CAMBRIAN AND ORDOVICIAN RIPPLE-MARKS 707 



Trenton limestone ripple-marks. — The extensive quarry in the 

 Trenton limestone at the cement mill one-half mile northwest of 

 Hull, Quebec, exposes a section with a vertical thickness of nearly 

 100 feet. Three or four horizons in this section show, in the verti- 

 cal walls of the quarry face, contacts between adjacent beds which 

 are very suggestive of ripple-marks of large dimensions. The im- 

 perfect exposures of the lower of these horizons leaves room for 

 some question as to their true nature but it appears clear that 

 the uppermost is an example of ripple-marks. Only the upper- 

 most of these horizons, which is admirably exposed for study 

 through the stripping of the stone to this level over an area of con- 

 siderable extent, will be considered here.^ This bed lies 10-15 

 feet below the top of the quarry in the part of the section called by 

 Dr. Percy Raymond^ the crinoid beds. 



The limestone here is a dark-gray rock lying in strata usually 

 6 inches to a foot in thickness. The regular horizontal contact of 

 these strata gives way at the horizon under discussion to a billowy 

 trough and crest contact. This is much less regular in profile than 

 the profile of the ordinary ripple-mark as seen in most sandstones. 

 The somewhat irregularly rounded summit of the ridges and their 

 variable width might suggest on brief examination an unconformity; 

 their parallelism and the identity of the fauna on both sides of this 

 horizon, however, render this explanation untenable. The ridges 

 trend approximately east and west and their axes are from 2 to 

 3 feet apart. No sharp crests are to be seen. All of them, as 

 shown in the photographs (Figs. 4 and 5), are rounded or somewhat 

 depressed on top. The troughs have an average depth of about 

 6 inches below the crest and frequently coalesce, thereby producing 

 many short ridges. An interesting feature connected with these 

 ripple-marks is the occurrence on them at some points of numerous 

 crinoid stems some of which have the heads attached as shown in 

 Fig. 6. These are often two feet or more in length and indicate 

 by their presence that the crinoids lived on this part of the sea 

 bottom shortly after the formation of the ripple-ridges in water 



^ These were brought to my notice by Mr. W. A. Johnson. 



2 "Excursions in Neighborhood of Montreal and Ottawa," Guide Book No. 3 

 (1913), p. 143. 



