Coastal Sediments of the Southeastern Shores of the Ría De Arosa (Galicia, NW Spain)

LEIDSE Vol. 15-10-1970 GEOLOGISCHE MEDEDELINGEN, 37, pp. 147—167, separately published

Coastal sediments of the southeastern shores of the Ría de Arosa (Galicia, NW Spain)

J.D. de Jong andH.H. Poortman

Abstract

The beaches of the southeastern shore of the Ría de Arosa in small and enclosed between are general rocky capes; especially

those along the inner ria are not subjected to strong wave action. Nonetheless, the beach sands are often coarse-grained, being

partially derived from the coarse-grained intrusive granites exposed in low cliffs along the major part of the shore. Generally,

the sands along the low-water mark are finer than the better-sorted sands along the high-water mark, since the backwash is not

able the In derived from left the to transport coarse grains. some cases, however, granules weathering granite are behind at

low-water mark because the weak them the beach. waves are too to carry higher up on In protected embayments tidal flats

whose sediments much finer than those of the originated are grained beaches, although they are relatively coarse-grained as

compared with tidal-flat sediments in general. At some places there is a supply of pebbles deriving from colluvium and from

raised beaches.

The heavy minerals partially reflect the composition of the basement rocks exposed in low cliffs (epidote association, tourmaline

association). The hornblende association is supplied by the two main rivers and is found near their mouths, but the hornblende

decreases due The the have content rapidly to weathering. metamorphic association, occupying largest area, may to some extent

been colluvium from xenoliths in the but supplied by (derived granite), a greater proportion was probably supplied by rivers

(Ulla and small rivers on the opposite shore), which deposited sediments on the ria bottom (as shown by seismic profiles), from

where it was washed ashore during the late-glacial rise of the sea level.

The quartz/feldspar ratios are highest where the beaches are most exposed.

Sumario

Sedimentos litorales del borde suroriental de la Ría de Arosa (Galicia)

Las del borde suroriental de la Ría de Arosa son en su encerradas playas mayoría pequeñas y están entre cabos rocosos. Las

situadas lo de la ría interior la fuerte acción las olas. No las de que estan a largo no están expuestas a de obstante, arenas las

son frecuentementede siendo derivadas de intrusivos de playas grano grueso, parcialmente granitos grano grueso expuestos en

lo de la del borde de la ría. las lo de la línea de escarpas bajas a largo mayor parte Generalmente, arenas a largo marea baja

son más finas las clasificadas a lo de la línea de debido el “backwash” de que mejor largo mareaalta, a que no es capaz trans-

los En los derivados de de la línea de portar granos gruesos. algunos casos, granulos granito alterado son dejados detrás marea

las olas son demasiado débiles más altas de la En bahías baja ya que para transportarlos a partes playa. protegidas se originaron

de las cuales los sedimentos de más fino plataformas marea en son grano que en otras partes, aunque grueso en comparación los sedimentos de de con plataforma marea en general. En algunos casos las playas se suministran de cantos provenientes de

coluvio de y terrazas costeras.

Los minerales la de las de basamento de pesados reflejan parcialmente composición rocas expuestas en escarpas bajas (asociación

asociación de La asociación de hornblenda es suministrada los dos ríos se epidota, turmalina). por principales y encuentra cerca

de sus el contenido de hornblenda decrece desembocaduras, pero la rápidamente a causa de la alteración. La asociación meta-

el area mas ser en suministrada coluvio de xenolitos el mórfica, que ocupa grande, puede parte por (derivado en granito), pero

ha sido suministrada ríos ríos de la los cuales en gran parte seguramente por (Ulla y pequeños costa opuesta) depositaron

sedimentos en el fondo de la ría es demonstrado del fue arrastrado la durante la (como por perfiles sísmicos), que a playa última elevación del nivel del mar.

Los altos más las coeficientes cuarzo/feldespato son mas cuanto expuestas son playas.

Contents

Introduction 148 Cliffs 150

Tidal flats 150 Morphology 148

Granulometric 151 General features 148 composition

Geology of the hinterland 149 Remarks on methodology 151

The beaches 149 Grain-size characteristics of the beaches 152

Beach profiles 149 Grain sizes at the high-water and low-water marks 154

* Department of Geology, Leiden State University (The Netherlands) 148 J. D. de Jong & H. H. Poortman

of related Grain-size composition the beaches as to c. Playa de las Sinas sub-association 162

that of the cliffs 156 d. Jidoiro sub-association 162

characteristics of the tidal flats 157 Grain-size .... El Grove sub-association e. 163

Eolian sands 157 Hornblende association 163

Tourmaline association 163 Morphometry 157 Epidote association 164 Roundness of sand grains 157 Provenance of the heavy minerals 164 Roundness of pebbles 158

Mineralogy: Light minerals 165 Mineralogy: Heavy minerals 159 Quartz/feldspar ratio 165 Introduction 159 Carbonate content 165

