Glacial Period

Chambers's Encyclopaedia, Volume 5: Friday to Humanitarians, p. 222–223

Glacial Period, or ICE AGE, is a term used in geology to designate that period the records of which are included in the Pleistocene System (q.v.). 'Glacial period' and 'Pleistocene period' are in fact synonymous as regards all northern and temperate regions—the former term being used when the prominent climatic characteristics of the period are thought of, while the latter is employed with reference to its life. The chief geographical and climatic changes of this period, and the general features of its fauna and flora, will be considered under PLEISTOCENE SYSTEM. But here a short account may be given of the relics which furnish evidence of former glacial conditions having obtained in many regions that are now in the enjoyment of temperate climates. It is chiefly in the northern parts of Europe and North America, and the hilly and mountainous districts of more southern latitudes, that the glacial deposits, properly so called, are developed. These deposits consist partly of morainic materials, erratics, &c., and partly of marine, fresh-water, and terrestrial accumulations. The most important member of the series is Boulder-clay (q.v.), or, as it is often termed, till. This is an unstratified clay, full of ice-worn stones and boulders, which is believed to have been formed and accumulated under glacier-ice. Several distinct and separate sheets of boulder-clay have been recognised, divided from each other by intercalated 'interglacial beds,' which last are often fossiliferous. The lowest and oldest boulder-clay covers vast areas in the British Islands and northern Europe—extending south as far as the Bristol Channel and the valley of the Thames in England, and to the foot of the Harz Mountains, &c., in middle Germany. Boulder-clay of the same age spreads over the low grounds of Switzerland, and extends from the great Alpine valleys for many miles into the circumjacent low-lying regions. Similar ground-moraines have been met with in all the mountainous and hilly tracts of Europe, as in central France, the Pyrenees, the Spanish Sierras, the mountains of Corsica, the Apennines, the Vosges, the Black Forest, the Erzgebirge and other ranges of Germany, the Carpathians, &c. The rock-surfaces on which the boulder-clay rests are often smoothed and striated, or much crushed and broken, while the hills and mountain-slopes in regions where boulder-clay occurs give evidence of having been abraded and smoothed by glacial action (see ROCHES MOUTONNÉES). At the time the boulder-clay was formed, Scotland, Ireland, the major portion of England, Scandinavia, Denmark, Holland, the larger half of Belgium, Germany as far south as Leipzig, and vast regions in Poland and Russia were covered with a great mer de glace. Contemporaneously with this ice-sheet all the mountain-regions of the central and southern regions of the Continent nourished extensive snowfields and glaciers, which last flowed out upon the low ground often for very great distances. Thus, Lyons stands upon old moraines which have been carried down from the mountains of Dauphiné and Savoy. The interglacial deposits point to great changes of climate when the snowfields and glaciers melted away, and temperate conditions of climate super- vened, as is shown by the geographical distribution of these deposits, and by the character of the plant and animal remains which they have yielded. The youngest boulder-clay, overlying, as it does, such interglacial beds, proves that the glacial period closed with another advance and final retreat of the Scandinavian ice-sheet and the great glaciers of the Alps, &c. The terminal moraines of the last ice-sheet do not come so far south as those of the first and greatest mer de glace. These moraines show that the ice covered the Scandinavian peninsula, filled up the Baltic, invaded north Germany, and overflowed Finland and wide regions in the north of Russia. Similarly in the Alps, &c., the last great extension of the glaciers was not equal to that of the first. See EUROPE.

The boulder-clays are not the only evidence of glacial conditions. Besides those accumulations and the scratched and crushed rock-surfaces already referred to, we encounter numerous erratics (see BOULDERS, ERRATIC), eskers or kames (see ÄSAR), Giants' Kettles (q.v.), clays with Arctic marine shells and erratics (in Scotland, Prussia, &c.)—the organic remains associated with the glacial deposits often affording strong evidence of cold conditions. The following table shows the general succession of the glacial deposits in several parts of Europe:

SCOTLAND—

  1. 6. Valley-moraines and fluvio-glacial gravels = small local glaciers.
  2. 5. Kames, erratics, fluvio-glacial deposits, laid down during retreat of last general ice-covering.
  3. 4. Clays, &c., with Arctic marine shells, occurring up to a height of 100 feet = deposits belonging to the period of retreat of mer de glace, and contemporaneous to a large extent with those of 5.
  4. 3. Upper boulder-clay = moraine profonde of latest mer de glace.
  5. 2. Interglacial beds = disappearance of cold conditions; clothing and peopling of the land-surface with temperate fauna and flora; subsequent submergence to not less than 500 or 600 feet below present level.
  6. 1. Lower boulder-clay with intercalated interglacial fossiliferous beds = the product of more than one mer de glace. The lowest clay marks the period of greatest glaciation.

ENGLAND AND IRELAND—

  1. 6. Valley-moraines and fluvio-glacial gravels.
  2. 5 and 4. Kames or eskers, erratics; fluvio-glacial deposits.
  3. 3. Upper boulder-clay of last mer de glace.
  4. 2. Interglacial beds, marine and fresh-water. Disappearance of glacial conditions; land-surface at first; subsequent submergence to considerable extent.
  5. 1. Lower boulder-clays with intercalated aqueous deposits, indicating probably same conditions as 1 in Scottish series.

NORTHERN EUROPE—

  1. 4. Sand and gravel; erratics; shelly marine clays (in Baltic area).
  2. 3. Upper boulder-clay and terminal moraines of last mer de glace.
  3. 2. Interglacial beds, partly fresh-water and terrestrial, partly marine.
  4. 1. Lower boulder-clay = greatest extension of ice.

SWITZERLAND—

  1. 4. Fluvio-glacial gravels in terraces.
  2. 3. Moraines and upper boulder-clay of last great glaciers.
  3. 2. Interglacial beds, with mammalian remains, &c.
  4. 1. Lower boulder-clay.

CENTRAL FRANCE—

  1. 4. Fluvio-glacial gravels.
  2. 3. Moraines.
  3. 2. Interglacial beds, richly fossiliferous.
  4. 1. Ground-moraines (Mont Dore).

In North America glacial deposits are developed upon a great scale, and there, as in Europe, the boulder-clays are separated by interglacial deposits. The northern part of the continent was drowned in ice during the greatest extension of the mer de glace, the ice flowing south into New Jersey, whence its front extended north-west through Pennsylvania, after which it trended south-west through Ohio and Indiana to reach the 38th parallel of latitude in Illinois. It then appears to have swept away to the north-west in the direction of the Missouri valley. The latest American mer de glace did not come so far south—its terminal moraines being well developed in Minnesota, Wis- consin, Michigan, &c. Evidence of former excessive glacial conditions has been met with in many other parts of the world—old moraines, &c. having been detected in the Caucasus, the mountains of Asia Minor, the Lebanon, the Himalayas, &c. in Asia; in the Atlas, the Kagu and Krome Mountains, &c. in Africa; in the Andes, Tierra del Fuego, &c. in South America; in New Zealand, &c. The probable cause of the glacial period is discussed under PLEISTOCENE SYSTEM.

Source scan(s): p. 0233, p. 0234