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BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l. 15, pp. 369-378 A u g u s t 13, 1904

POST-GLACIAL CHANGES OF ATTITUDE IN THE ITALIAN AND SWISS LAKES

BY FRANK BURSLEY TAYLOR

(Presented before the Society January 1, 1904)

CONTENTS Page Introduction...... 369 T he structure of torrent deltas...... 370 L ake Maggiore...... 371 ...... 375 L ake G arda...... 376 L ake ...... 376 ...... 377 L ake Lucerne...... 377 Conclusions...... 378

I ntroduction

During a trip abroad in the summer of 1894 the writer spent about two weeks studying the lakes of northern in search of evidences of change in the relative attitude of the lakes and the land. In the autumn part of another week was spent in similar studies on lake Geneva and two days on . The results attained seem to have some interest not only in their local bearings, but also in their general rela­ tions to the . This paper, however, is necessarily of a preliminary nature, for two weeks was much too short a time for an exhaustive study of the four examined. It was found that all three of the larger lakes of — Maggiore, Como, and Garda—formerly stood in different attitudes toward the land from those in which they now stand. Their surfaces were rela­ tively higher at the north and sloped to the south about 1 foot to the mile, as compared with their present surfaces. It might be thought that the abandoned shorelines of lakes of such magnitude—30 to 37 miles long, and with average widths of from 2 to 5 miles—would show wave-made beaches, but, excepting the southern ex­ panded part of , which reaches a width of 10 miles, no certain evidence of wave work was found above the present shorelines. The main evidences of the former attitudes of the lake surfaces are the old

L—B ull. Gkol. Soc. Am., Vol. 15,1903 ( 3 6 9 ) Downloaded from gsabulletin.gsapubs.org on August 2, 2015

370 TAYLOR— POST-GLACIAL CHANGES IN ITALIAN AND SWISS LAKES delta cones of mountain torrents. The torrents descend from high moun­ tains ; their fall is rapid, and in time of freshet they carry large quantities of coarse detritus. attains a depth of nearly 2,800 feet, Como over 1,900 feet, and Garda upward of 1,000 feet. The subaqueous walls of their basins are in some places nearly vertical, in a few places overhanging, and their slope is generally as much as 30 degrees, and is often much more. Such steep slopes afford a poor foundation for delta building. The deltas are relatively steep conical forms, resembling somewhat the alluvial cones of our arid western regions. They are, in fact, compound forms, partly alluvial cone and partly delta. Every torrent of any size has a conspicuous old delta deposit of this kind at its former mouth, and in most of these old deltas the present stream has cut a deep trench in order to reach the lake at its present lower level. In a few cases the old delta has been so warn away by erosion that it was difficult or impossible to find any certain evidence of a former higher lake level.

T h u S t r u c t u r e o f T o r r e n t D e l t a s

As is well known, deltas are built of sediments arranged in foreset and topset beds. By continual additions of foreset beds at its outer margin or front, a delta grows in horizontal extension; by the addition of suc­ cessive layers to its top it gradually aggrades or builds up its surface to higher levels. The principal growth is accomplished by the foreset de­ posits, especially where the offshore slope is steep. As the area filled in by these beds grows larger it forms a platform on which the topset beds are laid down. When the topset beds are composed of very coarse materials, the shape they give to the subaerial part of the deposit may resemble a detrital cone. The torrent deposits of the Italian lakes are mostly of this character. In the original process of building the old deltas, when they were aggrading, the streams did not enter the lakes through deep trenches in older delta deposits as they do now, but in shallow distributaries choked with coarse detritus and frequently shifting their places. The coarser sediments were mostly dropped in the headward parts of the distributaries and added to the building up of the detrital cones. The greater part of the finer sediments were carried down to the lake to be added to the foreset beds. The coarseness of the detritis and the steep­ ness of the surface slope both decreased toward the lake, so that at the shore the material was mostly pebbly gravel and sand and the slope was more nearly horizontal. A small quantity of sand and fine gravel was deposited at the mouths of the distributaries on the shallow water mar­ gin of the deltas, and shore currents and wavelets distributed and re­ arranged them to some extent along the delta fronts. Downloaded from gsabulletin.gsapubs.org on August 2, 2015

LAKE MAGGIORE 371

Some of the larger old torrent deltas show flattening toward their outer edges quite clearly, giving them the appearance of flat-topped terraces, when seen from the lake, but closer inspection shows the top to be a gentle slope steepening rapidly back toward the ravine. These old delta terraces now stand with bluff fronts facing the lake. The top of the bluff in such a delta is the outer edge of the original flat border­ ing the shore, and the presence at that place of a layer of sand and gravel may be taken as reliable evidence that the lake stood at that level when the delta was made. These related characters of form and composition were used as the principal criteria for the determination of the former attitude of the lake surfaces.

