LOWER CHALK—SOtJTH DORSET. 
97 
concur in this view, afterwards publishing a note thereon in the 
Geological Magazine , from which the following remarks are taken*: 
—“ The Lower Chalk becomes extremely impure in its lower part, 
and contains much glauconite ; it thus graduates insensibly down¬ 
wards into a gritty glauconitic sand. The sand contains a few 
phosphatic casts, more or less worn or corroded, scattered through¬ 
out it, but has a well-marked nodule bed, crowded with these casts, 
at its base ; other fossils, with the shell preserved and not filled in 
with phosphate, occur also throughout the whole bed.” The total 
thickness of very sandy chalk and glauconitic sand is about 15 feet, 
and it is throughout rather hard. The nodule bed at its base is in 
contact with a sandy glauconitic clay, which forms part of the Gault, 
so that the whole of the Upper Greensand is absent at this place. 
At the top of the cliff, 200 yards west of the section on the 
beach, the same bed is again seen, with about the same thickness, 
but resting on soft Upper Greensand, of which about 40 feet is 
seen. The question arises as to whether these appearances are 
caused by faulting or by erosion. Mr. Strahan considers that there 
is evidence for both, but thinks that the amount of erosion must 
have been slight, because of the absence of any fragments of 
chert in the nodule bed. To us, however, this seems a small 
difficulty, since there is no chert in the Upper Greensand of 
Worbarrow Bay, though it does occur at Lulworth. We are 
disposed to think that there was very considerable erosion of the 
“ Greensand” during the formation of the basement bed, and that 
the phenomena are largely due to erosion, though probably in 
part also to a fault. We have little doubt that the glauconitic sand 
which here forms the basement bed of the Chalk has been derived 
from the Upper Greensand, and that it occupied a trough or 
channel eroded in that Greensand by current action, though, 
owing to the co-existence of a fault, we are unable to ascertain to 
what depth this erosion extended. 
Messrs. Hill and Strahan re-measured the Lower Chalk of Mupe 
Bay during their joint visit in 1901, but report that it is difficult 
to be sure of the total thickness, because of the broken condition 
of the chalk and the occurrence of at least one fault, which occurs 
just below the Belemnite Marl, and appears to have a throw of 
about 10 feet. Allowing for this, Mr. Hill thinks the thicknesses 
of the several parts are approximately as follows:— 
ft. 
Pale bluish-grey marl (Belemnite Marl) 7 
White chalk in regular courses separated by marly 
bands, about ------- 40 
Massive white chalk, becoming greyer below - - 30 
Massive greyish chalk, containing scattered flinty 
concretions, and passing into the next - - 15 
Sandy glauconitic chalk, passing down into rough 
sand, with a layer of phosphatic nodules at base 15 
]C7 
4*219. 
* Geol. Mag., Dec. iv. Yol. viii. p. 319. 
G 
