|Date sent: Tue,
21 Jul 1998
John Shaw <John.Shaw@ualberta.ca>
Here are my responses to your comments.
How did melting occur under an ice sheet?
Melting at a glacier bed is a result of several energy sources:
i) Geothermal heat conducted from the Earth's interior;
ii) Frictional heat generated by sliding at the ice bed;
iii) Advected heat from surface water or groundwater; and
iv) Frictional heat produced by viscous dissipation in flowing water.
What prevents a sheet flow from being concentrated
Nothing. That's exactly what the field evidence tell us.
The sheet flows are transient and, after the formation of drumlin or Rogen
fields or hummocky terrain, sheet flow is concentrated in tunnel channels.
Convergent vs divergent flow patterns?
Patterns of drumlins do converge into tunnel channels near the former
ice margins (e.g., the Finger Lakes, N.Y.) Elsewhere, patterns diverged
as water flowed radially outwards towards distant ice margins (see Prest
et al. 1968, Glacial Map of Canada). In some locations flow diverged
over high ground; e.g., the Livingstone Lake drumlin field (Shaw and Kvill
Shifts in flow direction
Flow direction in a pressurized system is determined by gradients in
the hydraulic head (i.e. the pressure in subglacial meltwater). If the
pressurized system bursts, flow will be towards ruptures, for example breaks
in a seal around the periphery of the Laurentide ice sheet. Initially flow
will occur through all outlets. Later, when the hydraulic head falls, the
higher outlets will be resealed and flow diverted towards the lowest outlets.
Hence drumlins show cross-cutting patterns related to the changing availability
of outlets as outburst events evolved.
Flow from out of the sea?
If the ice were thickest over the sea then the hydraulic head would
be highest there and flow would be towards what is now land. For
example, the Laurentide ice sheet flowed uphill from Hudson Bay to the
Milk River Ridge in Alberta.
Size of vortices forming drumlins and Rogen moraine
In the meltwater hypothesis vortices forming cavity fill drumlins are
presumed to be on the scale of the landforms. Horseshoe vortices
forming erosional drumlins, Beverleys, may be of much smaller diameter
than the height of the obstacle (drumlin).
Contemporaneous erosion of lake basins and formation
Yes this appears to have been the case, since lake basins, for example
the Lake Ontario Basin, contain only thin or no surficial sediment on bedrock.
Areas adjacent to the present lake have thick deposits, for example the
Oak Ridges Moraine, Ontario.
Drumlin fields are about the same age
Yes, this seems to be the case for the fields related to the maximum
extent of the Laurentide ice sheet. I have inferred from this that
there was some external effect, such as a rapid climate change, on the
ice sheet that caused build up of meltwater storage on, within or beneath
What about the imprint of the ice sheet?
This is a fascinating question. Airphotos, satellite images and
Digital Elevation Models (DEM's) illustrate regional-scale bedforms with
very little subsequent reworking. Consequently, if the meltwater
hypothesis is correct, the ice sheet must have been let down gently on
the bed as pressure fell in the meltwater system. At that point,
because the ice had been floating, there would have been insufficient slope
on the surface to drive ice flow. Thick ice must have stagnated and
melted away. Hence recess. Where did the water come from?
C. Warren Hunt's comments are out of date; I discuss the origins of
the meltwater (Shaw 1996) including references to Shoemaker's papers on
the storage and release of subglacial meltwater. Furthermore, it
is quite clear that we realize that the ice sheet could not be floating
at the margin. It is for this reason that I show tunnel channels
in the marginal area of the ice sheet (Shaw 1996, Fig 7.45).
As I have written before, the similarity of our conclusions regarding
the mechanisms of formation of drumlins and Rogen moraine is remarkable,
given that we worked completely independently of each other. I appreciate
your respectful questioning of the meltwater hypothesis and hope that you
find my responses equally respectful.