32 
MUSEUM BULLETIN NO. 25. 
Observations and photographs of ripple-mark phenomena at 
Annisquam, Mass., accompanied by a sketch map and cross 
section which were kindly placed at my disposal by Mr. G. K. 
Gilbert 1 illustrate some of the ripple-mark patterns which result 
from the joint work of wave and current action when opposed 
to each other. Mr. Gilbert writes regarding the Annisquam 
ripple-mark: “When the currents are strong the ripples assume the 
character of subaqueous dunes with rounded crests and foreset 
beds at the angle of instability; and when the currents move in a 
different direction from the wind waves the ridge crests break 
up in a rhythmic way, giving a sort of imbricated pattern. I 
had an opportunity to study this phenomena at Annisquam, 
Mass., where I spent the summers of 1911 and 1912. 
“Figure 5 shows the relations of a sand spit to a broad shoal 
on one side and a tidal channel on the other. There is a large 
estuary to furnish strong tidal currents, and these currents not 
only follow the channel but cross the bar obliquely — an arrow 
shows the direction of the outgoing tide, and another arrow the 
general direction of approaching waves and ground swells. 
On the shoal are ripple-marks of ordinary type. On the bpit, 
each storm from the north, and each period of heavy ground 
swell, develops large ripples and these nearly always assume 
the imbricated pattern. When exposed at low tide they are 
asymmetric, with the lee side toward the ocean, and thus facing 
obliquely against the formative water waves. They are formed 
under 5 feet or more of water, and their size varies with the depth. 
That is they are smallest at C and grow larger toward D (see 
cross section, Figure 5). I was able to trace them to about 10 
feet below low tide level. Often they are covered by ripple- 
marks of smaller size which I assume to have been formed after 
the water had become shallow on the spit; and in the hollows of 
the pattern are still smaller ripple-marks. I enclose photo- 
graphs. The oar is 7 feet long. The photograph (Plate 
XIV A) looks along the spit toward rocks at E, while Plate 
XIV B looks in the opposite direction. The largest examples 
of ripple-mark seen measured about 15 feet from crest to crest 
and about 15 inches in height." 
1 Letter to the author, Jan. 16, 1914. 
