Thick Till Research Articles

Nature and Rate of Basal Till Deposition in a Stagnating Ice Mass, Burroughs Glacier, Alaska

Rates of basal till deposition ranging from 0.5 to 2.5 cm year-' have been obtained from the Burroughs Glacier in southeast Alaska. Unlike minimum rates of till deposition which are derived by dividing the thickness of till deposited by the length of time the area was ice covered, these values are believed to more closely represent an actual rate of deposition.

Nature and Rate of Basal Till Deposition in a Stagnating Ice Mass, Burroughs Glacier, Alaska

Rates of basal till deposition ranging from 0.5 to 2.5 cm year–1 have been obtained from the Burroughs Glacier in southeast Alaska. Unlike minimum rates of till deposition which are derived by dividing the thickness of till deposited by the length of time the area was ice covered, these values are believed to more closely represent an actual rate of deposition.Because of the emergence of hills during deglaciation, a change in ice flow direction of up to 90° has occurred near the southeast terminus. Since this change is recorded by maps and photographs dating to 1892, a rate of change of ice flow direction can be estimated. Till fabric measurements and till composition at two or three depths in the till at seven localities reflect this change. Estimates of the rate of till deposition are obtained by assuming that the fabric azimuth represents the ice flow direction at the time the till was deposited.Most till deposition seems to have taken place during late stages of deglaciation since at two locations fabric of till just above bedrock or a paleosol records a post-1892 flow direction. The rate obtained by dividing the average thickness of till by the number of years the area was ice covered is an order of magnitude lower than those calculated above.

Periglacial Landslide Topography of Canjilon Divide, Rio Arriba County, New Mexico

On the higher parts of the Canjilon Divide, New Mexico, occur thick deposits of glacial till which are believed to represent the maximum extension of the oldest, or Cerro, glaciation of the San Juan Mountains of Colorado. On the lower parts of the divide and on the gently sloping upland west of it, there are numerous discontinuous and comparatively thin bodies of till which form a distinctively hummocky topography. The topographic forms and the distribution of till are satisfactorily explained only as having been formed by landsliding of bodies of till derived from a thick till mantle capping the remnants of the high-level erosion surface. The sliding took place on gentle slopes, now stable, and is believed to have resulted from intense frost action during glacial stages of the Pleistocene, when periglacial climatic conditions must have prevailed. Two widely separated periods of sliding are suggested by the comparative sizes and the distribution of the various landslide masses. These are correlated, tentatively, with the Durango and the Wisconsin glacial stages of the San Juan Mountains.

The distortion of crystals of aluminium under compression. Part II.―Distortion by double slipping and changes in orientation of crystals axes during compression

Changes in the orientation of crystal axes during compression of a disc cut from a single crystal of aluminium are discussed. They are in accordance with the prediction made on assumption that the crystal slips as determined by distortion measurements. As in case of tensile test pieces, crystal axes always take a position where two possible planes of slip are symmetrically disposed in relation to the stress, but in this case, the orientation of crystal axes relative to normal to flat surfaces of specimen is quite different from their orientation relative to axis of tensile test piece. After the axes have taken the symmetrical position, Laue photographs show that they remain there, even when distortion is very great. Tests were continued till thickness of specimen was only 0.28 its original thickness. Distortion during period when crystal axes remain in symmetrical position is due to slipping on two symmetrically disposed planes of slip. The bearing of the results on structure of rolled metals is discussed and distortions of cubical blocks of material, due to various types of double slipping, are compared, to find out which is most likely to occur.

III.—What must be Explained before the Preservation of Deposits under Till is Explained

As I am one of those who find difficulty in understanding the preservation of stratified deposits beneath the Till, I would like to point out to Mr. J. Geikie that he has not correctly stated the difficulty in his paper (Geol. Mag. Feb. 1878) on “The Preservation of Deposits under Till or Boulder-clay.” The difficulty really lies in the explanation of how the Till itself was deposited: were that clear, the preservation of beds beneath it might be at once intelligible. Admitting the passage of an ice-sheet across the country, admitting further that the Till is the moraine profonde of such an ice-sheet, the accumulation of 100 feet of Till beneath the ice-sheet is a phenomenon which has not been explained, and cannot be passed over as unimportant. A moving mass of ice, 2000 feet thick, seems a formidable agent of erosion; yet where I now write nearly 100 feet of Till intervene between the surface and the Carboniferous rocks, which, where exposed, shows striations having the usual compass bearing for this neighbourhood. The greater portion of the Till hereabouts is derived from the Carboniferous rocks of the district, but numerous boulders of granite, felstone, schist, and of Old Red Sandstone sedimentary and volcanic rocks, show that the ice had brought materials from a wider area. The limits of the Carboniferous series are about twelve miles to the west and northwest of Glasgow, that being the direction whence the ice came. As the ice of a modern glacier pushes the results of erosion before it, we may conclude that the old ice-sheet with a pressure of about 1000 lbs. to the square inch did the same; indeed, Mr. Geikie has already accepted this “idea as to how an ice-sheet would behave,” but without explaining how the alternate exposure and concealment was effected in the presence of very thick Till. It may be said that the ice pushed its moraine profimde before it, but then Hugh Miller's “pavements” have always been referred to as proofs that the ice overrode not displaced the Till already accumulated.