Darryl (23 July 2009). "river brent | 853". 853blog.wordpress.com. Retrieved 30 July 2015.
archive.org
Wooldridge, S.W. and Linton, D.L. (1955), Structure, surface and drainage in South-East England (first published in 1939), Philip, London (text online at archive.org): "At Finchley ... the surface of the boulder-clay" (where it lies on Dollis Hill Gravel) "varies in height from 250-300 feet and the sub-drift surface is at 224 feet at Church End." 224ft = c. 68 metres.
Gibbard (1979) found deposits of Dollis Hill Gravel up to six metres thick in the vicinity of the Finchley Gap. According to the British Geological Survey (Lexicon of Names Rock Units), Dollis Hill Gravel generally can be up to 15 metres thick.
These isolated hills are mostly on London Clay and have a protective cap of Dollis Hill Gravel. Their isolated forms bears witness to the fluvial erosion which was carried out on all sides of each hill, notably by the pro-glacial lake. They include Horsenden Hill, Wembley (Linden Avenue), Barn Hill, Dollis Hill and Hendon (Sunningfield Road). Another such isolated hill, at Colindale (Wakemans Hill Avenue), does not have a cap of Dollis Hill Gravel, but it probably did have one until a recent geological date, because its summit is at about the same altitude (90 metres) as the isolated hill with a Dollis Hill Gravel cap at Hendon, a short distance to the east. In all these cases, the permeable Dollis Hill Gravel has protected the easily-eroded London Clay below from being removed. It is not uncommon to find isolated London Clay hills in the London area which are capped by a protective layer of sand or gravel. Other examples include Forty Hill (capped by Boyn Hill Gravel) and Shooter's Hill (capped by Stanmore Gravel).
Deposits of "Black Park Gravel" (see BGS Lexicon of Names Rock Units) (which are identified on current geological maps (see BGS Geology Viewer) and which were laid down by the Thames during the latter part of the Anglian stage after it was diverted southwards, indicate the approximate route then taken by that river, between Uxbridge and Richmond. However, the Thames at that time (as during subsequent cold periods of the Pleistocene) was probably flowing as a broad, braided watercourse, in a kilometres-wide flood plain (see Ellison, R.A. 2004, Geology of London, British Geological Survey, pages 54 and 59. Online at pubs.bgs.ac.uk).
See Geology of Britain Viewer (British Geological Survey) for more information on the distribution of all the geological formations mentioned in this article.
Deposits of "Black Park Gravel" (see BGS Lexicon of Names Rock Units) (which are identified on current geological maps (see BGS Geology Viewer) and which were laid down by the Thames during the latter part of the Anglian stage after it was diverted southwards, indicate the approximate route then taken by that river, between Uxbridge and Richmond. However, the Thames at that time (as during subsequent cold periods of the Pleistocene) was probably flowing as a broad, braided watercourse, in a kilometres-wide flood plain (see Ellison, R.A. 2004, Geology of London, British Geological Survey, pages 54 and 59. Online at pubs.bgs.ac.uk).
largeimages.bgs.ac.uk
1:50,000 geological map, Sheet number 255, Beaconsfield, 2005, British Geological Survey.
Deposits of "Black Park Gravel" (see BGS Lexicon of Names Rock Units) (which are identified on current geological maps (see BGS Geology Viewer) and which were laid down by the Thames during the latter part of the Anglian stage after it was diverted southwards, indicate the approximate route then taken by that river, between Uxbridge and Richmond. However, the Thames at that time (as during subsequent cold periods of the Pleistocene) was probably flowing as a broad, braided watercourse, in a kilometres-wide flood plain (see Ellison, R.A. 2004, Geology of London, British Geological Survey, pages 54 and 59. Online at pubs.bgs.ac.uk).
This is the deposit of "Black Park Gravel" at Hanger Lane. See Gibbard, P.L. (1985), The Pleistocene history of the Middle Thames Valley, Cambridge University Press, pages 23-24. This gravel contains a wide variety of material which could only have been transported to the area by the ice sheet which reached Finchley, including granite, quartzite, Carboniferous Limestone and red sandstone. See Bromehead, C.E.N. and Dines, H.G. (1925), The geology of North London. Explanation of one-inch geological sheet 256 new series, British Geological Survey, pages 39-40; online at pubs.bgs.ac.uk.
Gibbard, P.L. & Lewin, J., 2003 ("Gibbard & Lewin 2003"), The history of the major rivers of southern Britain during the Tertiary, Journal of the Geological Society, 160, pages 829-845. Tectonic map and Palaeocene sections. Online at www.qpg.geog.cam.ac.uk.
Bridgland, D.R. and Gibbard, P.L. (1997), Quaternary River Diversions in the London Basin and the Eastern English Channel, Géographie physique et Quaternaire, vol. 51, n° 3, 1997, pp. 337-346. Online at www.erudit.org/fr/revues/. See Figure 1.
Bridgland D.R. and Gibbard P.L, 1997, Quaternary River Diversions in the London Basin and the Eastern English Channel, Géographie physique et Quaternaire, vol. 51, n° 3, 1997, p. 337-346. See Figure 1. Online at www.erudit.org.
essexfieldclub.org.uk
Wooldridge, S.W. (1927), The Pliocene Period in western Essex and the preglacial topography of the district, Essex Naturalist, 21, page 247; online at www.essexfieldclub.org.uk.
The description on this page of the history of the formation of isolated, mostly gravel-capped hills such as Horsenden Hill, is, like the description offered for the major drainage reversal, a simplification of what must have happened. But a similar combination of factors, involving pro-glacial lakes, meltwater flows, and subsequent evolution under a primarily periglacial regime, also led to the formation of a number of relatively small, isolated, gravel-topped hills adjacent to the eastern Fenland margin in East Anglia, in the Mildenhall - Lakenheath area (albeit in a different time frame and in a different geological setting). See Gibbard P.L., West R.G., Hughes P.D., 2018,Pleistocene glaciation of Fenland, England, and its implications for evolution of the region, R. Soc. open sci. 5:170736. and Gibbard, P. L. and others, 2009, Late Middle Pleistocene glaciation in East Anglia, England, Boreas, Vol. 38, pp. 504–528.
Gibbard, P.L., Wintle, A.C. and Catt, J.A., 1987, Age and origin of clayey silt 'brickearth' in west London, England, Journal of Quaternary Science, Vol. 2, pp. 3-9. Downloadable from www.researchgate.net.
