6. Except for two small inliers of Lameta, Gondwana, and
Metamorphic rocks along the northern
boundary of this District and occasional
patches of laterite, the only geological formations represented are Deccan Trap, and the alluvium of the Purna valley. A large portion of the District is occupied by plains, but there are two hilly tracts, both formed of Deccan Trap rocks. These have already been described.
The following is a list of the formations, which will be described in order:-
7. Concerning the cotton soil, A, B. Wynne writes. [Rec. Geol.Surv.Ind. Vol. II, part I, page 5 (1868).] ' The cotton soil or black soil of the Purna valley, although common enough,
as is usual in these trappean districts, has no
geological peculiarity here requiring attention. To its
development, however, and the fertile nature of 'soils derived from the trap, may be traced doubtless the name
which this country has obtained as a cotton producing district.'
8. Below the Gawilgarh range lies the Payanghat or
valley of the Purna river, ranning through the Amraoti, Akola and Buldana Districts of Berar, before entering Khandesh. It is described by Wynne as follows [Rec,Geol.Surv,Ind- II, part I, page 1, (1868.) The words given in brackets in the following quotations are additions to the original.]:-
'The valley of the Purna possesses but little variety of geological interest and is principally distinguished by monotonous repetitions of features observable in crossing the Deccan from the seaward to this locality, where each hill and ghat and undulating slope or plain exhibits similar kinds of nearly horizontal flows of gray amygdaloidal trap, with here and there a bed of harder texture of columnar structure, or of bright red bole, or alternations of these; the traps sometimes containing numerous zeolites.
' In the river valleys, and where superficial " rain-wash" has accumulated, a light brown " kankry" alluvium is associated with sub-recent calcareous conglomerate below and black cotton soil above, one being quite as occasional and accidental as the other, the conglomerate or concrete being perhaps the most persistent along the river courses, the brown alluvium or (?) " soda soil" more universal and the cotton soil occurring, subject only to the rule that it is always uppermost.
The alluvium of this great plain, although of very considerable depth and occupying so large an area, is as completely isolated from that of the neighbouring rivers as such a deposit can be. A section crossing the valley from the Ajanta ghats, by Edlabad (Khandesh) across the Purna river, to the western termination of the Gawilgarh range, would show the ordinary trap of the Deccan, forming the high ground at either end, and an undulating country between, which viewed from above or from a distance has a plain-like aspect, but frequently exposes the rocks of which it is formed, consisting of the usual traps,
here and there covered only by slight detrital accumulations of the same kind as those of the Deccan. Except on the very banks of Purna no considerable quantity of alluvial matter would be found, and this does not extend far from the river at either' side. North and south through Malkapur (Buldana) a different section would be obtained. Here a wide space, chiefly on the south side of the Purna, is occupied by fine brown calcareous alluvium with " kankry ", and is connected by a narrow neck, at Piprala, with the great alluvial deposit of this valley which in thickness may exceed 150 feet; and nothing else, save varieties this, of is to be seen in or near the river from Dadulgaon (Akola) on its south bank eastwards up the stream nearly to the "sangum" or junction of the Phairli river, (Akola District), except two or three small exposures of trap in its bed near Piprala Pulsod (Akola).... The Purna changes its course from the N. N. E. at the junction of the above-named tributary, and thence takes a westerly direction:-the alluvium on its south side seldom extending beyond an average of ten miles from the river and nearly coinciding along its southern boundary with the Nagpur extension of the Great Indian Peninsula Railway, while on the north it reaches nearly to the base of the mountains. On the east its rather arbitrary and indefinite boundary closely approaches the watershed of Ellichpur, and bending southward traverses undulating country, eventually reaching the flanks of the hills near Amraoti.
