Page:EB1911 - Volume 27.djvu/654

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630
UNITED STATES
[GEOLOGY


differentiated are the following, numbered in chronological order: (5) Wisconsin, (4) Iowan, (3) Illinoian, (2) Kansan, (1) Sub-Aftonian, or Jerseyan. Of these, the Kansan ice-sheet was the most extensive, and the later ones constitute a diminishing series.

Essentially all phases of glacial and aqueo-glacial drift are represented. The principal terminal moraines are associated with the ice of the Wisconsin epoch. Terminal moraines at the border of the Illinoian drift are generally feeble, though widely recognizable, and such moraines at the margin of the Iowan and Kansan drift sheets are generally wanting. The edge of the oldest drift sheet is buried by younger sheets of drift in most places.

Loess is widespread in the Mississippi River basin, especially along the larger streams which flowed from the ice. Most of the loess is now generally believed to have been deposited by the wind. The larger part of it seems to date from the closing stages of the Iowan epoch, but loess appears to have come into existence after other glacial epochs as well. Most of the fossils of the loess are shells of terrestrial gastropods, but bones of land mammals are also found in not a few places. Some of the loess is thought to have been derived by the wind from the surface of the drift soon after the retreat of the ice, before vegetation got a foothold upon the new-made deposit; but a large part of the loess, especially that associated with the main valleys, appears to have been blown up on to the bluffs of the valleys from the flood plains below. As might be expected under these conditions, it ranges from fine sand to silt which approaches clay in texture. Its coarser phases are closely associated with dunes in many places, and locally the loess makes a considerable part of the dune material.

Much interest attaches to estimates of time based on data afforded by the consequences of glaciation. These estimates are far apart, and must be regarded as very uncertain, so far as actual numbers are concerned. The most definite are connected with estimates of the time since the last glacial epoch, and are calculated from the amount and rate of recession of certain falls, notably those of the Niagara and Mississipi (St Anthony Falls) rivers. The estimate of the time between the first and last glacial epochs is based on changes which the earlier drift has undergone as compared with those which the younger drift has undergone. Some of the estimates make the lapse of time since the first glacial epoch more than a million years, while others make it no more than one-third as long. The time since the last glacial epoch is but a fraction of the time since the first—probably no more than a fifteenth or a twentieth.

Outside the region affected by glaciation, deposits by wind, rain, rivers, &c., have been building up the land, and sedimentation has Non-glacial. been in progress in lakes and about coasts. The non-glacial deposits are much like the Tertiary in kind and distribution, except that marine beds have little representation on the land. On the coastal plain there is the Columbia series of gravels, sands and loams, made up of several members. Its distribution is similar to that of the Lafayette, though the Columbia series is, for the most part, confined to lower levels. Some of its several members are definitely correlated in time with some of the glacial epochs. The series is widespread over the lower part of the coastal plain. In the west the Quaternary deposits are not, in all cases, sharply separated from the late Tertiary, but the deposits of glacial drift, referable to two or more glacial epochs, are readily differentiated from the Tertiary; so, also, are certain lacustrine deposits, such as those of the extinct lakes Bonneville and Lahontan. On the Pacific coast marine Quaternary formations occur up to elevations of a few scores of feet, at least, above the sea.

Igneous rocks, whether lava flows or pyroclastic ejections, are less important in the Quaternary than in the Tertiary, though volcanic activity is known to have continued into the Quaternary. The Quaternary beds of lakes Bonneville and Lahontan have been faulted in a small way since they were deposited, and the old shore lines of these lakes have been deformed to the extent of hundreds of feet. So also have the shorelines of the Great Lakes, which came into existence at the close of the glacial period.

Much has been written and more said concerning the existence of man in the United States before the last glacial epoch. The present state of evidence, however, seems to afford no warrant for the conclusion that man existed in the United States before the end of the glacial period. Whatever theoretical reasons there may be for assuming his earlier existence, they must be held as warranting no more than a presumptive conclusion, which up to the present time lacks confirmation by certain evidence.

The following sections from selected parts of the country give some idea of the succession of beds in various type regions. The thicknesses, especially where the formations are metamorphosed, are uncertain.

West Central Massachusetts

Triassic.
  Chicopee shale  200  ft.  (?)
  Granby tuff  580  ”
  Blackrock diabase (cones and dikes) 
  Longmeadow sandstone 1000  ”
  Sugarloaf arkose 4660  ”
  Mount Toby conglomerate
Unconformity.
Devonian.
  Bernardston series 1950  ft.
Unconformity.
Silurian.
  Leyden argillite  300  ft.
  Conway schist
  Amherst schist
  Brinfield fibrolite-schist
5000  ” (?)
  Goshen schist 2000  ” (?)
Unconformity.
Ordovician.
  Hawley schist 2000  ft. (?)
  Savoy schist 5000  ” (?)
  Chester amphibolite 3000  ” (?)
  Rowe schist 4000  ” (?)
  Hoosic schist 1500  ” (?)
Unconformity.
Cambrian.
  Becket gneiss 2000  ft. (?)
Unconformity.
Proterozoic.
  Washington gneiss 2000  ft. (?)
   (Base not exposed.)

The above section is fairly representative for considerable parts of New England.


West Virginia, &c.

Pennsylvanian.
   (Top of system removed by erosion.) 
  Braxton formation 700  ft. 
  Upshur sandstone 300- 500  ”
  Pugh formation 300- 450  ”
  Pickens sandstone 400- 500  ”
Unconformity.
Mississippian.
  Canaan formation 1000-1300  ft.
  Greenbrier limestone 350- 400  ”
  Pocono sandstone 70-  90  ”
Devonian.
  Hampshire formation 1500-1800  ft.
  Jennings formation 3000-3800  ”
  Romney shale 1000-1300  ”
Unconformity.
  Monterey sandstone 50- 200  ft.
Silurian.
  Lewiston limestone 550-1050  ft.
  Rockwood formation 100- 800  ”
  Cacapon sandstone 100- 630  ”
  Tuscarora quartzite 30- 300  ”
  Juniata formation 205-1250  ”
Ordovician.
  Martinsburg shale 800-1800  ft.
Middle and Upper Cambrian.
  Shenandoah limestone 2400  ft.
   (Base not exposed.)

This section is fairly representative for the Appalachian Mountain tract, though the Cambrian is often more fully represented.


Ohio

Permian.
  Dunkard formation c. 25  ft. 
Pennsylvanian.
  Monongahela formation 200- 250  ft.
  Conemaugh formation 400- 500  ”
  Alleghany formation 165- 300  ”
  Pottsville conglomerate 250  ”
Unconformity.
Mississippian.
  Maxville limestone c. 25  ft.
  Waverley series—
   Logan group 100- 150  ft.
   Black Hand conglomerate  50- 500  ”
   Cuyahoga shale 150- 300  ”
   Sunbury shale 5-  30  ”
   Berea grit 5- 175  ”
   Bedford shale 50- 150  ”
Devonian.
  Ohio shale 300-2600  ft.
  Olentangy shale 20-  35  ”
  Delaware limestone 30-  40  ”
  Columbus limestone 110  ”
Silurian.
  Monroe formation 50- 600  ft.
  Niagara group 150- 350  ”
  Clinton limestone 10-  50  ”
  Medina shales (?) 50- 150  ”
   (Belfast bed.)