A Geological Sketch of the Tertiary Formation in the provinces of Granada and Murcia, Spain. With notices respecting primary, secondary, and volcanic rocks in the same district and sections

Front matter

A GEOLOGICAL SKETCH OF THE TERTIARY FORMATION IN THE PROVINCES OF GRANADA AND MURCIA, SPAIN. WITH NOTICES RESPECTING PRIMARY, SECONDARY, AND VOLCANIC ROCKS IN THE SAME DISTRICT AND SECTIONS. By CHARLES SILVERTOP, Formerly Captain in the 14th Light Dragoons, retired Brigadier in the Service of H. C. M., K. of the Royal and Distinguished Order of Charles III., and F. G. S. ILoniron : LONGMAN, REES, ORME, BROWN, GREEN, AND LONGMAN 1836. LONDON : WALTER SPIERS, PRINTER, 399, OXFORD STREET.

Preface

. As little is accurately known of the Geological Struc ture of Spain, it is hoped that the following sketch of a widely-disseminated deposit of tertiary beds, in the provinces of Granada and Murcia*, accompanied by a general view ofthe primary, secondary and volcanic rocks observed in the same extensive district, may be accept able to geologists, and useful to those who shall here after visit this southern portion of the Peninsula. To separate the tertiary deposit into distinct groups, must be left to the minute investigation of future observers ; but I have noticed localities where such appearances are indicated, although in this, as in various other points, it is very possible my conclusions are pre mature. The term transition is applied, under correction, to the slates and sub-crystalline limestone that compose various ridges in the southern part of the province of Murcia : founded upon mineralogical character and the apparent order of superposition, the future discovery of organic remains may, perhaps, assign to these rocks a much more modern date.

The sections given to elucidate the text, have no pre tensions to scientific accuracy, difficult to attain without * An abstract of this sketch, as far as regards tertiary deposits, was read at a meeting of the Geological Society, in 1833, and sub sequently withdrawn by permission of the council.

PREFACE.

. a regular survey, requiring prolonged residence and unlimited powers of locomotion, to which Spanish policy has hitherto been extremely adverse ; but they represent, faithfully, the general configuration of ground, relative height of ridges, and superposition of strata. They are termed General and Local, as the former 1 epresent considerable — the latter, minor-tracts of country, or a single hill or ravine. Geologists are well aware of the many obstacles to detailed examination in the present state of Spain, — the inaccuracies of maps — the absence of a single house over large districts — the few artificial excavations or roads, &c &c. &c. — but I would rather dwell upon the facilities it was my good fortune to meet with, in the universal kindness and attentions I received during various excursions made to collect ma terials for the little geological sketch I presume to offer the public.

Spain, in a geological sense, as in many others, remains to be deciphered. It is full of interest in every point of view, moral and physical, but the delicta majorum immeritus lues, hang upon it, and until the ordeal be passed it will continue an engima to the great loss of all civilized Europe. The country teems with genius and latent energy, and when the incubus, which for so many centuries has pressed them down, shall be fully removed, it will be one of the brightest stars in the European constellation of nations.

Chapter I

A SKETCH OF THE TERTIARY FORMATION ON THE NORTHERN FLANK OF THE SIERRA NEVADA CHAIN, OR IN THE INTERIOR OF THE PROVINCE OF GRANADA. The various localities along the southern base of the Sierra Nevada chain, or along the coast-line from Malaga eastwards as far as the port of Vera*, where beds be longing to the tertiary epoch are met with, were pointed out in a memoir read before the Geological Society of London, and published in the Edinburgh New Philoso phical Journal for October, 1833. I shall now attempt to show that similar deposits have extended over large areas on the opposite or northern flank of the same chain of primary and transition moun tains, whose ruins and imbedded organic remains indicate the former existence of a tertiary sea, which, at some remote epoch, not only covered a great portion of the province of Granada, but also of the contiguous one of Sevilla towards the west. * These localities, from Malaga to Vera inclusive, belong to the coast-line ofthe province of Granada u The three towns in the province of Granada, in whose respective vicinity the tertiary marine formation is most extensively developed are, Alhama, Antequera, and Alcala la Real: the two first, contiguous to the inland zone of secondary rocks that fringe the northern flank of the primary transition chain of mountains which borders the Mediterranean — the last, or Alcala la Real, many miles distant from it in the interior of the province*.

