468 
Geology of Norfolk. 
disseminated chalk rubble, for the plastic clay below it, and the 
crag for the sand and gravel which abound in the lower part of 
that formation. This error, as regards the outliers of Poring- 
land and Strumpshaw, has been partially continued in the 
map which accompanies Woodward's ' Geology of Norfolk,' 
in which they are represented as detached masses of clay, instead 
of as detached masses of gravel upon a continuous bed of clay. 
Professor Phillips, in his Geological Map of the British Islands, 
following the authority of his illustrious relative, has extended the 
plastic clay over the whole district occupied by the strong soils 
of Smith's County Map, adding in a note, that it is difficult to 
ascertain the boundaries of that formation in Norfolk. After 
very diligent search I have been unable to find a single outlier of 
the London or plastic clay within the area indicated. Even their 
bouldered fossils are by no means abundant, and the small quan- 
tity met with occur chiefly near the borders of Suffolk. 
The phenomena presented by the junction of the soil and sub- 
soil are very remarkable. The surface of the subsoil is indented 
with furrows and pierced with conical and cylindrical cavities 
similar to those which are common on the surface of the chalk, 
whether covered by the tertiary strata of all ages, from the plastic 
clay to the Norwich Crag inclusive, or by the more recent strata 
of the drift, which may be called the erratic tertiaries. 
Almost every excavation opened throughout the district shows 
this furrowed surface of the subsoil, the depth of the cavities, 
except in some extreme cases, varying from 2 to 4 feet. It is 
most striking when the subsoil consists of a bed of transported 
chalk or of till, but is not confined to them. The furrows abound 
in subsoils of sand and gravel, as well as of transported chalk and 
till : the cylindrical and conical cavities are best developed in the 
two latter, but 1 have seen them in sand. 
Section VIII. is a sketch of the junction of the soil and subsoil 
over a pit of the most chalky variety of till near Langham. In 
this case the greatest depth of indentation is 3 feet, and tiie depth 
of warp or surface soil varies in the space of a few yards from less 
than 6 inches to 3 feet. 
Section IX. is a sketch of the junction of the soil with a subsoil 
of more clayey till than the last, near Hardingham. In this case 
the depth of the furrows and pipes varies from 3 to 6 feet, and 
one of the pipes extends to the depth of 9 feet. If all the pro- 
jecting points of till were removed to a level with the dotted line 
A B, it would give a soil with a regular depth of 3 feet. 
This furrowing of the surface of the subsoil is a fact of con- 
siderable importance in pure geology, with reference to the 
agencies concerned in the formation of the surface soil or warp 
of the drift ; it is of no less importance in its bearing on practical 