L a k e M a g g io r e

On lake Maggiore very little evidence of a higher lake level was found in its southern third, south of Pallanza. On the opposite shore of the bay, south and southwest of Pallanza a narrow terrace, mostly very faint but fairly well developed at and , stands about 30 feet above the lake, and might suggest wave action, but no certain evi­ dence of such work was seen. It is formed by a lateral blending of small torrent deltas. At and Arona, farther south, there are sim­ ilar faint forms about 25 feet above the lake. North of Meina a number of sharp gullies descend the drift-covered slope and end at the faint ter­ race level, 25 to 30 feet above the present shore. On the level plain south of Arona and from there to , at the outlet of the lake, noth­ ing definite was seen. The east shore, south of Laveno, was not exam­ ined. The country on that side and also on the west side below Arona is relatively low, and no streams of any size enter. The cut of the through the great moraine between Borgo Ticino and Somma was seen from a distance to be deep and narrow. A fine series of ter­ races, which form an amphitheater around the south end of the lake, are cut on the back slope of the great moraine, and are probably due to ice- border drainage. Just south of Lesa, which is about half way between Arona and Pal­ lanza, there is a quite extensive deposit of gravel. It has the form of a. delta terrace, but from its greater horizontal extent was probably built in shallower water than those farther north. Its flatter part, where the sedi­ ments are finest, is now 30 feet above the lake. This part was presum­ ably at or perhaps a little above the old lake surface when the delta was formed. The pebbly gravels composing this terrace are well displayed in the bluffs on the shore back of the old castle, a mile south of Lesa. The lanes which lead down to the castle are on the flat delta surface. Southwest of this is the modern delta and stream bed at a lower level. Between Pallanza and Intra there is a beautiful level plain. At its Downloaded from gsabulletin.gsapubs.org on August 2, 2015

372 TAYLOR----POST-GLACIAL CHANGES IN ITALIAN AND SWISS LAKES

F i g u r e 1 .—Sketch Map of Lake Maggiore. Downloaded from gsabulletin.gsapubs.org on August 2, 2015

LAKE MAGGIOKE 373 north end it merges with the delta of two torrents which enter the lake in parallel beds at Intra. Going up the delta from Intra, on the road between the two streams, the ascent steepens more and more rapidly until near the hamlet of Torbasso, 2 miles back, the deposit becomes a mass of cobbles and small boulders, over 150 feet above the lake. The north stream is the San Giovanni, and the south stream, which is much the larger of the two, is the . From its hor­ izontal extent this delta evidently does not represent a deep filling. My aneroid readings make the level plain south of the San Bernar­ dino about 50 feet above the lake and the same for a similar area in the north part of Intra. The flatness of the plain, the fineness of its soil, and the apparent relation of the plain to the San Bernardino cone suggest that at least the surface or top dressing of the plain is the work of distributaries from the San Bernardino carrying only fine sediments. One mile northeast of Intra there is a small but very prettily formed tor­ rent delta, and its edge at the front of the terrace stands at about the same height as the flat in the village. The next deposit examined was at Luvino, on the east shore. At this place delta deposits are beautifully developed in terraces at two levels, but the situation and the history of these deltas are quite different from those of typical torrents. The deltas at Luvino are not torrent deltas, but are deposits of what one may call a less strenuous stream, the , which carries the overflow of . The Tresa descends about 250 feet in 7 or 8 miles, but nearly all of its fall is in its lower half. It receives no tributary torrents, except three or four very small ones in its upper half. The head of the main or upper delta is at the upper mill at Creva, a small village at the falls of the Tresa, about one and a half miles southeast of Luvino. The head of this delta is about 100 feet above lake Maggiore and its front, rather illy defined, about 80 feet. It is composed of a rather fine grade of gravel, and largely also of sand> and is thus strongly contrasted with the very coarse materials of the torrent deltas. Below the main terrace, about 45 feet above the lake, is another smaller, less conspicuous bench. Considering the character of the Tresa and its relatively scanty sources of detrital supply, it seems probable that the heavy upper terrace at Creva is associated with a glacial river of larger volume than the present Tresa and acting when the glacier was still present in the northern half of the Maggiore basin. The lower terrace was probably built by the Tresa at normal volume mainly out of materials excavated from the upper deposit. Three miles north on the east shore is . Here is a torrent delta in typical development. The torrent issues now through a narrow ravine cut in the south part of the old delta. The main part of the vil­ Downloaded from gsabulletin.gsapubs.org on August 2, 2015