Alhama. See general section I. ,+ The existence of a tertiary marine deposit in the neighbourhood of this town, was noticed in a memoir almost exclusively dedicated to describe a lacustrine * There is another extensive tertiary basin in the province of Granada, noticed in a former memoir as the Lacustrine Basin of Baza, which, as it is presumed to be entirely filled up or occupied by freshwater deposits, does not belong to the present subject. The deposit near Baza consists of two beds ; the lower one formed of horizontal strata of indurated gypsiferous marls, abounding in sulphur at certain localities — the upper one, which has been very generally denuded, of whitish compact limestone, also in horizontal strata. In the former, or gypseous marl beds, the Cypris Fava — in the limestone (Paludinæ) were found. — See the Edinburgh New Phi losophical Journal, July-October, 1830, Art. xvii. f From Velez-Malaga to Alhama is nearly S. and N., and Granada is about N. E. from the latter.

A. Mica slate, with superjacent grauwake beds towards the N. B. Argillaceous red sandstone, here much decomposed, but a few miles towards W., dipping at a high angle under C.=secondary Jura limestone. 1, 2, 3, 4, horizontal beds of the Basin of Alhama, 1 being tertiary marine the others lacustrine. From the Venta de Huelma to C, or the Jura limestone ridge that bounds the basin towards the N., gypsum, a, a, &c., in places variously inclined and contorted, and its associated marls generally prevail, belonging I believe to a more ancient epoch. In the valley of the Genii they have been removed and replaced by earthy conglomerate. formation it supports, which was published in Professor Jameson's Edinburgh Philosophical Journal, for the month of October — December, of the year 1830, Art. vii. The area partially occupied by the fresh water beds was termed the basin of Alhama, and stated to be chiefly surrounded by primary rocks towards the south and east and by ridges of secondary limestone towards the north and west.

This great depression, which is about fifty miles long and forty broad, is divided by the river Genii into two unequal parts, the smaller of which lies between its right bank and the mountainous barrier of secondary limestone towards the north; — the other, or larger portion, between its left bank and the circling line of primary and secondary mountains and ridges that confine the basin towards the south and west. It is in various parts of the latter more extensive and southern portion of the depression that the tertiary marine formation can be traced in the present day, as well indeed as those lacustrine beds overlying it, which formed the subject of the memoir 1 have just alluded to ; in the minor, or northern portion of the basin, neither tertiary marine nor lacustrine beds have come under my observation.

At some former period, however, the tertiary marine deposit probably occupied the whole of this great depres sion confined by a high and marked natural boundary of primary and secondary rocks, for remnants of it may be seen at the height of three thousand feet above the level b ofthe sea, on the slope of the Sierra Nevada, which for a certain space bounds the basin towards the east, and at a comparatively low level in its opposite or western portion, near Loxa, fifty miles distant from the former locality. In the southern division of the depression, the tertiary beds have not been equally subjected to the action of those causes which have completely effected their denu dation in the low tract immediately bordering the Genii, and between its right bank and the confining district of secondary limestone towards the north.

Hence this southern portion, although extremely broken up previous to the lacustrine period, still exhibits various fragments, patches, or ruins of the tertiary marine deposit, which may be traced discontinuously, and at most different levels, from one extremity of the basin to the other. These ruins are exhibited along an interrupted semi circular line, the general direction of which is nearly from east to west, and which, commencing with the fragment above alluded to, on the slope of the Sierra Nevada, and passing by the villages of Escuzar and Alhama, terminates near the Pueblo de Salar, about three miles distant from the town of Loxa, in the most western portion ofthe basin. The different points in this discontinuous band, where more or less considerable fragments of the tertiary marine formation may be examined, are, in proceeding from E. to W., at the following localities. 1.