Fractionated heavy-mineral analysis 159

Heavy-mineral associations 159 Conclusions 167

Metamorphic association 161 References 167

Isla de Arosa sub-association a. 161

b. El Vado sub-association 162 Plates I-IV (in backflap)

INTRODUCTION land, sloping gradually towards the ria, and the

inundated part of the ria is formed by an indented Among the graduate students of Leiden University coastline. this coastline of Along outcrops intrusive who carried out fieldwork as part of the Ría de Arosa and metamorphic rocks of the basement complex, investigation during the summer monthsof 1962, 1963, forming small capes, alternate with sandy beaches and the author of the article 1964, junior present whereas in (Fig. 1), some embayments or otherwise (H.H.P.) worked on the beaches along the left (i.e. protected coastal parts, tidal flats have developed. southeastern) shore of the ria (Poortman, internal The middle section of the coastline is more or less Mrs. Liesbeth Bisdom studied the report, 1969), protected by a large island, the Isla de Arosa, con- coastal sediments and the morphology of the El Grove sisting of granitic hills connected by tombolcs covered Mr. Wiersma Peninsula (internal report, 1967), D.J. by windblown sand. The island shows a girdle of worked in the Ensenada del Grove, the tidal-flat area beaches all around, wider along the eastern than in the embayment north of the tombolo connecting along the western shore. Along the W coast some small the El Grove Peninsula with the mainland (internal tidal flats with salt-marsh vegetation have also devel- and Mr. A. L. E. Amstelveen worked report, 1967), oped. At the E side, facing the mainland, a cuspate on weathering products of basement rocks east of the spit has developed (Phot. 1), most probably as a Ensenada del Grove (internal report, 1965). Their result of the meeting of material driven southward by the data for the reports provided present paper, northern winds in and material strong summer, by compiled by the senior author who directed and driven northward by strong southern winds in winter, supervised most of their works. although the tides may also contribute to this effect In this paper frequent reference will be made to the (Diez Taboada, 1967). Numerous small islands, C. S. H. M. who papers by E. Arps & Kluyver, have beaches and mostly granitic tors, no are simply described the opposite (northwestern) shore of the ria bare rocks washed by the waves of the ria. W. S. (this volume, p. 135—145), Dr. Koldijk, who Before the outer section of the southeastern ria coast studied the bottomsediments 77 — (this volume, p. 134), lies the still larger Peninsula del Grove, a former and Dr. E. B. A. Bisdom, who worked on weathering mainland the island, now connected with the by phenomena in the area adjoining the southeastern tomboloof La Lanzada. The S coast of this peninsula shore de Arosa of the Ría (this volume, p. 33—67). and the S shore of the tombolo face the Atlantic The authors are indebted to Professor A. J. Pannekoek and Ocean and are subjected to stronger wave wind for his critical reading of and comments on the first action than the coasts of the inner ria. Mr. M. draft of this paper and to L. Brittijn for his In the large embayment N of the tombolo, the able draughtmanship. Ensenada del Grove, which is protected on three sides

(by the tombolo, the E coast of the peninsula, and the MORPHOLOGY kilo- mainland), a tidal-flat area of about 12 square Generalfeatures this be dealt with in metres has developed ; area will a

The between the hinter- section of this boundary weakly undulating separate paper. Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 149

Phot. 1. El Vado foreland towards cuspate seen the Isla

de Arosa.

mostly mica schists, are also found S of the ria, along the open ocean. The Caldas granite is, however, not homogeneous, and

contains numerous schist-like xenoliths. Diez Taboada

(1965, 1967) mentions the local occurrence of meta-

morphic rocks from the Isla de Arosa, which was

confirmed by one of us (H.H.P.), but whether these

are xenoliths or larger bodies could not be determined.

The beaches

The beaches consist mainly of sand; locally, small

concentrations of pebbles are found, but shingle

beaches are absent. Most of the beaches are not more

than some hundreds of metres long, the longest in this beach between area being the 2 km long Villagarcia and Carril, the Playa de Compostela, and the 3 km

long beach of the tombolo of La Lanzada. The width of the beaches exceeds the hardly ever 100 m, average

being about 50 m.

Whether not beach has to or a developed appears

depend partially on the angle between the most

important joint direction in the basement rocks and

the coastline: where the joints cut the coastline more

or less perpendicularly and loose material is available, beaches between where the arcuate capes occur;

angle with the major joints is small, as is the case for

part of the coast between Villanueva and Cambados,

the shore consists of parallel ridges of bedrock and no

beach has developed.

Small tombolos occur where islets of bedrock, mostly

granitic tors, are found near the shore in a sandy, 1. Distribution ofbeaches the southeastern shore Fig. along shallow area, and where the deposition of sand is not of the Ría de Arosa. scale Approximate 1: 150,000. the of tidal prevented by occurrence strong currents.

