ORIGINAL ARTICLES. 
On Cycles of Sedimentation in the Eocene Strata of the 
Anglo-Franco-Belgian Basin.! 
By L. Duprey Stamp, M.Sc., A.K.C., F.G.S., King’s College, London. 
I. INTRODUCTION. 
HE term “ Cycle of Sedimentation ”’ or “‘ Cycle of Deposition ”’ 
is not nearly so familiar in England as it is on the Continent. 
Whilst the teaching of the general principles forms part of most 
courses in geology, the application of these principles to actual 
successions in this country has been somewhat neglected. Attention 
has recently been drawn to the seasonal or rhythmic changes seen 
in some sediments, and to avoid confusion it is desirable to outline 
what is meant by the expression “‘ Cycle of Sedimentation ” 
If one considers a sea transgressing over a steadily sinking land, 
the first stage in its progress will be marked by coarse, conglomeratic 
deposits of coastal (not necessarily beach) origin. This basal “‘ con- 
glomerate”’ is the “‘ gravier d’immersion”’ of French authors. It 
passes laterally in a seaward direction into sands and finer deposits. 
In a later stage the coastal deposits will overstep the former, and 
deeper-water sediments—sands and clays—be laid down in direct 
succession to the coastal deposits of earlier date. This process con- 
tinues during the transgression, and so in deposits of a simple overlap 
of this nature one has, at any point, a vertical transition from 
pebble-beds through coarse sands to finer sands and clays. That 
the basal conglomerates are not of the same age throughout is proved 
by a study of the faunas. One finds a paleontological zone to pass 
laterally from clays into sands and coarse sands and probably into 
the coastal pebble-beds. Such zones are represented in the diagram 
(Fig. 1) by horizontal layers marked Z 1, Z 2, Z 3, etc. 
When the transgressive movement of the sea ceases there is 
generally a tendency for the basin to become “‘ silted up”. This is 
especially the case when quantities of sediment are being brought 
into the basin from adjacent continental areas. In the neigh- 
bourhood of a delta one finds that the marine deposits pass gradually 
upwards into estuarine beds with a brackish-water fauna. These 
are succeeded in turn by lagoon (brackish or fresh water), lacustrine, 
fluviatile, or even wolian deposits. For the sake of brevity, all - 
sediments of this nature, that is, other than marine, are referred to as 
continental. 
In other parts of a basin of deposition where little sediment is 
_ being received from continental sources, the shallowing or, it may be, 
1 Paper read before Section C (Geology) of the British Association for the 
Advancement of Science, Cardiff Meeting, August, 1920. 
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