On the ascent, from Granada, up the Sierra Ne vada, to the Peak, or Picacho de Veleta. After traversing a portion of the beautiful and highly cultivated valley of Granada, which intervenes between this city and the base of the Sierra Nevada, a path or horse-road ascends a broad expanse of one of the western slopes of the latter elevated mountain, flanked by the Rios de Monachil and Dila, which, taking their rise in the zone of snow near its summit, pour their ice-cold waters down two corresponding ravines, that constitute the boundaries towards the north and south of the space under description. The acclivity, for the first three or four miles, displays a bed of conglomerate, and has generally a pretty even surface — a sort of inclined plain covered with short herbage and wild plants Little, low groups of whitish-grey semi-crystalline limestone, whose strata dip towards the north at about 30°, occasionally make their appearance in the subsequent two miles of ascent ; when a sort of bank, formed of whitish-yellow zoophytical freestone and rising some fifteen or twenty feet above the general surface, was observed to constitute an irregular narrow band intersecting the whole space enclosed between the two ravines just alluded to.

The strata are about six feet thick and dip at an angle of 38° towards the S. W. Minute fragments of corals, pecters, balani, and other obscure testaceous remains, mixed up with calcareous matter, compose the general mass of the rock ; but the following entire shells were b discovered in a portion of it, falling into decomposition and almost reduced to sand, which Monsieur Deshayes had the goodness to examine and name. a. Cardita squamosa, varietas. b. Dentalium Bouii. c. Turritella Sabangulata. d. Caryophylla indeterminable. The northern extremity of this bank of tertiary free stone is about two miles distant from a cottage called El Cortijo de Guanes, situated in the broad and deep ravine down which the Rio de Monachil flows, whose opposite bank is bordered by a ridge of semi-crystalline white and sky-blue limestone, a ramification from an elevated and picturesque mass of the same rock termed La Sierra de Trevenque.

Towards the south, the bank is interrupted or cut off by the ravine or barranco de Dila, so that its course at the present day is confined to the interval of from five to seven miles that separates these two great fissures, although a minute examination of this intricate district might perhaps trace other patches of the beds that com pose it to a ridge of the same formation which will be immediately noticed near the village of Escuzar. The elevation of three thousand feet and upwards above the level of the Mediterranean, at which this tertiary bank is seen, is worthy of remark. At this height, the deposit, whose ruins alone are now visible, would not only occupy entirely the great depression under consideration, but must have extended far beyond its northern boundary ; and as at the distance of about forty miles in the latter direction similar tertiary beds are met with, attaining about the same elevation above the level of the sea, at Alcala la Real, as will subsequently be shown, the hypothesis may perhaps be in dulged, that the whole intervening tract was once occu pied by a tertiary sea.

If this view of their former extension be correct, an immense denudation must have taken place, for no tertiary beds were observed between Granada and Alcala. 2. Another considerable remnant of the tertiary marine deposit is observed near the little village of Escuzar, about eight miles from Granada, in the form of a narrow insulated ridge, whose length may be estimated at about six miles, and summit at about one thousand five hundred feet above the level of the sea. It is composed of hori zontal strata of calcareous freestone of a yellowish colour, made up of comminuted coral and testaceous remains, with occasional entire pectens, fragments of ostreæ, and echinital spines. The road from Granada to this locality proceeds on that to Alhama as far as the village of La Mala (See local section 1*.) where there is a brine spring, worked by the government, in the gypseous deposit that occupies this part ofthe basin.

At La Mala the road to Escuzar separates from that to Alhama, and in the intervening gently rising ground * A. Secondary limestone. B. Gypsum. C. Tertiary marine. 1. Granada on the right bank of the Rio Genii. 2. La Mala. 3. Village of Escuzar. B g gypsum is observed in every little fissure and ravine ; and constitutes between the village of Escuzar and its adjacent ridge of tertiary freestone, a considerable hill. It extends also westwards to the Venta de Hueltna (marked in general section 1.) The gypsum is of a granular structure, of a whitish grey colour, and divided into lamina: or strata from two to four inches thick, with a dip towards the north of between 20° and 30°, from which, and other circumstances that will subsequently be noticed, I am inclined to believe it does not belong to the tertiary deposit.

Indeed the dislocations some of these gypseous beds have undergone (as shown in local section I., between the Rio Genii and La Mala), probably took place previous to the tertiary epoch, for the strata which compose the freestone ridge at Escuzar are horizontal aud undisturbed. 3. The high banks of the ravine that conntiL-s the Rio de Alhama exhibit, for nearly two miles on the load from Alhama to Arenas, the finest sections of the tertiary marine formation in the whole area of the basin. An alternating series of horizontal beds, composed of zoo phytir.al limestone and calcareous sandstone, constitute the bold perpendicular escarpments of this fissure, and demonstrate the natute of the lowest portion of the deposit, which is seen to rest at this, and some other localities near Alhama, upon inclined strata of secondary limestone.

