INFERENCE FROM GIANT RIPPLES 



139 



Medina ocean was agitated by storm Avaves sixty feet high. As great 

 waves require broad and deep bodies of water for their generation, sucli 

 a result would demonstrate the association of the Medina formation with 

 a large ocean. 



Associated Cross-bedding 



Figure 3. — Cro>>s-bed<tiii 

 iag of Ripple Sijsf( 



1/ piodiiced by slow Shift- 

 la (luring Deposition. 



The relation of sand-ripples to cross-bedding was studied first in the 

 quarries exhibiting the giant ripples, and afterward, to better advantage, 

 on the beach of lake Ontario, where blocks of sandstone, smoothed by 

 beach-rolling, exhibit small ripple-marks and the associated oblique 

 structures in e\Qry variety of combination and section. 



It appears that sediment may be added to a rippled surface witliout 

 any disturbance of the pattern, but that there is usually a coincident, 

 gradual bodily shifting of the pat- 

 tern in some direction. In order 

 that sediment may be brought it is 

 necessary that there be a general cur- 

 rent in addition to the oscillating 

 current caused by waves, and it may be safely assumed that the shifting 

 of the ripple pattern follows the direction of this general current. Just 

 as the direction of the general current ma^' make any angle, or no angle, 

 with the direction of the oscillatory movement, so the direction toward 

 which the pattern is shifted may bear any relation to the trend of the 



ripples. The shifting of the rip})le 

 profile during the accumulation of 

 sediment makes the accumulation 

 unequal on the two sides of the 

 trough (figure 3), and if the ratio of 

 shifting to deposition exceeds a cer- 

 tain amount, there is deposition on 

 only one side of the trough and ero- 

 sion on the other (figure 4). In each case an oblique structure or cross- 

 lamination results, and in the second case the general structure given to 

 the sand mass includes two systems of oblique planes. One system is 

 produced by deposition, and the laminse represent the successive posi- 

 tions of the! concave ripple profile. The other system is produced by 

 erosion, and its planes represent the progress of the profiles of the troughs 

 along certain tangents. This structure, which was frequently observed, 

 may be called compound cross-bedding. The tangent planes are some- 

 times so gently inclined that they might easily be confused with true 

 bedding. They are broadly exhibited in quarry floors at Lewiston and 

 Lockport. 



Figure -t. — Coinpound Ci'osa-bcddiiifi produced bi/ 

 Deposition and partial Ervsion asvociafed with 

 shiftinr/ Sand-ripples. 



