BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l. 17, PP. 285-294, PLS. 85-36 JUNE 26, 1906

GUADIX FORMATION OF GRANADA, SPAIN

BY WILLIAM HERBERT HOBBS

(Presented, by title, before the Society December 29, 1905)

CONTENTS Page Introduction ...... 285 Description of the Guadix formation...... 287 The Block formation ...... 287 The Alhambra form ation...... 287 The Guadix formation of von Drasche...... 288 Origin of the formation ...... 289 Agé of the deposits...... 290 'torrential deposits of southern Italy ...... 290 Probable torrential origin of many sandstones and conglomerates...... 292 Acknowledgment ...... 294

I ntroduction

The fertile Vega of Granada and the great plain of Guadix lie re- spectivel}' off the western and northern flanks of the Sierra Nevada and are separated from each other by the much lower Sierra Harana. Level almost as a floor, on their borders their surfaces incline vallevward at low angles, producing a topographic feature most common in Spain—the filled valley out of which steep mountain ranges rise abruptly. The material with which these valleys are filled merits a fuller consideration than is here possible, but a record of somewhat hurried observations, with conclusions drawn therefrom, may be of value, since they apply to a region which has received but little attention from geologists. Von Drasche, who visited the province more than a quarter of a century ago, has furnished the best description of the deposits, a portion of which he has relegated to three formations, namely, the Alhambra conglomerate, the Block formation, and the Guadix formation.* In the Block forma­ tion, which occupies the valley of the Genii, he found Miocene fossils,

* Von Drasche : GeologlseUe Sktzze des Hochgebirgstheiles der Sierra Nevada in Spanien. Jahrb. d. k. k. geol. Relehsanst., Bd. 29, 1879, pp. 93-122, pis. vli-xii.

XXV— B ull. Geol. Soc. A m ., V o l. 17, 1905. ( 285)

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and, as he believed the beds to dip beneath those of notably different type on which the Alhambra is built, he gave to the latter deposits the name Alhambra formation. The Guadix formation is areallv separate from both the others, though partaking of the characters of each. The map of the Spanish Geological Commission* shows the greater part of the three formations as diluvial, and the resemblance of the Block formation in particular to a glacial deposit is certainly most striking. Until the writer had examined numerous localities he was compelled to adopt the view that local glaciers from the Sierra Nevada had deposited the ma-

F i g u r e 1 .— Map of the Guadix and neighboring Formations. Modified from the map by the Spanish Geological Commission. The mass of *ne Sierra Nevada Is composed of crystalline schists of Silurian or greater age. J , Jurassic ; T, Triassic ; N, Neocene ; P, Pliocene ; G, Guadix formation.

terial, and it appears that every geologist who has studied the district has given prominence to a supposed glacial origin for the formation. Mo­ raines in the valley of the Genii were even described by Schimper.f Bertrand and Kilian J state that the lower laj^er of the Block formation at Talara contains calcareous pebbles which are polished and striated, and they quote Taramelli and Mercalli as holding this opinion of a

* Sheet no. 52, Granada, second edition. f Schimper : Voyage géologique botanique au Sud d'Espagne, 1849. t Bertrand et Kilian : Études sur les terrains secondaires et tertJares dans les provinces de Grenada et de Malaga. Mem. de l’Acad. de Sciences de l’Institut National de France, t. zxz, p. 520.

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glacial origin. Von Dräsche, while explaining away the moraines of Schimper, describes a great block scratched and polished which was found near the Camino de los Neveros.*

D e s c r i p t i o n o f t h e G u a d ix F o r m a t i o n

THE BLOCK FORMATION For a study of the Block formation one has only to go from the Al­ hambra of Granada, past the Cemiterio, into the valley of the Genii. Immediately after passing the cemetery on the crest of the divide be­ tween the Bio Darro and the Rio Genii the change in the deposits is apparent. From a loosely consolidated deposit of gravel containing large and small, well rounded pebbles, mainly of a single type, with maximum size a few inches in their longest dimension, one here encoun­ ters a deposit the pebbles of which are dark colored diorites and gabbros, together with garnetiferous schists, fissile mica-sehists, and many other crystalline types of rock foreign to the immediate neighborhood. The size of the pebbles also varies from a fraction of an inch to boulders whose dimensions must be measured in feet. There is, further, to be observed a noteworthy frequency of faceted boulders which in shape are not to be distinguished from those found in a bank of glacial till. Many blocks are polished, but the writer was unable to discover any on which glacial scratches could be made out. On the dump from a mining shaft sharply faceted blocks were found in abundance which are with little doubt derived from the lower layers of the formation, and these are pol­ ished in a noteworthy manner. The surface of these blocks gives, how­ ever, the impression of slickensides rather than of glacial polish, and, in view of the description by von Dräsche of such forms in the Sierra Nevada, any other explanation would seem to be superfluous.f The source of the blocks is clearly the slopes of the Sierra Nevada on which the Genii has its source.

