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Coupled ice–till dynamics and the seeding of drumlins and bedrock forms

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Abstract:

The geomorphological effects of ice sliding over till, internal deformation of till and till sliding over bedrock are considered. Two questions are examined: (1) is the till-sheet flow unstable, i.e. is a layer of uniform thickness maintained or not, and (2) does the slip of till over bedrock cause amplification of relief of the bedrock? Such instabilities seem to be necessary to explain such features as drumlins and whaleback forms.

It is found that the answer to (1) and (2) depends on the position of the system in a parameter space, defined by the till rheology, and applied shear stress, the effective pressure at the ice–till interface, the thickness of ice and till and the wavelength of the instability. Two configurations are considered: one where the wavelength of the perturbation is much less than the the ice-thickness, which is related to the classical Nye–Kamb solution for flow over bumps; and one where the wavelength is much greater than the ice thickness, where the mechanics are described by the shallow-ice approximation. In both cases, substantial areas of parameter space, where till-sheet and bedrock modes are unstable, are found. The conceptually related Smalley–Unwin bifurcation is re-examined.

The physical mechanisms by which ice and till flows couple are examined. At very short wavelengths (∼10 m), the ice is so rigid that it forces till waves to move at the ice velocity; while at long wavelengths (∼1000 m), the flows become essentially uncoupled and till waves move at the kinematic velocity. At intermediate wavelengths (∼100 m), high growth rates occur ; this is postulated to be the scale of drumlin seeding.

Document Type: Research Article

DOI: http://dx.doi.org/10.3189/172756499781821931

Publication date: January 1, 1999

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  • The Annals of Glaciology is a peer-reviewed, thematic journal published 2 to 4 times a year by the International Glaciological Society (IGS). Publication frequency is determined and volume/issue numbers assigned by the IGS Council on a year-to-year basis and with a lead time of 3 to 4 years. The Annals of Glaciology is included in the ISI Science Citation Index from volume 50, number 50 onwards.

    Themes can be on any aspect of the study of snow and ice. Individual members can make a suggestion for a theme for an Annals issue to the Secretary General, who will forward it to the IGS Publications Committee. The IGS Publication Committee will make a recommendation for an individual themed Annals issue, together with a potential Annals Chief Editor for that issue, to IGS Council. The IGS Council will make the decision whether to proceed, taking into account the spread of topics and the overall capacity for publication of pages in Annals.

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