Geology of the hinterland Beach profiles

the the SE the The of the In contrast to opposite shore, coast of relatively coarse-grained upper parts ria is situated over nearly its whole length in only one beaches dip about 10° near high-water and spring-tide the type of rock, the coarse-grained, intrusive, late- marks, whereas dip near the relatively fine-grained hercynian Caldas de Reyes granite (Floor, 1968). low-water mark is generally less than 2°. between Only at the northern end, N of Villagarcía de Arosa, Each high tide spring tide and neap tide do other the i.e. leaves behind some rock-types occur along coast, a relatively coarse-grained ridge at a two-mica granites and some gneisses. Other rocks, somewhat lower level than the preceding one. During 150 J. D. de Jong & H. H. Poortman

Where the following week, between neap tide and spring cliffs are absent, a berm has usually developed tide, when the daily high-water level lies some 15 cm at the spring-tide level; behind the berm there is

higher each day, the bulk of the newly deposited occasionally a lagoon 20 to 50 m long. material is carried landward and is Wind-blown sands in the finally deposited occur locally coastal area,

the berm tide. from the beach and thin on at spring deriving directly forming a

The tides in the ria follow the calculated cycles cover on the cliffs and the hinterland. Along the S Storm floods known the of the El Grove the closely. are not along coasts, coast peninsula, on tombolo, and

contact as shown by the simple fact that houses have been on mainland areas in direct with the Atlantic the level and the built very near high-water no measures Ocean, dunes have formed. In south-eastern part taken. from shallow of the Peninsula del wind-blown sands to prevent flooding are Apart some Grove, have embayments the ria is relatively deep (more than 50 m been found at heights up to about 100 m . in the central channel, less than 30 m in the inner ria) and water driven ashore by the wind has plenty of Tidalflats holds of tidal-flat found room to flow back. This also for the action The largest area is in the Ensenada since del Grove the off-shore winds, upwelling currents can directly (Fig. 2), deep embayment enclosed by

for a loss of water. the the El Grove and the compensate mainland, peninsula, tom- of the ditches bolo the with the To study the internal structure beaches, connecting peninsula mainland; this about 20—50 several In contains island and cm deep were dug at places. area one larger (La Toja) many these ditches a bedding lying parallel to the surface or smaller ones. of 5—15° observed. The tidal which tidal dipping seaward at an angle was flats, are separated by channels,

However, because of intensive digging for shellfish occupy strips 50—350 m wide along the coasts and the by the local population, the original bedding has been islands. The large channels in the lower tidal flats disturbed At locations black have widths about 50 and of 3 at many places. some a up to m depths to 4 m, humic is in the ria the smaller in the 2 —10 wide layer exposed beaches, probably ones higher parts are m and often than half representing a soil dating from a time when the sea- not more a metre deep. level still below the The Ensenada sediments show of was present one. a large variety grain

sizes. Sand is found in the lower parts and channels of

the tidal flat and on beaches; silt, dark in colour Cliffs beneath the surface and ofH in the smelling 2S, occurs

In several places the mainland is bounded at the ria higher parts. In these higher parts of the tidal flats,

Most Arenicola — with its characteristic small side by a low abrasion cliff. of the cliffs show heaps of

weathered at the surface — is Gravel is strongly intrusive or metamorphic bedrock, excrements common. whereas occasionally a layer of well-rounded pebbles found near remnants of terraces, near outlets of small overlain by head (colluvium) occurs (Phot. 2). Some rivers, and in some deep channels. low cliffs (2 —3 m) consist wholly of colluvium with The tidal-flatsequence shows the usual fining-upwards which alternation of pebbles, derived from Pleistocene beach deposits. picture, in an silty and clayey laminae with Wave sandy layers may occur. ripples the occur in sandy layers. Large numbers of pele- and cypods gastropods in layers 5 to 15 cm thick lying

about 10 to 20 cm deep are common. The occurrence

of shells at the surface has strongly decreased because

of the local habit of collecting shellfish, which has

also led to the obliteration of the original sedimentary

structures.

The subsoil of the Ensenada del Grove consists of

1 non-marine kaolinitic clay . In the eastern part and the on adjoining mainland, where it is worked in clay pits, the intercalation of sandy and gravelly

layers indicates a sedimentary origin. This clay prob-

ably forms part of a bedded slope deposit washed down from the neighbouring hills during the Pleisto-

cene (Nonn, 1964, 1966; Pannekoek, 1966). In the of the of fresh western part Ensenada the occurrence and only partially weathered feldspar and biotite

points to weathering in situ of the underlying granite minor or to very transport. Sedimentary structures

were not observed here. The tidal-flat area is disturbed

1 In the adjoining salt marshes illite and gibbsite are the

Phot. 2. Ancient beach overlain collu- main from kaolinite pebbly deposit, by clay minerals, apart some (report by vium, on top ofbedrock. Isla de Arosa. Dr. P. Hartman,Department ofGeology, LeidenUniversity). Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 151

2. Sketch of the tidal flats of the Ensenada del Grove, modified after Wiersma Fig. map D.J.

(1967). The boundary lines are approximate. Scale about 1:40,000.