(See local section 2.*) " CC. Tertiary marine strata. D. A ma«s ot conijlomeiiiie formed of primary rocks. The zoophytic limestone seems to be made up entirely of finely comminuted coral remains, with an occasional, but very rare minute fragment of mica slate. The fragments of shell seen in it are so small that it is impossible to ascertain their genus, but some echinital spines are sufficiently entire to be distinguishable. Testaceous remains appear to be principally confined to the calcareous quartrose freestone beds, but as far as I had time to observe, afford little variety, none being discovered but what belong to different, mostly undescribed, species of Pecten. Concealed by alluvion, or by beds of lacustrine origin, the fragment of the tertiary marine deposit visible at this locality, can only be traced for a short distance beyond the right bank of the ravine, but it may be discontinuously followed through Alhama, which stands upon its left bank, nearly half way up the ascent to the high ridge of secondary limestone that confines the basin towards the S.W., and over which a horse-road proceeds to Velez-Malaga on the Mediterranean coast. 4.

On the horse-road from Alhama to Loxa, between the former village and a hill called Majar de en Medio, the beds under consideration are observed, for short distances, at two or three different points, as may be seen by an inspection of the local sections, 3, 4, and 5. From the ravine in section 5, there is a long gradual ascent to the summit of the table-formed hill, called Majar de en Medio, crowned with thick strata of compact lacustrine limestone. The bed of the ravine, as well as the lower part of its banks, are composed of secondary limestone, whose strata dip externally at con siderable angles, whilst the higher part of the left bank is formed of horizontal beds of the tertiary marine deposit resting immediately upon the former. In some parts of the left bank, the secondary limestone is however nearly horizontal.

The tertiary beds at this point are formed of thick strata of a coarse agglutinated coral sand, of a dirty whitish colour*, and extremely hard. 5. The last locality within this basin where a fragment of the tertiary marine deposit was observed, is in its western extremity, close to the little village of Sal ar, about three miles from the town of Loxa. * The horizontal table-land that borders the depression, or valley of the Rio de Alhama, in local section 3, towards the W.S.W. and E.N.E. is composed of lacustrine limestone, D.D. being gypseous marl beds under it. C. Tertiary marine. A. Secondary limestone, forming the banks of Rio de Alhama near Los Banos, an old Moorish building erected as the point where a powerful thermal spring gushes out from the latter rock. 1. Is the eminence called Majar de en Medio, men tioned in the text. 2.

The high table land on the road from Alhama to Cacin. In local section 4, A is the secondary limestone ridge that borders the basin of Alhama towards the S.W., a patch of which comes to day (A) within the basin. C.C. Tertiary marine. D. Gypseous marl beds under the lacustrine limestone of Majar de en Medio. Local section 5. A. Secondary limestone. C. and D. being the same as in preceding section. Local section 6 intersects the basin at its western extremity. A.A. Secondary limestone ridges that border it towards N. and S. C., a group of tertiary marine beds, alluded to in the text, near the little village of Salar. D. Lacustrine limestone. The local sections, 3, 4, 5, 6, show the patch-like manner in which the tertiary marine beds present themselves in this part of the basin. Denudation has excavated the narrow valley in which the Rio de Salar flows, and having swept away the lacustrine beds, still seen to crown the contiguous low table-formed eminences, has laid bare a fragment of the subjacent marine deposit, which displays itself in the form of a little hill gently sloping down to a flat, covered with a fine olive wood, on the left bank of the above named stream.

(See local section 6.) The marine beds, constituting this little tertiary fragment, are entirely composed of a mass of comminuted shells, amongst which, however, numerous small pectens, fragments of ostreæ, of balani, and echinital spines, are sufficiently entire to be referred to their respective genera. The beds are slightly inclined towards the valley, which is no doubt owing to subsidence, as may be fairly in ferred from their horizontality in every other part of the basin, with the exception of those high up the flank of the Sierra Nevada (noticed under No. 1.), whose angular position seems to indicate some great local disturbance at this point. These are the different points within the limits of the great depression or basin, in which the city of Granada, the populous village of Alhama, and numerous other rich and flourishing villages are situated, where I have observed small tracts, ruins and fragments of that ter tiary marine formation which at one time appears to have occupied its entire area.

