C O N T E N T S .
i I. Prefatory Remarks
{ 11. Description of Uie Local Circumstances of tlie East Devon Undereliff
4 III. Nmativeof the recent Founder or Landslip which began on Christmas-day 1839
i n'. Particular description of the Great Chasiu and Elevated Submarine Reef; with observations on the Application of the Theory of Landslips
or Founders to explain their Formation
} IV. n. Of the Great Chasm
^ IV. b. Description of the Elevated Submarine Reef, and remarks on the probable causes of its elevation . . . .
i V. Account of the Subsidence of the Land and Elevation of the Beach near the Base of mitlands CliiF. Plates 11. & IX
i VI. Account of the Great Subsidence of a Portion of the Chalk Cliffs, and Elevation of Submarine Rocks near Beer, in the spring of 1790
PART II.
Enumeration of similar Founders and Landslips which have been recorded to have happened at previous periods and in other localities . . . .
} 1. Founders
j 2. Account of Landslips {strictly so called) in the highly incUned Silurian Rocks in Shropshire and Herefordshire. . .
5 3, Alpine Ecroulemens
i 4. Conclusion-—Recapitulation of Arguments evincmg the convulsion of the Bendon and Dowland's Cliffs not to have
been produced by an Earthquake
APPENDIX.
No. I.—Account of the Subsidence or Founder at Hawkley Hanger
No. n.—Earth Falls at the Undercliff. Isle of Wight
No. 111.—On tlie Relations of other similar Geological Phenomena
ACCOUNT OF LANDSLIPS AT AXMOUTH ON THE SOUTH-EAST COAST OF DEVON, ETC.
§ 1. PREFATORY REMARKS.
THE following Memoir has been undertaken in order to lay before the reader a distinct account
of the most remarkable example ever recorded to have occurred within this island, of
that class of disturbances affecting the configuration of portions of the earth-s surface wlucU
TOults from the undermining agency of water. Earthquakes, properly so called, which all geologists
are now agreed, on the surest evidence, to rank aiuoug the phenomena connected with
volcanic forees, are, as we shall have occasion hereafter to show, known in this countrv only by
le feeble tremors of tlic extreme and cxpirins undulations of those wide-spreading vibratory
ts of the eai-th's surface, evidently originating in deeply-seated causes, and c
extending over vast districts, including hundreds and even tliousands of square miles. Tlie undermining
agency of water is obviously, in opposition to all this, a cause superficial, local, and
limited; and iherefoi'e if we estimate physical causes according to the scale of their operation
and the sublimity of terror, the latter phtenonicna must undoubtedly be placed in a far humbler
class. But if we co.npure them under another point of view, with reference only to the remarkable
variations produced in the configuration of the local surface immediately affected, we shall
find that the more deeply-seated and diffusive cause usually leaves behind it, at any particular
spot, traces fiir inferior in magnitude of features and picturesque grandeur of effect, to those
whicli the superficial and local causc produces within the narrower sphere of its own activity.
It seems, indeed, not at all paradoxical to infer that the magnitude of local effect mast, under suc'li
circumstances, vary inversely to the profundity and diffused agency of the disturbing cause.
Disturbances of the surface arising from the agimcy of water percolating through the strata,
and causing dislocation and motion among tiiem, may be produced either by its directly undermining
agency, or by moistening the softer interstrata, iargillaceous way boards, as they are
oaUed,) and thus causing tliem to slide one over another, when previously loosened and rifted;
and this loosening and rifting may sometimes be produced near the surface by the expansive
force of freezing water. All these disturbances have usually been denominated indiscriminately
by the designation of landslips; but this term undoubtedly appears more peculiarly appropriate
to the sliding motion of the superstratum over the substratum j in this case the strata must be
considerably inclined. Witl. tlie «ITeets so produced every traveller who has witnessed the
gigantic ecroulemens of the Alps must have his memoiy deeply impressed; and some remarkable
and celebrated instances in our own country will hereafter be mentioned.
The subsidences arising from the undermining agency of water among more horizontal strata,
have in the undercliff of the Isle of Wight received the a|>propriate and descriptive name of
Pounders; and it may be convenient for the purposes of distinctness generally to apply that term
to tliis class of dislocations; but it is almost unnecessary to add, that both tlie landslip (strictly so
called) and the founder often occur intermingled. The founder most usually occurs where tlie
following triple combination of strata affords the most favourable cii-cumstances: l.a superstratum
of porous rock; 2. an iuterstratum of loose sand; and 3. an ai'gillaceous substratum
impervious to water. In such an an angeiiient of beds, the luin penetrating through the poroub
rock, and arrested by the impervious clay, must be impounded, as it were, in the intermediate
sand, often nearly converting it into a mass of qnicksand. Where the slope of adjoining escarpments
exposes this quicksand on the surface, copious kind-springs will gush forth, and carry away
in different seasons greater or less quantities of tlie loose material thi-ougli wtiicli tiiey (low; and
thus in process of time the superincumbent rock will become partially undermined. In seasons
of extraordinary wetness, like the winter of 1839-40, these operations will be accelerated and
aggravated; and from the same causes the superincumbent porous rock, at the very time when
it is thus undermined and deprived of support, will become thoroughly saturated with moisture,
and have its weiglit and pressure by consequence greatly increased. Hence numerous fissures
will open through its mass i and tlie disjointed fragments will subside, partly finding cavities already
excavated beneath them, and partly finding the treacherous quicksand on which they are
based, yield to their pressure, being, from its want of consistency, easily squeezed into the various
hollows which the under drainage has before produced.
Such are the causes which have been generally recognised by all write« on the subject to
have originally produced the whole of the romantic aggregadons of snbslded terraces constituting
the undercliffsof the Isle of Wight and East Devon, the latter the site of the remarkable convulsion
wc are about to describe: this indeed has only added one new, though mobt stupendous
feature of rain, to those of precisely similar characterwhich before marked every acre throughout
lines of coast and undercliffs extending in both instances more than six miles; and we shall also
have occasion to cite numerous instances, mostly subsequent to the beginning of the last century,
in which the increasing attention to physical facts has occasioned records to be preserved of
convulsions under geological circumstances, identical in character, though usually inferior in
magnitude, and less striking in the effects produced.
m