THE ALHAMBRA FORMATION The Alhambra formation occupies the valley of the Darro, which has its rise not in the crystalline rocks of the Sierra Nevada, but in the Triassic dolomite of the Sierra Harana. The uniformity in petrographic type of its pebbles and their smoother contours, find in this their explana­ tion. The thickness of the Alhambra formation can be little short of

* Loc. clt., p. 121. t “Diese Schiefer sind ungeraein mürbe; ein Hammerschlag auf einer grossen Block lässt denselben in ein Haufwerk von Kinmmfiächrigen Flatschen zerfallen, die auf der gewölbten Fläche stets eine Art Seidenglanz zeigen. Es scheint als ob die ganze Schiefermasse in sich selbst verrutscht wäre.”

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1,000 feet, mainly a conglomerate having distinctly water-worn pebbles which vary from a fraction of an inch to six inches or more in length. Interbedded within the formation are layers of fine sand and loess-like material which changes most abruptly to the conglomerate. In the finer material stratification shows plainly, and, while generally horizontal, dips as high as 25 degrees occur; but these dips are in the narrow portions of the valley and are apparently uniformly in the direction of its bottom. In ascending the valley of the Darro along the old post road from Granada to Guadix, one sees great thicknesses of the loess-like material, and at other times merely thin layers a fraction of an inch in thickness may be followed for many rods along a horizontal plane within the con­ glomerate. Not infrequently lenticular forms are noticed in the section. A little above the village of Huetor-Santillan the Alhambra formation comes into contact with the Upper Triassic dolomite of the Sierra Harana to the north of the road. The Darro valley has now narrowed to such an extent that the same dolomite soon appears npon the other side of the valley as well and the Alhambra formation presently comes to an end. Its steeper dips toward the valley are here noticeable, and the weathering of the dolomite is most instructive in considering the origin of the Guadix formation. Rising in steep slopes, the dolomite takes the form of beetling crags whose divisional plains have been largely determined by joints, while at the base of the cliffs is found an aggregation of larger and smaller blocks already arranged imperfectly in trains along the bot­ toms of small gullies or barrancas. The first heavy shower will carry a portion of these blocks down into the Darro valley, rounding the angles on the soft material. An observation of von Drasche, made to the east of the divide, is that the larger blocks are generally found near the bor­ ders of the formation.

THE GUADIX FORMATION OF VON DRASCHE Passing over the divide between the Darro and the Farde near El Molinillo and descending into the valley of the last mentioned stream, after passing Diezma we obtain a view of the broad plain of Guadix, which stretches away for 25 miles to the base of the Sierra de Baza and appears as level as a floor. In this deposit of soft material the Farde has cut broad valleys with level floors and steep walls, revealing abun­ dant sections of the material, which has characters in many respects the same as those found in the valleys of the Darro and Genii. It is, how­ ever, as a rule finer in texture, with larger proportions of loess. This loess is locally filled with roots and brush (“Noah’s brush heap” ), is yel­ low in color, and altogether quite similar to the loess of the upper Missis­ sippi valley. Throughout the loe«s is very perfectly jointed on vertical

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F ig u r e 1.— J u n c t io n o f t h e G u a d ix F o r m a t io n avith t h e S i e r r a N e v a d a a t A l q u if e

Mountain of hem atite in m iddle distance

F ig u r e 2.— “ B a d L a n d s ” T o p o g r a p h y in t h e V a l l e y o f t h e F a r d e