Subsequent to the retreat of the tertiary sea from which these beds were deposited, a fresh-water lake of great depth and considerable extent has prevailed, if not over the whole, at least over the southern portion of the basin, confined probably towards this quarter of the compass, and towards the west, by the actual moun tainous border of primary and secondary rocks, and towards the other quarters of the compass, by a boundary which is no longer in existence. In all probability, however, the lacustrine waters extended over the whole surface, and had for limits the great natural boundaries of the depression*. In concluding these observations, I beg leave to be allowed to say a few words upon the memoir in which, some years ago, I gave notice of the existence of a lacustrine deposit in this basin, and of vestiges of similar beds in other parts of the Peninsulas.

It might be understood by what was stated in that memoir, and by an inspection of a section which by mistake was incorrectly represented, that the gypsiferous horizontal marls which intervene between the subjacent * List of the organic remains met with in the tertiary marine beds of the basin of Alhama. Balani. Pectens, different species. Fragments of ostreæ. Echinital Spines. Corals and other Zoophytes. Cordita Squamosa-Varietas. "\ Dentalium Bouii. I Turritella Subangulata. t * Caryophylla-indeterminable. J f See Edinburgh New Philosophical Journal, for October — De cember, 1830, Art. tertiary marine deposit and the overlying compact lacus trine limestone, and which in the neighbourhood of Alhama and Cacin, are coextensive with the latter, are contemporaneous with the gypsum observed between the ridge immediately to the N. E. of Cacin, and the village of Gavia, on the road from Alhama to Granada ; and consequently, that the brine spring at La Mala, a small village between those of Cacin and Gavia, is also of tertiary origin.

(See general section 1.) I confess that I formerly leaned towards this opinion, but a subsequent visit to Granada and different localities within the limits of the depression under description, has induced me to consider these two gypseous deposits as belonging to two distinct and distant geological epochs. Of the compa ratively modern date of that near Alhama and Cacin, there can be no doubt ; but from the reasons I am about to state, I am induced to refer the gypsum seen from a locality called La Venta de Huelma, eight miles from Cacin on the road to Granada, to the village of Gavia, to the great secondary marl-gypsaliferous forma tion, that underlies, or perhaps alternates with, the secondary or Jura limestone, throughout the whole pro vince of Granada. In the section which accompanied the memoir upon the lacustrine basin of Alhama, the slope from the northern boundary of secondary limestone to the river Genii was marked as unknown ; but it was then conceived to belong either to the tertiary marine, or to the lacustrine deposit.

During my last visit to Granada this slope was examined, and was found to con sist of red marl containing beds of gypsum, the former being well displayed in several immense ravines that intersect this tract. The deposit terminates at the base of, and passes under, a ridge of secondary limestone, which, as far as it extends westerly, is the boundary of the depression towards the north ; but the limestone ridge being shortly afterwards denuded for some three or four miles in its prolongation westerly, the red marl, rising up into high ground, becomes the boundary of the basin for the interval between the western end ofthe alluded to ridge, and a high insulated hill of similar limestone, called La Sierra de Elvira, with which the former has been once connected. Its subordinate gyp sum, accompanied by a mass of greenstone, characteristic of the secondary gypsum saliferous formation, again makes its appearance in a ravine or hollow, that separates a small portion from the main body ofthe limestone that constitutes the Sierra de Elvira.

These facts have in duced me to conclude that the slope or ground marked as unknown in the section that accompanied the alluded to memoir on the lacustrine basin of Alhama, belongs to the great secondary gypsum saliferous formation so gene rally spread over the province of Granada. Now this slope is only separated from the hilly undulating tract of Gypsum, that may be followed continuously from near Gavia, by La Mala, to Escuzar (as seen in sectionl.), and to La Venta de Huelma, on the road from La Mala to Alhama, by the valley of the Genii, and it may therefore be fairly inferred that the two were once continuous. By an inspection ofthe local section 1., it will be seen that the gypsum from Gavia to La Mala is inclined at various high angles and reversed, and that from La Mala to Escuzar* its strata are also considerably inclined to the horizon ; whereas the gypsiferous marl beds, which in the neighbourhood of Alhama always intervene be tween the tertiary marine deposit and the compact lacus trine limestone, are universally horizontal.

