358 C. A. COTTON 



and finer bands in the conglomerate itself, and fairly regular 

 sandstone bands are universally present. These are clearly exposed 

 in the gorge of the Dee, where the series is well stratified, with 

 bands of coarse conglomerate and fine conglomerate, and thin 

 bands of sandstone. In the Mead gorge a similar stratification is 

 well marked, especially in the higher beds, and here the beds are 

 seen to be lenticular, many of the sandstone beds especially thinning 

 out to a feather edge. 



In no case, on the other hand, has a distinct false bedding been 

 noted, nor an arrangement of foreset beds that would indicate 

 beach or delta conditions of subaqueous deposition. The beds 

 were evidently laid down nearly horizontally and such deposition 

 of coarse material appears to be impossible in standing water. It is 

 practically certain, therefore, that the shallowing water of the 

 Grey Marl sea was immediately filled when deposition of the con- 

 glomerate began, and that subsequently accumulation went on 

 under subaerial conditions. 



The character of the conglomerate supports the view that it 

 accumulated under fluviatile conditions, and it presents many 

 striking analogies with the terrestrial deposits in Owen's Valley, 

 California, described by Trowbridge,^ who has set out a list of 

 criteria for the recognition of such deposits. Of these criteria 

 the following, which appear to be the most valuable, are satisfied 

 by the conglomerate: 



1. In alluvial fans coarse material has a wide distribution as against 

 confinement to a narrow zone near shore in standing-water deposits. 



2. Textural range in single exposures is large in fan materials. 



3. Fan materials are not in general so well sorted as deposits in standing 

 water. 



6. Fan material has a lens and pocket stratification, as against a sorting 

 into more or less uniformly thick horizontal layers, as in lakes or seas. 



7. Huge bowlders widely distributed vertically and horizontally in a 

 deposit indicate that it was deposited by running water, and with a large pro- 

 portion of fine material; that is, they indicate that tlie material is part of an 

 alluvial fan deposit, except in cases where glaciers have affected it, or where 

 standing waters could have received icebergs, or where basal conglomerates 

 are formed near shore. 



' A. C. Trowbridge, Jour. Geol., XIX (1911), 706-47. 



