Washington

Observed till layers deforming

Direct obvs (Boulton, 1986)

Deformation structures observed in glacially-overriden sediments

e.g. faults

Deforming sediment structure

Yield stress ↑ with effective pressure (N), ∴ N ↑ with Z

∴ Critical depth exists beneath which sediments do not fail

Thus
often see:

A) Upper till - deforming

B) Lower till = stable

i.e. doesn't deform below critical Z

Depth @ which deforms = f (rheology & hydrology)

Modelling sed def requires:

  1. Flow law for till
  1. Model for till hydrology

Both = very difficult to derive

Flow law
for till

Various studies tried to find relation
→ btwn driving stress + strain rate for seds

Complicated

by water flow effects in till

Key issue

whether till response is viscous or plastic

Depth @ which till experiences def f (...

Rheology

Permeability

Meltwater availability