CHARACTERISTIC TOPOGRAPHY OF THE GUADIX FORMATION

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planes. Within the finest loess one sees in vertical sections lenticular areas of finer or coarser pebbles. Small aprons of talus at close and fairly reg­ ular intervals border the river walls at their bases (plate 35, figure 2). Seen from the heights about the valley, not only is the surface of the formation notably horizontal, but several stages of lower and parallel planes produce sharply delimited shelves wherever the river has opened the formation in sections. The upper layer and some of the lower ones show a rich brown color, and on nearer approach are seen to be colored with soft hematite. As one looks to the southward beyond the formation toward the distant Sierra Nevadas, one sees rising at their base the black mass of the mountain of hematite at Alquife,* and it is difficult not to as­ cribe a common source to it and to the hematite of the Guadix formation in an earlier ferruginous deposit situated higher up and in the Sierras themselves. The color effect in the general view from below Diezma is un­ usually fine, the hematite layers standing out in the dissected valley floor like pencil lines, with the black iron mass of Alquife and the white dolo­ mite of the Alcohorra (each capped by a Moorish castle) sharply outlined against the gray background of the Sierra Nevadas. Where the Farde has cut its broad valley, patches and strips of green appear beneath the diversified and picturesque “bad land” topography which surrounds them. The labyrinth of loess columns, eaten into by the sudden rains, have been excavated locally to furnish homes for a large proportion of the peasant population in the district (see plate 36). Plate 35, figure 1, shows the abundance of irregular boulders upon the surface of the forma­ tion where it borders upon the Sierra Nevadas.

O r i g i n o f t h e F o r m a t i o n

The Guadix formation appears to be largely a torrential deposit of material derived from the neighboring Sierras, with local characteristics restricted to the individual valleys of the Genii, Darro, and Farde, and dependent on the rock material which is in place near the headwaters of those streams. The material is coarser near the borders of the formation, while loess and floating material, such as roots and brush, are more char­ acteristic of the central and presumably quieter areas. To account for the almost perfect horizontality of the beds within the central areas, it is necessary to assume the former existence of at least temporary lakes within the valleys, while the lenticular forms of the pebbly material in the sections indicate that streams once coursed over the floor of material below. Some part has undoubtedly been taken by the wind in depositing the formation, since it is active today in transporting the lighter ina-

* See an article by the author treating of the iron mines at Alquife.

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terial, but its role would appear to be secondary to the cloudbursts and the resulting torrents of the rainy season. Such an alternation of conditions as is indicated by the material of the Guadix formation is found today only in arid regions of high relief, where the rare but violent storms develop the torrent and the playa lake, and where the wind plays an important part in transporting the surface material. The climate within the province of Granada is today semi- arid—the fertile Vega of Granada being an oasis, which only accentuates the surrounding aridity.

A g e o f t h e D e p o s i t s

As already indicated, Miocene fossils have been found in the Block formation of von Drasche, and as has here been shown, torrential de­ posits are forming today in the valley of the Darro and elsewhere. It is probable that the Guadix formation includes beds extending without noteworthy interruption from the Miocene to the present. That similar conditions have prevailed from even earlier times might be inferred from the occurrence of a conglomerate of Triassic age at the base of Alquife hill, which is located on the exact border of the Guadix formation and at the foot of the Sierra Nevadas. This conglomerate includes angular boulders of iron ore whose dimensions are sometimes measured in feet.*

T o r r e n t i a l D e p o s i t s o f s o u t h e r n I t a l y

Deposits remarkably similar to those of the Guadix formation border much of the mountainous coast of the Italian peninsula and Sicily. Above Eeggio, in Calabria, blocks of granite two feet or more in their largest dimensions are found associated with similar blocks of several other crystalline types as lenticular forms within deposits of sand, gravel, and finer material, all of which has clearly been derived from the Calabrian Apennines immediately to the east. The bedding of these de­ posits is for the most part nearly horizontal, though angles as high as 30 degrees were observed. The topography of these deposits, wherever dissected, is that of the “bad lands”— the type for rain erosion. Such deposits may be seen to even better advantage to the westward of Messina, in Sicily, on the road to Castellaccio. The intermittent streams (torrenti or fiumare), which are so char­ acteristic of southern Italy, have dissected the deposits. These valleys are wide at their mouths, with broad, flat floors, which ascend at ex­ tremely low but ever increasing grades toward their heads, where the slope rises abruptly like a ivadi to form an amphitheater. On these

* William H. Hobbs : Mining in Spain. The Mining World, vol. 24, 1906, pp. 109-110.