' All round the margin of this alluvial tract is a belt of country that might or might not with propriety be included within it, although the surface deposits there do not conceal the underlying rock, the exposure of which was taken as the chief guide in determining the line of boundary. On the north and east, this tract of country is very stony, and it may be supposed that streams descending from the mountains and hills have frequently travelled across this space their courses subject to lateral deviation, covering the whole
of it with coarser fragment brought down by floods at a time perhaps when the water of a lake or the sea occupied the basin of the finer alluvium and arrested the boulder-bearing velocity of these mountain streams.
'In every part of the alluvium calcareous conglomerate or concrete is of common occurrence. It occasionally contains fragments of bone or fossil teeth of ruminants, but although sought for, no large accumulation nor even a large fragment of these fossils was observed. Yet enough was seen to show an identity of the conditions under which these deposits and those of the Nerbudda valley were formed. This sub-recent conglomerate is very frequent in the stony tract above mentioned. It was everywhere searched for worked flints but without success, although one flake was found in a quite similar deposit, forming the right bank of the Godavari at Paithan in the Deccan, at a considerable distance to the south.' [The industry has been prohibited.]
' A deposit of varying thickness (within three feet) and but small lateral extent, consisting of fine dazzlingly white sand finely laminated, occurs in the alluvial bank of the Purna at Paruth. It appears to be composed of comminuted or disintegrated crystals of felspars with a small admixture of clay. It did not appear to be formed of or to contain minute organisms, such as foraminfera, and was not elsewhere observed.
' Much of this Purna alluvium produces efflorescences of salts, of soda chiefly, and in many places the wells sunk in it are brackish or salt. Over a wide tract on each side of the Purna river, north of Akola and thence eastward towards Amraoti, wells are specially sunk for obtaining common salt from highly saturated brine.' '
That the alluvium of the valley is of considerable depth maybe perhaps inferred from the absence of numerous
exposures of rock, as well as from the depth of nullahs and
height of the river cliffs. The conglomerate, as usual,
occurs in its lower portions, but was observed in some
places west of Patulla at different heights in the sections exposed. Its constant or frequent occurrence beneath the rest of the alluvium would not prove its being contemporaneous in all places, as the trap-rocks, upon which these deposits lie, cannot be presumed to have had a surface sufficiently even to have permitted this.
9. ' Whether the whole of this alluvium was deposited in
a lake, or by the river travelling from
side to side of the valley under other
conditions than at present obtain,
does not appear. A former estuarine state of thing
may be indicated by the salt-bearing gravels, or a large
salt lake, but the even though interrupted surface of
the alluvium is against the probability of its having been
deposited by the Purna under present conditions; while
want of information as to the relative levels obscures
the possibility of determining whether the rocky country
about Edlabad may not have formed a natural bund flooding the country occupied by the alluvium [The formation of the alluvial tracts of the Purna, Tapti, Nerbudda, and Godavari, has been explained recently by Mr. Vredenburg as due to the formation of slight anticlinal axes in Pleistocene times; the rocks being ridged up along these axes so as to impound waters with the formation of lake basins that were subsequently filled up by alluvial deposits. See Rec.Geol.Surv.Ind. XXXIII, page 33, (1906).]; certainly
the stream through most of this is sluggish, but it
seems to be a rather strong assumption, that no greater
fall than the height of the river banks where it enters this
rocky tract-perhaps on an average not more than 30
feet-takes place within so great a distance as extends
between this and the upper end of the alluvium, about
south-west of Amraoti.'
10. With regard to the occurrence of laterite in this
District W. T. Blanford writes :- [Mem,Geol,Surv.Ind. VI, page 285, (1869).]
Two small tracts of laterite are met with, one just south of Rithpur, the other about four miles north of Amraoti. The latter is the largest and best exposed, some good sections of it occurring in the river bank. Above, it is
gravelly in texture, consisting of the usual small ferruginous grains in a red matrix; the grains when broken showing concentric structure. Beneath, it is more compact, but soft. In one place it was seen to rest on greenish-grey mottled mudstone, breaking into small cuboidal
fragments, with joint surfaces between, so minute that it is impossible to
obtain a fair fracture. This is probably decomposed trap.
