The Erratics of the Cambridge Greenland—Their Nature Provenance and Mode of Transport

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Summary The 163 erratics from the Cambridge Greenland in the Sedgwick Museum, Cambridge, comprise about 40 types of sedimentary, igneous, and metamorphic rocks. Two boulders may be matched with the paisanite of Mynydd Mawr, North Wales, others with the Uriconian rocks of the Midlands and Welsh Borders, whilst the majority, two of which are of types unknown in the British Isles, cannot be localized. The boulders were floated to the Cambridge region, probably in the roots of trees, during the long period of the formation of the Greensand deposit. They prove the existence of land in North Wales and other areas in Cambridge Greensand times. I. Introduction The Cambridge Greensand, a glauconitic marl 1-2 and one-half feet thick, rests on the Gault, and in places with obvious disconformity (Jukes-Brown 1875 ; Reed 1897, p. 96). Between 1854 and 1909 it was worked for phosphate nodules at many localities from Sharpenhoe (Bedfordshire) to Soham (Cambridgeshire) (Strahan 1916), a distance of over 50 miles. Sedgwick writes in 1858 : “Many hundred men are employed in turning out this (so-called) coprolite bed, and many steam engines are working to wash out the phosphate nodules” (Clark and Hughes 1890). “Immense tracts of land in the neighbourhood of Cambridge were turned over” (Jenyns 1867), the nodule bed being only a few feet from the surface, and Sedgwick (1846) states : “The Greensand though partly incoherent and of such inconsiderable thickness, yet appears to have protected the upper surface of the gait from denudation,

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