not only by digging for shellfish but also by digging GRANULOMETRIC COMPOSITION for the kaolinite underlying the tidal deposits, and by pollution with debris from neighbouring brick works. Remarks on methodology various the mainland and islands In places on some the tidal flats are bounded by low cliffs in terrace The grain-size composition of about 100 samples was deposits. These cliffs have supplied pebbles to the determined. Because different students worked on this adjoining beaches. The origin of these pebble deposits project in different years, the analyses were not will be discussed in the roundness. to uniform the chapter on pebble performed according a system over

Low cliffs are also foundwhere the sea has attacked the whole area. The grain-size analyses of the samples the Grove salt marshes occurring locally on the landward side of from the shores of the ria north of El Peninsula and from the northern ofthe Ensenada the tidal flats. From these places mud pebbles have part been supplied to the beaches and flats. del Grove were done on the whole sediment, i.e. the whereas the labora- The Ensenada del Grove is only a few centuries old; including gravel-size fractions, the sediments of the Ensenada could of the sediments and south of the fine-grained only tory analyses along of the Ensenada be deposited after the tombolo had formed. peninsula and of the inner part much found the concerned the material finer than 2 the Another but smallertidal-flat area is in only mm, deep embayment of Villanueva. Strong tidal currents percentage of the granule-pebble fraction of most beenestimated in the field. This has occur in the narrow channel connecting this area samples having no with the ofthe the formation effect for such open water ria, leading to fine-grained sediments, as occur locally of asymmetric megaripples. in the Ensenada, since they lack gravel-sized material 152 J. D. de Jong & H. H. Poortman

Fig. 3. Grain-size composition of four samples of the tidal flats of the Ensenada del Grove.

Samples 84, 98: fine-grained; samples 94, 97: coarse-grained (granules not shown).

(Fig. 3, samples 98 and 84), but when there is a Sands from small arcuate beaches between protruding substantial of and sometimes and well proportion granule pebble-sized tors capes are even fine-grained the cumulative shows cut-offtail sorted From beaches inother grains curve a (Fig. 3, (Fig. 5, sample 11). parts This of samples 94 and 97). also holds for the sediments of the world it is known that a longshore movement from El Grove the winds result in of Peninsula, grain-size analyses of water by may a longshore transport which also limited the sediment. Sand were to sand-silt-clay-sized grains may even bypass rocky points fractions, resulting in cumulative frequency curves and "continue along the shore provided that the water with tails the fine and ends off the is shore" at coarse (Figs. 4, samples points not very deep near (Shepard,

142, 143) and curves with a cut-off tail at the coarse 1963, p. 182; Ingle, 1966, p. 123). The great differences side (Fig. 4, samples 104, 107, 129, 136 and 139). For in grain size of neighbouring beaches show that in the such cases, the laboratory boundary of 1700 micron is Ría de Arosa such longshore currents are absent or not natural the of minor On beaches the a one. Consequently, parameters only importance. protected

derived from the grain-size analyses from the El Grove beach material is only affected by transversal waves Peninsula and the Ensenada and there will be lateral and are not directly compa- hardly any transport rable with those from the other ria sediments. mixing of material from neighbouring beaches. This conclusion also follows from the heavy-mineral

and is in with the of composition, agreement findings & the shore of the ria. Grain-size characteristics of the beaches Arps Kluyver ( 1969) on opposite Where surf beaches are most exposed to action, as on The ria beach sands are in general coarse-grained, the southern shores of Isla de Arosa, the grain-size

M-values of 1000—2000 micron being fairly common, composition has its modal class in the 850—1100 valuesof 2000 micron and micron size On the although —3000 even larger grade. protected beaches, along 420— occur as well. high-water mark, the 300—420 micron and/or In where various cases, as could be expected, beaches open 600 micron size grades are predominant; to the winds and thus beaches the prevailing to stronger wave are moderately open to prevailing winds, action, consist of coarser sand than protected beaches. 600—800 micron size grades reach the highest per- Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 153

Fig. 4. Grain-size composition of beaches of the Peninsula del Grove (samples 142, 143: fine-grained;samples 129, 136, 139: coarse-grained, granules not shown)

and from the island of La Toja (samples 104, 107).

Grain-size of beach de las The shaded column all size Fig. 5. composition a protected (Playa Sinas). represents

grades coarser than 3400 microns. 154 J. D. de Jong & H. H. Poortman

the of Protected beach centages. However, grain-size composition samples from the Lanzada beach, which is exposed to (S of Umia) the ocean swell, do not follow this rule (samples 142 Md and 143 in Fig. 4) : the average of five samples locality the mark is with Md along high-water (HWM) 373, an (Plate I) Qi O. So average sorting value of 1.46, and for five samples 63 1210 575 335 taken along the low-water mark (LWM) these values LWM 1.90 HWM 2710 1380 685 1.97 are 284 and 1.70 respectively. (tombolo) Table I shows some parameters of beach sediments of 66 LWM 1220 180 105 3.50 de las which the Playa Sinas, was exposed to strong HWM 2440 1360 740 1.82 northern winds at the time of sampling, and from the 69 LWM 520 185 143 1.90 beaches S of the mouth of the Umia River, protected HWM 2125 1385 885 1.55 by islands, where wave-action and depth are limited and the content of the fine size grades is larger.

