578 THE JOURNAL OF GEOLOGY. 



wash on slopes, and marsh. The Post-Columbia channels are in 

 greater part flooded by tide water, which, to the southeastward, 

 extends to slightly above the base of the Columbia formation. 

 These channels have a depth of 150 feet in the deepest portion 

 of Chesapeake Bay, but decrease in amount to the westward. 

 They are excavated through the Columbia deposits and are not 

 floored by that formation, as suggested by McGee. Throughout 

 the region, the later Columbia deposit caps cliffs at the waters 

 edge, and to the northward its base is usually considerably 

 above tide water and various subterranes appear below. Many of 

 these cliffs have been cut back more or less by lateral wave action, 

 but there are numerous others which are due entirely to vertical 

 erosion. All the tide water channels are cut deeply into forma- 

 tions underlying the Columbia deposits, and the base of the very 

 lowest Columbia deposits southward is but a short distance below 

 low tide. Several years ago I obtained a sample of the bottom 

 of the Chesapeake bay in twenty feet of water, a mile from the 

 west side of the lower bay, and it was typical Chesapeake clay 

 containing a perfect, very fragile shell of a characteristic tellina. 

 At Claiborne, on the east side of the bay, extensive dredgings 

 were made for a long railroad dock, and the lower diatomaceous 

 clays of the Chesapeake formation were found to be practically 

 bare in the bottom. In Patapsco river, the dredgings out of the 

 channel brought up typical Potomac clay at twenty-seven feet, 

 which was overlain by a few feet of river mud. McGee 1 states 

 that the boring on Spesutic island, near the head of the bay, was 

 through 140 feet of sands and silts, which were thought to be 

 neither Columbia nor Potomac. I believe, however, that in the 

 lower portion of this well some Potomac sands may have been 

 penetrated. The well, however, proves the existence of a deep 

 channel here with a great mass of alluvial filling. 



Owing to submergence now in progress, alluvial deposits are 

 mainly laid down in tide water. In the Piedmont region, and in 

 some of the smaller valleys, there are transient accumulations of 

 alluvium on freshet planes. Throughout the region, there are 



1 Loc. cit. 