374 TAYLOR— POST-GLACIAL CHANGES IN ITALIAN AND SWISS LAKES lage is built on the larger remnant north of the stream. The business and more compact residence portion is on the outer, gentler sloping part of the old delta, while the church and many dwelling-houses are higher up toward the head of the cone. The composition shown in some parts of the bluff front and in banks of the ravine are like those described above as being typical. A cross-section from the ravine lakeward shows a typical delta form with detrital cone superimposed and indicates a lake surface at timé of construction 40 to 45 feet above the present water. Across the lake at there is another good example of old delta, but in a more eroded, fragmentary state. The torrent passes out through a deep cut in the north side. The village is built on the old delta surface, the same as at Maccagno. The general level in the busi­ ness part of the town is gently sloping toward the lake and 60 to 70 feet above it. The outer edge of the gentler slope is 50 to 55 feet above the lake. This is probably not far from the old lake level, which in this case is not well shown. In the valley west of Cannobio, on the north side of the stream, there is a heavy terrace with large boulders. The body of the terrace is composed of till, and the stream now flows in a ravine around its south side. Along the stream, but somewhat below the main terrace, are fragments of narrow, secondary terraces cut in the till. The main terrace appeared to belong to the time of the higher old lake level. The secondary terraces were probably made while the stream was cutting down to its present level. The projecting part of the delta of the between and is low and broad and flat, and is composed of gravel and sand and silt. This is not an old higher level delta, but has been made since the lake came to its present level. The Maggia and its large tributary, the Merlezzo, are mountain torrents, but for several miles in their lower courses their rate of descent is moderate, so that they do not carry much coarse detritus to the delta. The lake is shallower here than farther south, and the present delta has been built out mainly in a horizontal direction, so as to reach half way across the lake. The general situation makes it evident that if the lake formerly stood, say, 50 or 60 feet higher than now, the delta would have been of the same character—mainly of the finer sediments. East of Ascona, where the delta laps around a pro­ jecting ridge, it has the appearance of a sharply cut terrace 40 feet above the lake, but is not in fact of that nature. A mile or more north of Ascona and west of the main road a considerable flat area of fine deposits suggests an old delta, but nothing more definite was seen. At Locarno and eastward there are many small torrent cones. The village itself is built on a number of these, which blend together at their sides, but they are much eroded and do not seem to show distinct evi­ dence of the place of the former lake surface. Where their slope is least Downloaded from gsabulletin.gsapubs.org on August 2, 2015

LAKES MAGGIORE AND COMO 375 rapid their lower edges are 60 to 80 feet abo\je the lake, but this slope is relatively steep. A well formed delta of a larger torrent is at , about 3 miles east of Locarno, which seems to show the old lake was 60 to 65 feet above the present. The Morobbia has built a cone or delta of considerable size below Guibiasco, but this was seen only from the train. The larger rivers—the Ticino above lake Maggiore and the west of Pallanza—were disappointing. In the brief examinations which I was able to make I found no clear evidence of a higher lake level. Their later delta deposits and the cones of the torrents which empty into their valleys appear to have overwhelmed any older deltas which may have been built. The phenomena below Pallanza led me to believe that, besides being tilted in post-Glacial time, lake Maggiore has cut down its barrier nearly 20 feet. If this be true, then the old lake surface now rises from nearly 20 feet above the present lake at Cesto Calende to between 55 and 65 feet at its north end, near Locarno—that is, the old surface rises 35 to 45 feet in 37 miles. But if the altitudes found near Intra are correct, there appears to be a local upbending of 10 or 15 feet in that vicinity. This chapter of the history of lake Maggiore is worthy of much more careful and detailed study than I was able to give it in one week. Though I examined most of the places on foot—a few while driving— the intervening stretches along the lake shore were not examined. The most valuable evidences that could be found are those of wave action, of which, however, I found none that were certain. A more detailed study might be expected to discover some evidence of this kind. Evi­ dence exclusively from deltas like those here described does not yield a basis for accurate determination of the old water surface. The best con­ clusions they afford are only approximations.

