I. On the formation of ripplemark

Abstract

Fossil ripplemarks are often appealed to by geologists as evidence that the winds and waves that formed them cannot have differed much in intensity from those that produce similar corrugations on the sands of our modern beaches; but, although considerable value is attached to the evidence afforded by these relics of ancient seas, authorities differ much as to their origin. According to Sir Charles Lyell ripplemark originates in the “drifting of materials along the bottom of the water,” and “is usually an indication of a sea beach, or of water from 6 to 10 feet in depth, though this rule he proceeds to say is not without exception, as recent ripplemarks have been observed at the depth of 60 or 70 feet. The crossing of two sets of ripples the distinguished author ascribes to the new direction in which the waves are thrown on the shore consequent on a change of wind (“Elements of Geology,” 6th edition, p. 19). Mr. J. Beete Jukes, in the first edition of his “Manual of Geology,” states, that current mark or ripple “is produced on the sea beach, not in consequence of the ripple of the wave impressing its own form on the sand below, which would be an impossibility, but because the moving current of water as the tide advances or recedes produces on the surface of the sand below the same form as the moving current of air produces on the surface of the water above. A rippled surface, therefore, to a rock is no proof of its having been necessarily formed in shallow water, though rippled surfaces are perhaps more frequently formed there, but simply a proof of a current in the water sufficient to move the sand at its bottom gently along, at whatever depth that bottom may be from the surface of the water.” Speaking of fossil ripplemarks the same author states that the distance from crest to crest of the ridges varies from half an inch to eight or ten inches, with a proportionate variation in depth between them (Jukes’ “ Man. Geol.,” p. 172). The article on ripplemark is recast in the third edition, edited by Dr. Geikie and published in 1872, but the views expressed therein are the same.