Erratic Block

A erratic block is, in Géologie and Géomorphologie, a fragment of rock of relatively important size and which was moved by a Glacier sometimes at long distances. At the time of the cast iron of the glacier, the erratic block is on the spot abandoned.

Description

Discovered erratic blocks and first assumptions

At the 18th century, the first geologists who arrive in the the Swiss Alps and the the Jura are intrigued by enormous granitic rocks several tons placed at the top of the hills, isolated in the middle of the plains, etc They are quickly called “erratic blocks” because it is not known how they arrived there, they are lost . Horace-Bénédict de Saussure is perplexe : “ the granites are not formed in the ground like truffles, and do not grow like fir trees on the rocks limestones! ”

Many theories and explanations are born then. Jean-Etienne Guettard supposes in 1762 that the erratic blocks of the plain of Northern Europe are all that remains of an old eroded mountain.

But the alpine origin of these blocks is quickly shown. The question of the transport of these blocks arises then. In 1778, Jean-Andre Of Luc puts forth the assumption of underground explosions due to air pockets which would expel blocks on kilometers. De Saussure does not adhere to this idea only it judges farfelue : there is no example of these explosions and the blocks would be pulverized while being crushed on the ground.

De Saussure notices whereas the blocks are in the axis of the alpine valleys. The step between erratic blocks and river transport is quickly franchi : the rocks would have been deposited at the time of enormous risings caused by the rupture of lakes or the brutal cast iron of the glaciers by hot gases. Leopold von Buch calculates even the force necessary so that these blocks are carted over the Jura.

But of others consider an origin marine : the rising of the Alps was so brutal that the sea which was with its feet would have carted the erratic blocks while being withdrawn. Others, on the basis of the observations on the ice-barriers, think that the erratic blocks would have been deposited by the ice-barrier or the icebergs melting when the sea recovered the area.

All these theories have their advantages, their defects, their defenders and their detractors but none achieves unanimity.

The glacial origin

Another track is thus explorée : glaciers. Indeed, at that time, the glaciers are in full rising because of what one will invite later the Petit Ice Age at the point to worry the Swiss authorities which fear the destruction of certain villages by the ices.

In 1821, Ignace Venetz, a Swiss engineer, studies the glaciers to include/understand their operation. It collects testimonys on the projection of the glaciers and notes a thing hitherto inconnue : the glaciers transport materials and push in front of them a remains accumulation forming of the characteristic hills. And it is by noticing these same hills with several kilometers downstream from the current face of the glaciers that it comes to the conclusion which the erratic blocks are neither more nor less than the traces of old moraines and than the glaciers moved back.

But this conclusion disturbs because in the Christian tradition, the Ground, since its creation, is cooling. The glaciers could not thus be more wide than today. For Venetz, that does not make any doute : the climate of the Earth oscillates between the heat and the cold and the projection of the glaciers indicate that the time is with cooling.

The veracity of its theory, Venetz will show even further. It reminds a meeting of 1818 with a mountain dweller hunter of chamois which affirmed to him that the glaciers recovered formerly the whole of the Alps as the striated rocks and the erratic blocks found in altitude proved it. It is posed a question then;: until where the glaciers they have advanced? While collecting indices in Switzerland, it comes to the conclusion that all the Swiss plate was occupied by a glacier.

In 1829, Venetz announces as of its observations and its conclusions to a ami : Jean de Charpentier. At the beginning skeptic, it is let persuade by the theory of Venetz which explains a crowd of phénomènes :

the aspect of the erratic blocks with their not blunted edges (unexplainable in the case of a river transport).
the distribution of the blocks in the vallées : blocks limestones on the line and of the granitic blocks on the left. A river cannot make such a sorting contrary with a glacier.
the absence of stratification or sorting of materials.
the trace of small lakes to the hollow of the old moraines in the form of localized laminated deposits.
formation of the moutonnées rocks, their scratches and their polishing by the ices.

But the projection of the glaciers is still explained by an important rising of the Alps which reached a height more important than today and which would have been packed.

Louis Agassiz takes note of the glacial theory east becomes one of its more enthusiastic defender. In 1837, while crossing its European research with those carried out in North America, it concludes that part of the northern hemisphere was found under the ices and speaks about Ice Age . Agassiz and Roderick Impey Murchison go in 1840 in Scotland and discover over there striated rocks and erratic blocks, signs of the old presence of a polar icecap. William Buckland is then convinced of the theory of Agassiz but the scientific world remains still divided.

It is only in 1862 that the glacial theory makes the unanimité : Thomas Jamieson reports the rupture of a Scottish stopping and the complete absence of the phenomena of scratches, erratic blocks, etc, which should have occurred.

Characteristics

The gauge of erratic materials goes from small stones to the large blocks of several thousands of tons (as it is the case, for example, of the erratic blocks of Okotoks, in the province of the Alberta to the Canada). But one holds the erratic term of block to the rock fragments of large gauges. The materials of smaller gauges form as for them Moraine S, Sandur S, Drumlin S…

The geologists can find the place of origin of an erratic block by carrying out petrographic analyzes . One shows thus that blocks can be transferred onto thousands of kilometers.

Erratic blocks everywhere are found where there were glaciers. There exist erratic blocks in Germany, with London, Lyon, the the United States, the Canada…

The geologists help themselves in general of the moraines to reconstitute the extreme projection of a glacier. But these formations, relatively fragile, often disappear with erosion. The erratic blocks can then be of a great help because it is rare that they are moved differently than by a glacier after their deposit. Thus, the erratic block of the hill of the Cross-Russet-red in Lyon (the " large Caillou") allowed to prove that the Glacier of the Rhone had crossed the the Rhone and the the Saone at the time of the Glaciation of Riss.

There exist also underwater erratic blocks: either because they were on the dry land at the time of their deposit and that the sea level increased thereafter, or because they were prisoners of a Iceberg which on the open sea slackened them at the time of its cast iron.

External bonds

History of the description of the erratic blocks

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