L a k e C omo

In two days spent on lake Como I found that the delta at Cernobbio, 2 miles north of the village of Como, stands very low with reference to the present lake. It seems to show a permanent, unchanged relation to the lake level. Steep cones at Argegno and Campo show a slight falling away of the lake—less than 10 feet. At the old delta is well developed and indicates a former lake level 12 to 15 feet above the present. Another, equally fine, at Bellano, on the east shore, shows about the same height. A delta at Dervio shows the old lake level at about 20 feet. Three fine specimens of torrent delta-cones occur at Dongo, Gravedona, and Domaso. They are so close together that they are connected by narrow terraces. The old lake level here is about 25 feet above the water. I did not go farther north, but at the same rate of rise the old level at the extreme north end would be about 30 feet, or Downloaded from gsabulletin.gsapubs.org on August 2, 2015

376 TAYLOR— POST-GLACIAL CHANGES IN ITALIAN AND SWISS LAKES

perhaps a little more. The old lake level in lake Como therefore rises 30 to 35 feet in 30 miles. L a k e G a r d a

The studies here, as on lake Como, were less detailed than on lake Maggiore, but some of the evidences found are much more satisfactory. The evidence from deltas on this lake is relatively poor, but in its southern expanded part there is a faint, but sharply cut wave-made beach rising toward the north. The south end of lake Garda expands in bulbous form to a width of 10 miles, and it lies outside of the mountains in the lap of the great moraine. I saw only the west shore of the expanded part. At Maderno there is a well developed delta which shows evidence of the old lake level at about 10 feet. There are faint evidences of wave action from here southwest to Salo at about the same level. On rounding the point east of Salo one obtains a good view of the shoreline from the steamer. It is here 7 or 8 feet above the lake, and its gradual descent south­ ward is easily noted. Its bench is 10 to 50 feet wide when cut in drift slopes, and the cliff at its back is not over 5 or 6 feet high, and usually lower. It is faint compared with beaches of our American Great Lakes region, and is unlike our fainter beaches in that it is a sharply cut bench wherever I saw it. Our fainter beaches are seldom cut, but generally take the form of a low, more or less broken ridge of sand or fine gravel. The cut bench is well shown on a small island, Isola di San Biagio, where the bench is cut all around, leaving a higher central part like the crown of a hat. It is well formed also on the Sermione peninsula, but on the rocks of the outer point it is scarcely distinguishable. It de- with clines apparent regularity to within a foot or two of the water at Desenzano, where it is not easily distinguished from the present beach. The west side of lake Garda, north of Maderno, is mostly precipitous and the streams are small, affording poor opportunity for deltas. On the east side at Ascenza and Malcesine there are faint marks of the old lake level at 18 to 20 feet. At the north end of the lake, between Riva and Arco, there is a tangle of torrent deltas built in by streams from several directions. No definite evidence of the old lake level was ob­ served here, but assuming its plane to be produced from the south it would be somewhere between 25 and 30 feet above the present lake, agreeing well with the level of the delta deposits back of Riva. Thus in 30 miles the old surface of lake Garda rises 25 or 30 feet.

L a k e L u g a n o

This lake is much smaller than the other three described. Its length in a north and south direction is about 9 miles, but its outlet is nearly midway of this distance. At the ends of its two south arms, at Downloaded from gsabulletin.gsapubs.org on August 2, 2015

LAKES GENEVA AND LUCERNE 377 and , the shallows grown with reeds seem to suggest a back­ ing up of the water. The delta fronts at Lugano and stand 3 or 4 feet above the water. Nothing more definite was found. A more promising subject, and the only remaining promising one in Italy for studies of this kind, is lake Iseo, but this was not visited.