TABLE I Grain sizes at the high-water and low-water marks

Open beach all the beaches taken from the For samples were top (Playa de las Sinas) LWM. layer near both the HWM and the Comparison

of the results of the analyses of the locality average grain-size HWM- and from the inner-ria beaches, (Plate IV) a. Md Qa So LWM-samples demonstrated the shows as by frequency curves (Fig. 6), between the 12 LWM 4450 2020 530 2.90 that there is hardly any difference amounts 15 HWM 4000 1600 860 2.15 that 75 micron of grains > 1000 micron, grains < are 17 LWM 2400 640 215 3.34 virtually absent, but that grains of 300—1000 micron 1650 23 HWM 4100 590 2.63 the occur on the average 7% more frequently at 28 LWM 3750 1380 350 3.26 HWM than at the LWM, whereas the percentages of 31 HWM 3300 1850 1120 1.71 75—300 micron grains are on the average 9% higher

mark sediments beaches of the Fig. 6. Average grain-size composition of high-water mark and low-water on than 3400 microns. inner ria. The shaded column represents all size grades coarser Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 155

Fig. 7. Triangle diagram of grain sizes of high-water mark

and low-water mark sediments ofthe inner ria.

at the LWM than at the HWM. A triangular diagram landward by the surf, whereas grains < 300 micron HWM- LWM. (Fig. 7) also shows the difference between the are washed back towards the These latter

which located toward the 300— washed farther down into samples, are mostly grains are not deep water,

1700 micron corner, and the LWM-samples, which which implies that also the relatively fine-grained sand found all the This bed are practically over triangle. grains are not transported in suspension but as difference may be explained by the fact that all sand load. grains, including those > 300 micron, are transported The differences in grain size and in sorting between

8. of mark and low-water mark sediments of beach 158 Fig. Grain-size composition high-water open (samples and 50 The shaded all size 3400 h, 1) protected beach (samples h, 1). column represents grades coarser than microns. 156 J. D. de Jong & H. H. Poortman

of the beach show bimodal HWM- and LWM-sands is shown by samples from samples a grain-size beaches 8, 158 1 and 158 distribution with to sand and various open (Fig. samples h) respect granule-size and from a protected beach, where the differentiation grades; the sand fraction itself, however, is mostly in the sands is less marked : Fig. 8, samples 50 1 and rather well sorted, as is shown by sample 11, a beach

50 h. Especially on the protected beaches S of the sand at the LWM taken from the central part of an of Umia River, Qg-values of LWM-samples show much arcuate beach between tors capes (Fig. 5). Sorting lower values than those ofHWM samples (Table I). sand is also moderate to good (1.21 —1.52 at the HWM, HW\f-and Great differences between LWM-samples 1.33—1.59 at the LWM) in sands free of coarse occur in the beach samples from La Toja (Fig. 9). The material ( > 2000 micron) along the La Lanzada

coast. FiWM-samples, as far as the material finer than beaches, i.e. at the ocean above the 2000 micron is concerned, all show a reasonable degree Where granite tors barely rise LWM, whereas the from of the of sorting, LWM-samples vary granules disintegrated granite are supplied near reasonably sorted to definitely badly sorted grain-size LWM. If wave action is weak, the beach samples taken the LWM much than those of the compositions. at are coarser

Fig. 9. Grain-size composition of high-water mark and low-water mark sands of the island of La Toja.

Grain-size composition of the beaches as related to that of the HWM. Examples of this situation are found on the

cliffs protected coastal section between Villanueva and Cambados :

Where beaches bounded the are by cliffs, grain-size TABLE II

composition is strongly dependent on that of the cliffs. Locality If these cliffs consist mainly of terrace material, their (Plate I) Q.1 Md Qs So shows resemblance granulometric composition a strong from the fact that in the to recent beaches, apart 49» HWM 1410 1110 885 1.26 found terraces, material smaller than 100 micron is in LWM 3700 1410 515 2.24

54 HWM 860 500 a higher proportion than on the beaches. If, however, 1320 1.62 the cliffs LWM 4000 1720 830 2.19 consist of weathered coarse-grained granite, a 55 HWM 915 685 485 1.37 certain supply of granules reaches the beach. Since LWM 5100 1610 510 3.16 these moved granules are not by ordinary waves, most Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 157

These show that these beaches with 25 <2 So figures clearly on marsh, per cent micron, = 3.3, and

not granules are moved by the waves and that only the Md =32 micron belongs to the worst sorted material sand fraction is washed and carried the in this out up to area. HWM. Accordingly, the sands are better sorted along Sands from a shoal in the Ensenada del Grove tidal the HWM the LWM. than along flats show a granulometric composition devoid of

