264
THE ESSEX NATURALIST.
drainage modifications in evidence at the 200-ft. level. North of
Sewardstone the broad valley of Cobbins Brook enters that of
the Lea, but on its further side the platform is continued,
occurring as outliers at Monkhams and St. Leonards, and thence
onward to Nazing and Roydon, in the Stort Valley. At Monk-
hams the hill-top retains a capping of glacial gravel erroneously
shewn on the old geological map (1 N.W.) as valley gravel of
the Lea. The platform is developed on the western side of the
Lea Valley as in the gravei-capped plateaux of Southgate and
Winchmore Hill, at Enfield, and at Broxbournebury. It is thus
evident that, although the glacial deposits attain greater heights
in the neighbourhood, as for instance, at Epping, and on the
north-east angle of the Hertfordshire plateau, the Lea Valley,
like its associates, had been excavated to the level of the 200-ft.
platform prior to the advance of the ice. Since the valley is
wide, we have no means of knowing at what level the pre-
glacial river actually flowed: probably it had sunk itself
slightly below the general level of the floor.
The valley of Cobbins Brook, tributary to the Lea, merits
brief consideration. The present stream is insignificant in
volume, but the valley is a large and striking feature. Along
its southern side a definite 200-ft. bench is present, continuing
the Sewardstone bench, past Wood Green and Copt Hall. On the
northern side a similar bench is developed eastward of Amies
Green. The amount of post-glacial denudation here has been
considerable, the Boulder Clay and the associated gravels
having been completely removed over wide areas. Small
tracts of the gravel remain on the 200-ft. bench, and the Boulder
Clay descends on to it north-west of Epping. Cobbins Brook
rises near Epping Upland, but the line of the valley is continued
north-eastwards through a broad col filled with Boulder Clay
at Thornwood Common. The surface level here is 252 ft. and
the thickness of the Boulder Clay is not known. The sub-glacial
floor of the col cannot, however, be higher than 220-30 ft. On
its further side the small stream known as Cripsey Brook—an
obvious misfit—continues the line of the valley north-eastward.
There can be no question that at the 200-ft. level the valley at
present occupied by these two brooks was a through-valley,
bearing a stream tributary to the Lea. The reversal of Cripsey
Brook is to be attributed to capture by the Roding. Pincey