L a k e G e n e v a

My observations on lake Geneva were fewer than on the Italian lakes, but, so far as they go, seem to indicate a similar direction of deformation, though less in amount. At the time of my visit I was not acquainted with Forel’s great work, and so, unfortunately, did not profit by his studies. Forel mentions two principal sets of terraces, one at about 10 meters altitude and the other at about 30 meters,* with a few scattered ones at higher levels. I saw some of these near Vevay, Lausanne, and Thonon. I did not observe wave-made features in connection with them, and so regarded them as products of ice-border drainage rather than true deltas made in the margin of open water. The breadth of lake Geneva in its broader part seems, by comparison with the broader part of lake Garda, to justify the expectation of a recognizable wave-made shoreline, provided the lake stood long enough at one level. Besides the deltas described by Forel, I found near Lausanne a feature which appeared to be a true old shoreline in the form of a sandy bench 12 to 14 feet above the lake. There appeared to be a faint wave-cut ter­ race also at Evian and Thonon on the south shore, but at these places only 7 or 8 feet above the water. Faint marks supposed to belong to the same stage of the lake were seen near Nyon and Rolle on the west shore. If these features are of the character supposed, they seem to show an old lake surface rising northward 12 to 14 feet in about 20 miles. Lake Geneva, like the lakes studied in Italy, has its outlet to the south, though its greatest extent in a north and south direction is only 20 miles. Forel’s observations suggest no deformation, though the fragments above 30 meters altitude were said to be at discordant heights. A slight differ­ ential tilt like that here recorded would be hard to determine in so short a distance as 20 miles by the study of old delta deposits. So small a factor might easily have been overlooked.

L a k e L u c e r n e

The remaining larger Swiss lakes—Neuchatel, Lucerne, Zurich, and Constance—have their outlets to the north. If they have suffered changes of attitude in the same direction as the Italian lakes and lake Geneva, the change has been equivalent to a depression of their south­ ward parts, and, except at their outlets, their waters must now stand at

* F. A. Forei : Le Leman, Monographie Limnologiqne, pp. 175-179.

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(i78 TAYLOR----POST-GLACIAL CHANGES IN ITALIAN AND SWISS LAKES

relatively higher levels than formerly ; but this kind of change is not so evident on ordinary examination, and its amount is sometimes hard to determine. The only one of these lakes examined was lake Lucerne. No evidence of a higher lake level was found, but, on the other hand, no very certain evidences of submergence were seen. Descriptions of lakes Zurich and Constance indicate the same conditions as for lake Lucerne. It seems probable that these lakes were affected in the same way, and that more detailed study would show southward submergence in all of them.

Co n c l u sio n s

The observations here recorded were not sufficiently accurate to deter­ mine the exact direction of tilting for the lakes mentioned, but they seem to show a differential elevation having a generally northerly direction. In another part of his work Forel discusses the origin of lake Geneva. He rejects the glacial theory and regards the basin of the lake as merely a prolongation of the Rhone valley, made when the mountains stood 1,000 meters higher than now. The lake he regards as having origi­ nated by a more recent general depression of the mountains to this amount, while the Swiss plain remained relatively unaffected. It would seem natural for one holding Forel’s view to explain the Italian lake basins in the same way. If this be done, then the deformation here recorded is due to a resilience of later date than Forel’s depression) and would seem to indicate that both depression and resilience were confined mainly to the central region of the Alps. To the present writer glacial agencies seem to have been, perhaps not the sole, but at least the main, cause of all the deeper Alpine lake basins. On this view Forel’s recent depression of 1,000 meters would be eliminated, and a cause of tilting must be sought elsewhere. In “ Das Antlitz der Erde ” Suess holds that the Adriatic and Black Sea depressions are of recent origin. The shores of the gulf of Genoa suggest a similarly recent depression. A study of the slopes of the valley and the floor of the shows that if the modern delta of the Po, reaching 40 or 50 miles westward from the present mouth of the river, be eliminated, the slope from to the head of the delta and thence on the Adriatic sea floor out to the bathymetrical line of 100 meters is so nearly uniform as to strongly suggest a subaerial or fluvial origin for the whole of it. If the recent movement of depression sup­ posed by Suess took place, it may have produced as one of its mani­ festations the observed deformation of the Italian and Swiss lakes. This seems to the writer a more likely cause than that suggested along the lines laid down by Forel, and it agrees with the fact that the Swiss lakes appear to have been tilted in the same general direction as the Italian. Downloaded from gsabulletin.gsapubs.org on August 2, 2015 Geological Society of America Bulletin

Post-glacial change of attitude in the Italian and Swiss lakes

FRANK BURSLEY TAYLOR

Geological Society of America Bulletin 1904;15, no. 1;369-378 doi: 10.1130/GSAB-15-369

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© 1904 Geological Society of America Downloaded from gsabulletin.gsapubs.org on August 2, 2015

Notes

© 1904 Geological Society of America