The above facts, which I was enabled to ascertain in my last visit to this district, seem to countenance the opinion that there are two distinct gypseous deposits in what has been termed the basin of Alhama ; one hori zontal and decidedly tertiary, in the neighbourhood of Alhama ; the other secondary, whose inclined beds may be seen from La Venta de Huelma to Gavia, and in the vicinity of the villages of La Mala and Escuzar. The latter, as before stated, appears to have been continuous at some remote epoch with the red marl containing beds of gypsum, that constitutes the high slope to the valley of the Genii, from the ridge of secondary limestone that bounds the basin towards the north in the neighbourhood of the village of Alfacar, and under which it passes, to reappear again in numerous vallies and depressions that * The dip of the white lameicated gypsum near this village and on the road from it towards La Venta de Huelma and to Agron, is towards the north east at from 20 to 30 degrees. intersect and inasculate amongst ridges of secondary limestone, over a district towards the north of between forty and fifty miles in breadth.

In the same memoir upon the lacustine basin of Alhama, the limestone forming the insulated Sierra de Elvira, near Granada, was presumed, with a doubt, to belong to the transition order. Having been informed of the discovery of amonites in this limestone, I was induced to revisit the locality, and from several impressions and casts of amonites, numerous flint nodules*, and the general mineralogical character of the rock, there can be no doubt that it belongs to the secondary limestone which generally bounds the basin towards the north, and from which it is an outlying mass. The beautifully polished tables and slabs I had seen in Granada, and the crystalline texture of the particular bed in the Sierra de Elvira, whence the blocks had been quarried, induced me to presume that the limestoue belonged to the transition series. * Flint nodules and bands, are characteristic of the secondary lime stone in the province of Granada. In the Sierra de Elvira they assume numerous imitative forms.

Chapter II. Antequera

Antequera is situated on the northern base of the mountainous district that borders the Mediterranean, at the distance of about thirty miles westerly from the basin of Alhama, and about forty miles nearly due north from Malaga. From the town of Loxa, placed in a gorge of the semi-circular band of secondary limestone that sur rounds the western extremity of the basin of Alhama, to Antequera, no vestige was observed of the tertiary marine formation, the intervening tract being occupied by the great marl gypsaliferous deposit, and by irregular masses of secondary limestone. The latter rock indeed forms a high boldly escarped ridge, which, with a direction from E. N. E. to W. S.W., ranges uninterruptedly between these two towns, a few miles to the S. of the line of road. But the tertiary formation reappears at Antequera, part of the town being built upon it, and constitutes in its immediate vicinity a series of insulated hills and eminences, and a band of conchiferous free stones which may be followed continuously between seven and eight miles.

Before commencing the detail of the tertiary beds seen c at this locality, I shall offer a few observations upon the geological structure ofthe adjoining district. The ridge of secondary limestone that has been stated to range continuously between Loxa and Antequera, passes the latter town at the distance of about two miles to the south, and is prolonged beyond it for many miles with a direction towards the south west. Opposite, or immediately to the south of Antequera, it is called La Sierra del Torcal, an elevated and characteristic mountain, contiguous to which, in its range towards the S.W., another still more elevated portion of the ridge is termed La Sierra de Chimenea, partially separated from Torcal by a Col or Puerto, over which a mule road leads to the village of Almojia, situated in the mountainous district of transition rocks, that intervenes between this ridge of secondary limestone and the Mediterranean Sea.

(See general section 2*.) Along the northern base of this elevated ridge, from Loxa to Antequera, and from Antequera onwards towards the S. W., for about twenty miles, there is a lower, broken, hilly tract, varying in width from three to ten miles, which belongs to the great marl gypsaliferous formation, and is characterised by numerous brine springs and projecting rocks of the greenstone and trappean family. Various indentures and recesses have been • From the Mediterranean, near Malaga, to the Sierra de Chimenea. A. Grauwake Slates flanked by Red Sandstone B.B. C. La Sierra de Chimenea, a. a. &c. Caps of Secondary Limestone. worked out in the inland or northern border of this tract, which, near Antequera, slopes down towards the north to a plain or champaign country, watered by the river Guadalhorce.