R. M. Deeley—The Thames Valley Gravels. 59 
a main valley in which the gravels were deposited over a wide belt, 
and other tributary watercourses flowing from the ice margin and 
forming fluvio-glacial fans of gravel and sand. A map showing the 
probable position of the ice margin, the fluvio-glacial gravel and 
sand beds in the tributary watercourses, and the area occupied by 
the fluvio-glacial deposits of the main stream, would furnish such 
a datum as we require. 
If the conditions suggested above ever existed in the Thames 
Valley, then some of the gravel and sand deposits which exist on the 
hill-tops and valley sides of the Thames may have formed portions of 
the fluvio-glacial deposits. 
The Map, Fig. 1, has been constructed to show what were probably 
the conditions obtaining when the ice which formed the Chalky 
Boulder-clay reached its maximum extension. 
A considerable portion of the northern area of the map (Pl. IV, 
Fig. 1) is occupied by the ice-sheet of the Chalky Boulder-clay. 
From the various lobes, A to H, powerful streams of water escaped 
_ down the valleys of the Chelmer, Thame, Colne, Brent, Lea, Roding, 
and Crouch. These streams formed extensive fluvio-glacial fans, 
and threw large quantities of water-worn rock debris into the old 
Thames Valley. ‘I'he probable area occupied by the gravels thus 
laid down is indicated by the coarse hatching. As the remnants of 
this deposit now occupy hill tops or shelves on the valley sides and 
are well developed on the hill on which Tilehurst stands, they have 
been called the Tilehurst Terrace Gravels. Some gravels, which are 
probably fluvio-glacial deposits of somewhat greater age, and lie at 
a higher level, are mapped as Higher Terrace Gravel. When the 
_ boundaries of the deposits are somewhat uncertain the lines have been 
dotted in. The dotted line, which runs along the Thames Valley 
from east to west, and is marked with a scale of miles, is the line of 
section along which Fig. 2 is drawn. 
The ice margin has been drawn so as to include the most southerly 
patches of boulder-clay known. They probably do show the actual 
limits of the true glacier flow, but some of the lobes may have projected 
further down the valleys. West of Luton the ice rested near the 
edge of the Thames watershed. East of Luton the ice crossed the 
watershed and was arrested by high land in the Thames Basin. 
Indeed, the outliers of Chalky Boulder-clay show that the margin of 
the ice-sheet was determined largely by the topography. 
Here and there lobes of ice pressed through the gaps in the hills. 
Working from west to east the main ice lobes are as follows :— 
A. Buckingham Lobe. South of Buckingham there is a gap in 
the watershed between the Ouse and the Ray in which boulder-clays, 
gravels, and sands are largely developed. ‘This seems to have been 
an important line of overflow; for from Stony Stratford past 
Buckingham to Charndon the overflow valley is well marked. 
B. Leighton Buzzard Lobe. Here the ice stream curved round 
the hill to the south of Woburn and travelled as far at least as 
Leighton Buzzard. 
C. Dunstable Lobe. At Dunstable there is a gap between the 
Chilterns and the high ground to the north-west, through which ice 
