244 PROCEEDINGS OP THE MALACOLOGICAL SOCIETY. 



It is difficult to classify Holocene deposits, but they may be 

 divided into seven groups, viz. : — 



1. River deposits. 



2. Lacustrine deposits. 



3. Swamp deposits. 



4. Buried land surfaces. 



5. Rain-washes. 



6. Cavern deposits. 



7. Ancient graves. 



Of these, all but the two last maybe considered as sealed deposits, 

 though the possibility of disturbance by burrowing animals must 

 always be remembered. River deposits need special consideration 

 for two reasons. They often represent the sweepings of a large 

 area, and the shells may have travelled a considerable distance, 

 whilst there is also the probability that they may contain shells 

 derived from earlier beds. The alluvial beds of the Thames and 

 Lea, the results of the slow intermittent sinking of the land, often 

 contain not only true river deposits but also old land surfaces and 

 swamp deposits, as was plainly seen in the New Albert Docks. 



Lacustrine and swamp deposits need no elaboration, the con- 

 tained mollusca having lived practically where they were entombed. 

 Swamp deposits are frequent in Essex and the Midlands, Copford 

 being perhaps the best known. In this group, too, may be classed 

 the various tufaceous deposits, such as Blaskenwell, in Dorsetshire, 

 and Totland Bay, Isle of Wight. 



Old land surfaces are principally found in sand dunes, but many 

 archaeological sites such as Grimes Graves, Norfolk, and Cissbury, 

 Sussex, may well be included. In these also the possible margin 

 of error is extremely small, and the evidence derived from their 

 fossils may be relied upon. Rain-washes are very common on 

 the slopes of the chalk-hills, and probably owe their, formation to 

 the destruction of the woodland on the ground above them, thus 

 exposing the soil and the already disintegrated subsoil to the action 

 of rain. Their accumulation was thus very rapid, and practically 

 ceased as soon as the vegetation covered the slopes again. Where 

 there is evidence that the slopes have not been disturbed, these rain- 

 washes are absent. Though not relevant to our subject, it may be 

 mentioned that the rain- wash at Otford is situate below the so-called 

 Pilgrims Way, which has been claimed to be a pre-Roman road. 

 Now, when the deposit was accumulated, there must have been an 

 unbroken slope of bare chalk soil, and the road cannot have existed. 

 Since the rain-wash can be definitely dated on archaeological 

 evidence as of Roman age, it follows that this part of the road is 

 not as old as enthusiasts would wish us to believe. As a rule, 

 mollusca are common in these deposits, and they furnish interesting 

 evidence as to former conditions. 



Cavern deposits are particularly liable to disturbance by 