The opposite is found on the cuspate foreland of El grains > 210 micron. which and Vado, is exposed to winds surf (Phot. 1). Three samples from a boring in the Ensenada del These show bimodal mixture of Grove of samples a granules reaching a depth 70 cm, have So-values of and coarse sand. Waves wash the spit, and material 2.3, 4.3, and 3.8, with Md-values of 1190, 150, 27 is the southern when the deposited on slope wind is micron from bottom to top, thus showing the fining- from the N and on the northern slope when the wind upwards characteristic of tidal-flat deposits. is from The found the the S. coarsest material is at Since the intertidal zone is flanked by a coastline

HWM. behind which granitic bedrock with a moderate

relief is exposed, the tidal-flat sediments of the

Ensenada del Grove and of other embayments in the TABLE III DeVries Klein's area belong to (1963) Fundy type. They differ texturally from those of the Dutch Wadden the Ensenada Qi M o. So Sea, being coarse grained in in contrast

to the fine-grained sediments of the Wadden Sea. This El Vado HWM 9500 3980 700 divergence is accounted for by the differences in LWM(S) 3700 700 270 3.70 coastal geology and geomorphology (DeVries Klein & LWM(N) 1850 430 215 2.92 Sanders, 1964).

Eolian sands

That the grain size is also dependent on other factors will be of eolian sand derived is shown by the composition along LWM and HWM Only one example given

from a beach the Isla de Arosa. The on the small sandy island of Jidoiro Arenoso. The coarse-grained on

shows that the wind out of a accumulation beaches show a bimodalsediment at the analysis picks grains very and that the dune LWM, whereas at the HWM the granule fraction is specific narrow grain-size range, so sand is much finer and much better sorted missing and a well sorted coarse sand is found. In all grained than the beach. probability, the beach sands are in this case derived adjoining from eolian sands in cliffs on this small island.

TABLE V

TABLE IV

Locality 0.1 M Q.3 So

Q.! M Qs So

150 HWM 2500 1660 390 2.53

320 2.35 Jidoiro LWM 1785 885 152 352 1.35 Arenoso LWM 3050 1900 325 3.06 dune 460 255

HWM 520 425 345 1.23

HWM 500 372 282 1.32

The frequency graph shows the difference between

these samples very clearly (Fig. 10).

Grain-size characteristics of the tidal flats (Ensenada del

Grove)

MORPHOMETRY The sédimentsofthe Ensenadadel Grove, i.e. the tidal- flat area (Fig. 2), show a wide variety of grain sizes; Roundness ofsand grains the kind of sediment is the of sub- dependent on type environment in the embayment. Fine-grained sand is For 18 samples from different localities, roundness found in the and sand in the values determined of 4 size of higher coarse-grained were grades quartz lower in the channels. Gravel is and 1700— parts, especially grains: 50—300, 300—850, 850—1700, found 4080 microns. mainly near scarps of terraces, as winnowed relict sediments, and in some deep channels, e.g. In the fine-sand size grade (50 —300 micron), apart island from with all betweenthe ofLa Toja and El Grove Peninsula. from one sample a cliff angular grains,

The best-sorted sands are those from a sandy beach samples showed subangular grains, the majority the with So = 1.4andMd= 1100 micron, and from channel toward angular side. The coarse-sand size grade with So and and Md-values of —860 is the deposits =1.4 1.6 290 (300 micron) subangular; very coarse- and 700 micron, respectively. Silt from the salt sand size grade (850—1700 micron) is subangular at 158 J. D. de Jong & H. H. Poortman

Fig. 10. Grain-size composition of beach samples (150 h, 1) and dune sand (152) from the Isla de Arosa. The

shaded column represents all size grades coarser than 3400 microns.

found the beaches where 15 and subrounded, toward the subangular side, at 3 Quartz pebbles are on only the latter bounded their landward side stations ; granules of 1700—4080 micron show similar are on by roundness values. These degrees of roundness indicate abrasion cliffs containing pebbles (Phot. 2). These the absence of of abrasion. in the cliffs either older any appreciable amount pebble deposits represent marine colluvium and older Along the Lanzada beaches, however, the rho-values, terraces or slope deposits determined for grains of 500—1000 micron, must be partially deriving from older marine terraces. classified as subrounded and rounded. The dune sands The highest (fluvio-) marine terrace in the area lies at about the mouth of the Umia along this coast, as well as the eolian sands on El Grove a height of 30 m near

Peninsula, even consist of rounded and well-rounded River (Nonn, 1958; Bisdom, 1967), but is not in direct grains. contact with the beach. Many marine terraces are These observations situated 12—15 and downward into permit a clear differentiation at m grade an between : extensive level only a few metres above the present considered (a) subangular beach sands, as derived from weathering sea level (Nonn, 1958). This level is to of intrusive and the level metamorphic rocks; represent Epi-Monastirian (last interglacial) The sub- (b) subrounded and rounded beach-sand grains from and is exposed in many of the low cliffs. the of the bedded high-energy beach of La Lanzada; angular quartz pebbles (torrential) rounded wind-blown sands which derived from (c) are slope deposits near Noalla, too, were partially re- this form beach. worked by the waves at the same time and now

