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Updated: May 17, 2025
The average height of the Jura is about one-third that of the Alps, and it is now entirely destitute of glaciers; yet it presents almost everywhere similar moraines, and the same polished and grooved surfaces. The erratics, moreover, which cover it, present a phenomenon which has astonished and perplexed the geologist for more than half a century.
Boulder clay of glacial period, with far transported erratics, some of them polished and scratched, 20 to 80 feet in thickness. 5. Contorted drift. 6. These cliffs vary in height from fifty to above three hundred feet.
So much was this the case, that the intention of Agassiz had been to embody the whole in a publication, the first part of which should contain the glacial system of Agassiz; the second the Alpine erratics, by Guyot; while the third and final portion, by E. Desor, should treat of the erratic phenomena outside of Switzerland. The first volume alone was completed.
Both ice sheets and valley glaciers drag on large quantities of rock waste in their ground moraines. Stones transported by glaciers are sometimes called erratics. Such are the bowlders of the drift of our northern states. Erratics may be set down in an insecure position on the melting of the ice. DEPOSIT. Little need be added here to what has already been said of ground and terminal moraines.
In the map, the dark shade expresses the land which alone remained above water. The area shaded by diagonal lines is that which cannot be shown to have been under water at the period of floating ice by the evidence of erratics, or by marine shells of northern species.
He found true erratics loose pebbles, granite, gneiss, and granitic sandstone, having no resemblance to any native rock in the vicinity scattered over the whole surface of the hill to its very summit. The hill itself had also the character of the "roches moutonnees" modeled by ice in the northern hemisphere.
Chronological Relations of the Close of the Glacial Period and the earliest geological Signs of the Appearance of Man. Effects of Glaciers and Icebergs in polishing and scoring Rocks. Scandinavia once encrusted with Ice like Greenland. Outward Movement of Continental Ice in Greenland. Mild Climate of Greenland in the Miocene Period. Erratics of Recent Period in Sweden.
The marks of glaciation on the rocks, and the transportation of erratics from Cumberland to the eastward, have been traced by Professor Phillips over a large part of Yorkshire, extending to a height of 1500 feet above the sea; and similar northern drift has been observed in Lancashire, Cheshire, Derbyshire, Shropshire, Staffordshire, and Worcestershire. Binney and Mr. Prestwich.
All geologists acquainted with the district now under consideration are agreed that the mountain ranges A, B, and c, as well as the adjoining valleys, had assumed their actual form and position before the drift and erratics accumulated on and in them and before the surface of the fixed rocks was polished and furrowed.
But these big erratics, says Puggaard, never enter into the fissures as they would have done had they been of date anterior to the convulsions. Fourth. After this subsidence, the re-elevation and partial denudation of the Cretaceous and glacial beds took place during a general upward movement, like that now experienced in parts of Sweden and Norway.
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