. Elementary physical geography . izontal;but in some cases these ancient shore lines are no longerhorizontal. Other evidences might be brought forward inproof of a change in the relation between the sea level andthe land. It may be asked whether this is proof of changes in thelevel of the sea, or of land movement. While there is reasonto believe that there have been changes in the sea level, theevidence is conclusive that the greater number of thesechanges in relation of land to sea, are due to actual move-ments of the land. Without entering into this subject indetail, it may be stated, that
. Elementary physical geography . izontal;but in some cases these ancient shore lines are no longerhorizontal. Other evidences might be brought forward inproof of a change in the relation between the sea level andthe land. It may be asked whether this is proof of changes in thelevel of the sea, or of land movement. While there is reasonto believe that there have been changes in the sea level, theevidence is conclusive that the greater number of thesechanges in relation of land to sea, are due to actual move-ments of the land. Without entering into this subject indetail, it may be stated, that the most conclusive evidencethat this change is due to land movement, is the fact thatmany of the rocks, which we know were formed as nearlyhorizontal layers in the ocean, are now found in mountainsin a folded and often broken condition. Disturbance of the Rocks. — In many cases, the rocksthat have been raised from the sea, to form a part of thecontinent, are still in nearly horizontal positions (Figs. 90 208 PHYSICAL GEOGRAPHY. Fig. rocks on the plains of Kansas. and 133 and Plate28). They have beenbodily raised withvery little disturb-ance. In mountains,and less prominentlyelsewhere, the rockshave been movedfrom their horizontalposition, and causedto assume inclinedattitudes, which areoften very complex. These changes commonly assume oneof two forms, either (1) fold-ing or (2) breaking, whichwe call faulting. Even the most brittle ofrocks may be folded. Thecause for the folding usuallyacts so slowly, and the rocksare under such pressure fromabove, that they bend, rather than break, when subjected to a strain such as that which comes from contraction of the interior. A simple kind of fold is that known as the monocline (Fig. 91), where the rocks are inclined in only one direction. When they are bent up in the form of an arch, the folds are si s I gl known as anticlines (Fig. 92), FlG 92 and the corresponding down Anticline. fold is known as the syncline
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