Cross Strata Anomalies
Configuration of Cross Strata Around Pebbles
Flume Experiments with Pebbles
Composition of the Drift
Expansion Effects in the Drift
Robert Jameson on Conglomerates
Related Links There is no sign of deflection of the cross strata around embedded pebbles in cross stratified drift, that would be expected due to stoss and lee effects. In an environment of rapid current flow, erosion of a horseshoe-shaped trough occurs on the stoss side of the pebbles, and elongated ridges form on the lee side, parallel to the current direction, but these effects seem to be absent in typical exposures of cross strata in drift and sandstone, where pebbles are present.
If it formed as a sediment, the presence of pebbles in sandy cross stratified beds would also cause bumps and wavyness in the cross strata, due to the elongated ridges or flutings on the lee side of the pebbles in the vicinity, but these effects are generally not present. This seems anomalous if the cross strata was formed in a sedimentary environment where rapid currents produced the cross strata and transported the pebbles.
Lack of the stoss and lee effects around embedded pebbles is a prediction of the disintegration theory. In this theory, the drift formed by a disintegration mechanism, and laminations of cross strata mark the successive surfaces of disintegration. No currents were involved, so there would be no stoss and lee effects around embedded pebbles.
The limestone bedrock in the area of the Peterborough drumlin field consists of Ordovician calcareous shales and limestones of the Black River and Trenton Groups along with basal sandstones. These overlie Precambrian granite and metasediments.
Across the Shield boundary, the Precambrian pebble content of the drift declines, and the composition of the sand and finer particles changes. Above the limestone bedrock, only about 14% of the pebbles are silicate, the rest are carbonate pebbles, presumably derived locally.
These results are reported by J.G. Cogley and co-workers [See Reference]. In the Abstract, the authors say:
...little of the till covering a portion of central Ontario was carried across the boundary between Precambrian rocks (up-ice) and Paleozoic limestone (down-ice). Seven eighths of the pebble fraction is local, from within 2-5 km of the site of deposition.From the point of view of the glacial interpretation of the drift, these observations seem surprising. The streamlined landforms in the area of their investigation consist of well developed drumlins and flutings, some extremely long. These clearly show the flow directions of the agent of streamlining. If drumlins were shaped by ice sheets, one would expect plenty of pebbles from the Shield to have been carried south and deposited over the limestone, but they are relatively few where they should be abundant.
From the point of view of the disintegration theory, the silicate pebbles and boulders in drift overlying limestone are interpreted as having formed in place, from minor components of the original sediment, or from interbedded units of shale and sandstone, during disintegration of the rock in former catastrophic conditions. Drumlins are believed to have been formed by very rapid currents that streamlined unconsolidated sediments.
The sand fraction in the drift over the limestone regions was on average about 61% non-carbonate, but highly variable, ranging from 100% to 20%. The insoluble component of the limestone bedrock was 5-12%.
The authors reported that limestone pebbles were present in the drift even in areas north of the Shield boundary, which is highly anomalous in a glacial interpretation. About 10% of the larger drift pebbles and 5% of the smaller pebbles were limestone. While this observation is particularly damaging to the glacial explanation, it is easily explained in the disintegration theory; a layer of limestone sediment was disintegrated together with some of the Shield rocks.
Movements due to expansion can explain striations on bedrock, and the formation of kames and eskers, in areas where expansion occurred, and where there was resistance from another drift layer, resulting in a pressure ridge of drift gravel and sand being thrust up between them.
William Buckland, who became Professor of Mineralogy and of Geology at Oxford (later appointed Dean of Westminster), opposed the views of Neptunists like Jameson. In a letter to Greenough, now in the Cambridge University Library, Buckland wrote in 1817:
In Bath I attended two lectures of a young disciple of Jameson, Dr. Galby who is very promising. He gave us Jameson's last theory of the coal formation, which is that all the substance of coal as well as the strata attending it is of chemical origin and unconnected with vegetable matter and that whin dykes are contemporaneous concretions in the rocks they transverse. The whole theory of denudation is to him terra incognita. All pebbles in every species of conglomerate are considered as contemporaneous concretions, with sundry other monstrous opinions which at this time of day astonish me in Jameson.Jameson apparently eventually converted to the views of James Hutton.
ReferenceRupke, N.A. 1983. The great chain of history. Clarendon Press, Oxford. p. 119 Raphael Pumpelly on secular rock disintegration
The Destruction and Shattering of the Chalk of Eastern England, by Sir Henry H. Howorth.
The Dislocation and Disintegration of the Chalk in Eastern England and in Denmark, by Sir Henry H. Howorth.
The Chalky and other Post-Tertiary Clays of Eastern England, by Sir Henry H. Howorth.
The So-called Middle Sands and Glacial Gravels of Eastern England, by Sir Henry H. Howorth.
Mystery of Pothole Origins
Pillars, Polystrate Formations, and Potholes