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F ig u r e 2.— R a in E r o sio n F o rm s a t B a r r io d k S a n tia g o

CHARACTERISTIC TOPOGRAPHY OF THE GUADIX FORMATION

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slopes are often found the steep pinnacles with pebble cappings so charac­ teristic of rain erosion. Other deposits and erosional forms were ob­ served about Taormina and are well shown from the summit of Mola. In the summer season the “fiumare” are dry, the pebble floors being generally utilized as highways of travel; but after the rains they become roaring torrents, which suddenly rise and as suddenly ^subside. At Co- senza, in Calabria, the writer was fortunate in witnessing the trans­ formation of the Busento (the ancient Buxentms) by one of the sudden cloudbursts characteristic of the region. Crossing the river by the famous Ponte Alarico in the face of an impending shower, the broad river floor showed a mere thread of water. When the storm had burst the steeply sloping Corso of the city became trans­ formed into a swift current bor­ dered by waterfalls where the F i g u b e 2 .— Type of Cross-bedding. steep side streets entered. Within Observed in the torrential deposits near Pontegrande, in Calabria. a half hour the storm had passed, but the Busento was swollen to a roaring torrent which filled its bed from bank to bank. A few hours later it presented almost the same appearance as before the rain. All about Cosenza are found torrential deposits, including round peb­ bles and boulders, not unlike the Alhambra conglomerate of Granada. Xearlv identical deposits were studied at Rossano and at Pontegrande, near Catanzaro, in Calabria. At the last mentioned locality the deposits can be little short of 1,500 feet in thickness, if they do not exceed that figure. The boulders included are of many petrographic types and often exceed a foot in diameter. Between markedly horizontal layers revealed by the finer material, cross-bedding of a type often seen in ancient sand­ stones is well displayed (figure 2). In this type we find between thick horizontal layers intercalated beds in which the bedding makes a nearly uniform but relatively large angle with the layers which inclose it. At Rossano the torrential deposits are locally faulted in the manner shown in figure 3. Through these deposits the crystalline formations occasionally project on steep walls. High up the valleys toward the crests of the older rocks the generally horizontal bedding is locally re­ placed by steeply dipping layers, which, like those observed in Granada, incline toward the valley. Angles as high as 45 degrees have been ob­ served. Where deep dissection has been produced by the “fiumare,” the steep walls of loosely compacted material become during the rainy season

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saturated with water until one of the earthquakes so characteristic of the province sets loose a large mass to slide down and be soon dispersed by the torrent below. The banks of many iiumari present great scars, the freshness of whose surfaces furnishes an indication of their relative age. On the fiumare Oliveri, between the Calabrian village of Aiello and the Tyrrhenean coast (a distance of about 10 kilometers), a number of these great scars were seen, the largest caused by a landslide during the past year. In this category must be placed also the mass of soft rock (which has been estimated at 7,000 cubic feet) which was precipitated from the castle rock upon the town itself by the earthquake of September 8, 1905. The torrential deposits which border southern Italy between the moun­ tains and the sea appear to be in part of Quaternary age (especially those bordering the straits of Messina); they are also in part Recent, and in part they are pre-Quaternary. Cortese, who has furnished the best re­ port upon the region,* ascribed much of the Recent and some of the

Quaternary deposits to a fluviatile origin through the agency of the torrenti, and stress is laid upon the difficulty of delimiting the several formations. The occurrence of these deposits in distinct pianos or shelves on the seaward side of the Apennines he explains by the existence of such topographic forms in the surface of the underlying crystalline terrane; and with this view the writer is in full accord.f Within the extensive arid regions of the western United States the present-day importance of the torrent and the desert lake in filling the valleys and fashioning the topography has been generally recognized by the geologists who have studied the region.

P r o b a b l e t o r r e n t i a l O r i g i n o f m a n y S a n d s t o n e s a n d C o n g l o m ­

e r a t e s

A terrane as heavy and as extensive as the Guadix formation, when buried beneath later deposits and cemented into sandstone or quartzite,

* E. Cortese: Pescrizione geologlca della Calabria. Memorle descrlttive della carta geologlca d’ltalia, vol. ix, 1895, pp. xxviii and 310, map and 4 plates. t See tbe colored plates of the work cited for the topographic character of the piani and figure 19a., on page 185, for a hypothetical section.