The roundness value of probably wind-blown sands in low marine terraces around the Ensenada del Grove the ria Isla de Arosa area itself, as one sample on (Fig- 2). and shows, hardly differsfrom thatof the beach sands. The quartz pebbles of the terraces at 12—15 m

lower down are probably derived from colluvium. the Bisdom (1967) mentions that many ofthe slopes in Roundness of pebbles hinterland are covered by colluvium containing For 45 the roundness of 50 veins in the localities, quartz pebbles angular quartz pebbles supplied by quartz in from 16 120 It have of these varying length to mm, was measured; granite. must been some quartz from these readings, Cailleux'sroundness index 2r/L X fragments that underwent marine reworking, most

1000 was calculated. recently during the last interglacial. Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 159

would the the ofthe and that Consequently, we expect pebble deposits heavy-mineral compositions samples different exposed in the cliffs to consist ofmixtures ofangular to areas with heavy-mineral associations can be this what the ria beaches. there well-rounded pebbles, and is we actually distinguished along Moreover, find in various cliff sections (Table VI, Fig. 12). The is often a marked difference in composition between

1 to 384 LWM- and medium roundness values vary from 206 . HWM-samples. Theobserved differencesin heavy-mineral composition

could result from differences in provenance but could TABLE VI also be due to differences in grain-size of the samples.

It should be kept in mind that counts of 100 grains in Locality Roundness size include the 50—500 micron range grains of

differentsizes but each grain counts as one, irrespective Beach Playa de las Sinas 302 of its size ; for instance, a small grain of zircon and a Beach Playa de las Sinas 354 of tourmaline both counted Average for beaches 339 large grain are as one grain. To find out whether the differences in heavy- CliffPlaya de las Sinas 294 i.e. mineral composition are primary, dependent on Cliff at location 62 345

the or result from the grain size of the Average for cliff sections 292 provenance, sample, the heavy-mineral composition of several size

grades of 34 samples was determined.

considerable action be Beaches exposed to wave can expected to show higher roundness values, since the Fractionated heavy-mineral analysis more angular fragments from the cliffs undergo the beach. various beach As for the grain-size analyses, the fractions chosen for further rounding on Indeed,

the fractionated heavy-mineral analyses are not samples (but not all) show lower percentages of the uniform for the whole since various students least-rounded pebbles. The medium roundness values area, worked on this subject in different The samples for pebbles fromsuch beaches lie between284 and 390. years. from the ria and from the islands were divided Fig. 11 shows that roundness ofbeach pebbles is lower proper of the ria the inner into 5 size grades: 50—75, 75—150, 150—300, in the more protected parts : ria, 300—420, and 420—500 microns, whereas the material the area protected by the Isla de Arosa, and the from the El Grove area was divided into 4 size grades : Ensenada del Grove. Only where terraces containing the beaches also 50—105, 105—210, 210—420, and 420—500 microns. many rounded pebbles are abraded do Plate II shows only some examples from the former show higher roundness values. Along the present coast not area. Some samples did contain sufficient material of the Ensenada del Grove, where roundness indices

of one or more size to low from a grades permit heavy-mineral are generally (92, 121, 205), one sample island shows analysis. marine terrace exposed on a small a Comparison of the composition ofdifferent size grades considerably higher index (291). This implies that of shows that various minerals marine influence when the terrace one sample are more or was stronger less bound size whereas other than it is to one or two grades, sediments were reworked by waves now. distributed various when there tombolo connection minerals are more evenly over size Formerly, was not yet a the had grades. It is well-known that zircon is generally between the mainlandand El Grove, ocean a

the Ensenada Since concentrated in the fine-grained size grades. Indeed, free entrance into present area. older and sample 68 LWM clearly shows that zircon, dominant there are only a few of these relatively cliffs influence of these in the 50—75 micron size grade, is virtually absent erosion is weak in this area, the

grains coarser than 300 micron. The reverse deposits of better-rounded pebbles on the present among be said of tourmaline, which seems to be con- beach-pebble assemblages is small. may centrated in 300—500 micron size grades and only

occurs in small amounts in the finer size grades (with

MINERALOGY: HEAVY MINERALS the exception of two samples extremely rich in tourmaline). Hornblende, in locality 68, is found in Introduction only small amounts in the 50—75 micron, and in the To investigate the origin of the beach sands, the much larger quantities in coarse size grades. 50—500 micron size grade of about 150 samples from Andalusite and sillimanite, and to a lesser degree mainland and island beaches were analysed to deter- staurolite, preferentially occur in sizes larger than 150

micron. on the other hand, to be minetheir heavy-mineral composition. As for the granu- Epidote, seems more lometricinvestigations, samples from the LWMand the frequent in the medium-size grades (75—300 micron). therefore shows the of HWM as well as from cliffs bordering the beach, were Plate II susceptibility various minerals variations in size. analysed; the heavy-mineral composition was deter- to grain mined by line-counting of 100 translucent grains. in It is clear from Plate I that there is great variation Heavy-mineral associations

