80 SIDNEY POWERS 
Cross-bedded sandstones may show very similar grooves and 
ridges when relief is accentuated by weathering (Fig. 6), but 
the two kinds of markings are entirely distinct in origin. 
CONCLUSIONS 
Parallel groovings on bedding planes such as described above 
have been shown to be markings on the original strand either in 
sandy clay or in a fine-grained sandstone. Preservation of either 
the original surface, as in the case of the sandstones, or of a cast 
of the surface, as in the case of the sandy clays, has failed to reveal 
both original and cast of any one set of markings except in the form 
of small fragments. 
Inspection of the various theories which may be advanced to 
account for the formation of such strand markings shows that they 
have been formed by the dragging of plants, probably stems and 
fronds of algae, over sand and mud in shallow water, probably 
by tidal or undertow currents. Ground ice and shore ice are 
shown to be incapable of having made these markings. 
Reasons for the conclusions above may be briefly summarized: 
(1) general distribution through formations of similar origin, but 
of different ages in various regions; (2) perfect parallelism over 
broad surfaces with notable undulations and sinuosities confined 
to certain of the markings; (3) curved, overlapping, and cross- 
striations; (4) absence of any disturbance within the strata; 
(5) intimate association with beach and shallow water markings 
as current, ripple, rill marks, worm borings, and algal impressions; 
(6) occasional association with heaps of shells found in the grooves 
and stratigraphic proximity of plant remains; (7) negative evidence 
of the presence of ice in any form or of glacial deposits in the 
containing formations; (8) complete gradation from straightness to — 
curvature to cross-markings and more irregular markings showing 
nodal and stemlike impressions; and (9) the outstanding delicacy 
of the whole, showing that gouging was subordinate to molding 
by objects of light weight. 