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must figure largely in the formations of the district when these have been exposed upon a later erosion surface. Such a formation should be bordered by a fringe of coarse breccia-like conglomerate giving the im­ pression that it is a basement layer, and perhaps also having a basement layer of the same or similar material. It should be characterized by in­ cluded lenticular areas of coarser or finer material and by layers of fine material in sharply defined films and plates which reveal its bedding. If undisturbed from its original attitude, its bedding should be generally horizontal, though with a gentle centrally directed inclination upon its borders, resembling in this the initial dip of a marine formation. Lo­ cally, at least, it may show a type of cross-bedding like that represented in figure 3, which, though often observed in ancient sandstones, is not adequately explained by the changing currents along a marine shore.* In its finer-textured portions a rectangular jointing will be likely to be found characteristic, and minor faulting dating from a period when the material was only slightly compacted is possible (see figure 3). The great variety of rock type, the range in dimensions of included pebbles and boulders, and the frequency of faceted forms among them may sug­ gest an origin of the formation through glaciation, as has been true of the massive conglomerates of the original Huronian of Canada. Having in mind the fact that arid conditions prevail today over about three-fifths of the earth surface, the study of the torrential, playa, and eolian deposits, which are characteristic of desert regions, must be brought into consideration before an adequate explanation can be found for the masses of sandstone and conglomerate, 1,000 to 1,500 feet and more in thickness, which exist within the ancient rock formations of the globe. It is but natural that the first explanation of these formations should have been baaed upon geological processes which are most famil­ iar— those concerned in the erosion of the land areas under humid conditions with deposition along the ocean littoral. Marine sandstones should, however, be relatively thin; for it would appear that during a transgression of the sea on the land the formation of sand should be limited to a depth not far below the wave base—a depth measured in tens rather than in hundreds or thousands of feet. To meet this difficulty, the theory of Hall, that depression is in areas of deposition adjusted to the material deposited, has been greatly strained. The dominance of ripple marks and the paucity of marine fossils just where marine life should have been most abundant are facts difficult to

* Such a structure is shown in great perfection over considerable areas by the Cambrian sandstone exposed in the picturesque “Dalles’* of the Wisconsin river near Kilbourn City, Wisconsin. XXVI—Bull. Geol. Soc. Am., Vol. 17, 1905.

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explain on the theory of marine origin for the great sandstone formations. The enormous expanse of sandstone formations fits better to a desert than to a marine theory of origin along the ocean borders. In the Tri- assic or Newark formation we find beds of clay of clastic origin lacking marine fossils, together with conglomerates, moraine-like deposits of debris, great beds of sandstone and conglomerate, colored clays, and beds of salt and gypsum, all alternating with marine sediments. Within the sandstones are found rain-drop impressions as well as footprints of ani­ mals, but without the animals themselves. In the Tertiary deposits of the Paris basin there is found such an alternation of marine sediments filled with mollusks, with clay, gypsum, and sandstone, including the bones of land animals, that it is little wonder Cuvier was impelled to adopt his theory of earth cataclysms to explain them. Just such alternations are, however, characteristic of the deposits in desert regions, where the torrent and the playa lake are found, where the surface deposits are continually shifted by the wind, and where the barrier from the sea is at times broken down. At such times the land fauna is either driven away or destroyed, and in the latter instance a “bone bed” is entombed beneath marine deposits which later may come to the light and their territory be again invaded by a land fauna. As regards the more ancient sandstone formations, the frequent occur­ rence of an abundance of angular feldspar fragments to form an arcose indicates a secular disintegration of granitic rocks under essentially arid conditions. Such an explanation has been applied by Pumpellv* to the Grey lock district of Massachusetts, and the writer has found striking illustrations of such deposits from near the Massachusetts-Connecticut interstate boundary, f The recent papers by Walther,J Passarge,§ and Davis | indicate that the part the desert has played in geological history is to receive greater attention in the future than it has in the past.

A cknowledgment

The writer desires to acknowledge his indebtedness to M. Pelsmacker, the Belgian consul at Granada, who is both a mining engineer and a com­ petent geologist, and whose familiarity with the geology of the province has been of great assistance to the writer in his study of the district.

♦ R. Pumpelly : Monograph xxi,. U. S. Geol. Survey. t Best seen on the summit of Collins hill, near New Milford, Mass. $ J. W alther: Das Gesetz der Wiistenbildung. Berlin, 1900. § S. Fassarge : Die Kalahari. Berlin, 1904. jj W. M. Davis: The geographical cycle in an arid climate. Jour. Geol., vol. xlii, 1905, pp. 381-407.

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