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1. Trays of gravel were implanted for 24 h periods in several chalk streams. The trays were variously modified to monitor colonization from different sources, particularly drift (D). other downstream movements (DSM). upstream movements (USM), and all sources combined (ALL).2. The abundance of invertebrates in the streams was high (maximum of 717 per O.()4 m-) and colonization of the trays was extensive, especially by Gammarus pulex, the predominant species in all but one of the streams (maximum number of individuals of G. pulex per tray: for D, 473; for DSM. 462; tor USM, 286; and tor ALL. 639). In some cases benthic densities were achieved in the trays in the 24 h exposure period, and in most cases any one source of colonization could provide benthic densities in a few days.3. Patterns of movement differed between species, between size classes of C. pulex. and between sites for particular species. No one source of colonization could be regarded as generally predominant although downstream movements in total (D+DSM) usually far exceeded USM. DSM was usually an important source of colonization.4. The extent and direction of movement appeared to be determined by local factors and were explained in terms of ihc high densities of invertebrates and the current and substratum characteristics of each site. 5. Estimates of the relative importance of different sources of colonization varied greatly according to the criteria used in determining propc)rtions.

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SUMMARY Recent excavations in Sidgwick Avenue, Cambridge, have exposed up to 5 metres of gravels of the Intermediate Terrace resting on Gault Clay. Low down in these gravels an almost complete bison skull was discovered associated with deposits containing non‐marine Mollusca, while a few yards away at the same level deposits rich in macroscopic plant remains and Mollusca were found. Although no countable pollen was present, the general nature of the flora and fauna indicates cool to cold conditions, probably the Early‐glacial phase immediately prior to the Last Glaciation.