Wynne [Rec.Geol.Surv.Ind., II, page 5, (1869).]' refers to the following lateritic occurrence:-
' The plateau upon which Chikalda stands and the surrounding summits have a strongly lateritic appearance, such as may be seen at Matheran, and other summits of the Western Ghats.'
11. The Gawilgarh range of hills is composed, as has
already been mentioned, of rocks of
the Deccan Trap formation. It is described by Dr. Voysey in the following terms
:- [AsiaticResearches, XVIII, page 189 (1833).]
' The principal part of the whole range is formed of compact basalt, very much resembling that of the Giant's Causeway. It is found columnar in many places, and at Gawilgarh it appears stratified-the summits of several ravines presenting a continued stratum of many thousand yards in length.
'The basalt frequently and suddenly changes into a wacken, of all degrees of induration, and, of every variety of composition usually found among trap-rock.'
' In external appearance, the columnar and semi-columnar basalt closely resembles that of the Giant's Causeway, possessing the same fracture, internal dark colour, and external brown crust. It is equally compact and sonorous.- Perhaps the basalt of the Gawilgarh range, more nearly resembles, in every respect, that of the Pouce mountain in the Mauritius.'
An interesting feature of this range is the existence all along its southern edge of a marked northerly dip (in places
as high as 15°) in the lavas of which it is composed. This is a very uncommon occurrence in the Deccan Trap, which is almost everywhere horizontal; it is due to the fault that brings up the Gondwana inliers noticed below.
With regard to the hill Deccan Trap formation south of the Purna alluvium Wynne [Rec.Geol.
Surv.Ind. Vol. II, page 3.] says:-
The hills and portion of the valley south of the Purna river have been stated to consist of trap similar to that of the Deccan. All the usual varieties of amygdaloid, zeolitic, columnar, hard, gray, and softer ashy-looking traps occur, their stratification being very perceptible and always nearly horizontal.'
12. Along the northern boundary of the District, lying partly in Amraoti and partly in Betul
there are some inliers of the Gondwana
age concerning which Medlicott and
Blanford write in the first edition of the Manual of the Geology of India, page 224, (1879), that along the southern scarp of the Gawilgarh Hills:-
there is, north and north-east of Ellichpur, a line of fault, running east-north-east to west-south-west, and having a considerable down-throw to the south. Along the northern or upthrow side of this fault, sedimentary beds appear in places, from beneath the Deccan trap, forming the whole of the surrounding country, and extend for a considerable distance (in one case for several miles) along the base of the hills. These exposures are but 30 miles south-south-east of the sandstones in the Tapti west of Betul.
' Themost western ofthese inliers occurs about 8 miles north of Ellichpur, and extends east andwest between 6 and 7 miles. For 16miles to the eastward no sedimentary rockis seenin place, butin onespot, 3miles west ofNarha, some blocks ofsandstone occur, and there may be a small outcrop. At Narha, about 22 miles east by north of Ellichpur, the sandstones reappear north of the fault, and extend
for 15 miles. They then disappear again, but two small inliers, each about a mile long, occur at short intervals just beyond.
'In these inliers Lameta (cretaceous) beds occur immediately beneath the basaltic traps, and are succeeded in descending order by about 500 feet of strata comprising felspathic sandstones, white and brown conglomeratic beds, occasional ferruginous beds, and thin layers of white and purple shale. It has not been decided whether these rocks are of Kamthi age, or whether they should be referred to the Mahadeva series, no distinguishable fossils having been found.
' No beds of Barakar or Talchir age have been detected,
and the base of the sedimentary beds is not seen, whilst an
attempt to discover coal by boring proved unsuccessful.
Metamorphic rocks appear in one place along the southern
edge of the sandstone, and are apparently brought up
between two faults with their throws in opposite directions As these faults
coincide at each end of the strip of metamorphics, there is evidence in this instance of two throws in
opposite directions having taken place along the same line of weakness.'