When the vertical columns of Plate II considered 1 are The highest roundness value, 428, was found onpebbles in de las Sinas. in other words when the ofthe a black humic layer, at the eastern end ofPlaya separately, composition 160 J. D. de Jong & H. H. Poortman

Fig. 11. Average roundness values of beach and cliff pebbles of the south-eastern shore of the Ría de Arosa. Approximate scale 1:100,000. Coastal sediments of the southeastern shores of the Rla de Arosa (Galicia, NW Spain) 161

Fig. 12. Frequency distribution ofpebble-roundness values. Upperrow: cliffs, lowerrow: beaches

of andalusite same size grades of different samples is compared, it is dominance (67 —86%), with garnet in

clear that there differences that be second but at and are great must place (3 —27%) only LWM,

primary and thus related to provenance. Sample 1 staurolite third (1 —18%). This assemblage was only LWM and HWM shows dominance of found in this coastal and additional from clearly a area, heavy-

tourmaline; samples from locations 68-tombolo, 68-1, mineral investigations will be required to determine sub-association. and 68-h have a high proportion of hornblende. In whether it represents a separate of found in the sample 54 epidote (+alterite), although not dominant, Relatively high percentages garnet were

occurs in larger quantities than in the other samples. western part of the El Grove Peninsula (localities 127,

In most of the remaining samples the metamorphic at the HWM, 131 at both the LWM and the HWM, mineral andalusite, silli- and at the group (staurolite, kyanite, 133, only HWM). is subdivided number manite) is distinctly dominant, although the propor- The M-association into a of of these minerals within the sub-associations based the relative tions metamorphic group regional on amounts

of the various minerals and on the vary. metamorphic differences admixture of other minerals. The Since the major in heavy-mineral com- some heavy composition are related to certain regions in the ria area, positions of the sub-associations are, however, largely the as is clearly shown by Plate I, various heavy- overlapping. Sometimes even a sample occurring mineral be into within the of certain sub-association shows assemblages may grouped regional region a a associations (Koldijk, 1968, p. 111). On the grounds heavy-mineral composition belonging to a different indicated above, we distinguish four main associations: sub-association. Such cases should be attributed to

particular local conditions. The areas occupied by the 1. metamorphic association (M) therefore show sub-associations only a general trend 2. hornblende association (H) in their compositions rather than sharply defined 3. tourmalineassociation (T) of The percentages certain minerals. following sub- 4. epidote association (E). associations within the M-association could be

distinguished :

a. Isla de Arosa sub-association

Metamorphic association b. El Vado sub-association

c. Playa de las Sinas sub-association

Ifzircon is excluded, more than 50 per cent of the d. Jidoiro sub-association minerals the of heavy belong to group metamorphic e. El Grove sub-association. minerals: staurolite, kyanite, andalusite, and sillimanite. Of these minerals, staurolite is generally — dominant in the ria area proper, whereas in the a. Isla de Arosa sub-association. This sub-association, the sides of the with the El Vado beaches at southern and western El together sub-association, represents M-association Grove Peninsula, andalusite is dominant. the in its purest form. In this sub- staurolite dominates the The beach sands from the rocky coast S of Playa de association, strongly over la Lanzada (localities 146, 147, and 148) show a other metamorphic minerals. 162 J. D. de Jong & H. H. Poortman

Composition range: with epidote and sometimes with zircon, the latter in

one case reaching 59%.

percentages average

Composition range: metamorphic minerals 65—81 73

staurolite 39—73 53 percentages average kyanite 2—14 6 andalusite 5—22 13 metamorphic minerals 25—85 60 sillimanite 0— 3 1 staurolite 17—54 32

kyanite 0—21 7

andalusite 1—29 11

sillimanite 1—29 10 The association is found along the E coast of the Isla

de Arosa, N of the El Vado cuspate foreland.

Fractionated analyses show (Plate II, locality 181, at The adjoining land samples (from colluviated terrace LWM and in that the the cliff) among metamorphic deposits) show similar heavy mineral compositions minerals staurolite is slightly concentrated in the (Plate IV), although some are aberrant (very high that does in coarser size grades, kyanite not occur epidote or zircon contents). grains larger than 300 micron, that andalusite is found The occurrence of zircon together with a high content mainly in the 150—420 interval, and sillimanite, of metamorphic minerals makes this sub-association although ubiquitous, is most strongly represented in susceptible to granular variations. This explains the the coarsest size grade. Epidote is found in the finer, marked difference between samples collected at the and alterite in the medium-grained size grades. HWM and the LWM. Four fractionated heavy- mineral analyses again show that staurolite is mainly

found in the size that b. El Vado sub-association. — The El Vado sub-association relatively coarse-grained grades,

shows of kyanite is only found below 420 micron, that andalusite also high percentages metamorphic minerals, is and sillimanite concentrated in the but staurolite is less dominant and therefore the other slightly strongly

coarser size grades. minerals of this group are better represented. The Playa de las Sinas sub-association should not be

confounded with Koldijk's Sinas sub-association of the

Composition to range: sands on the ria bottom, which belongs an epidote-

hornblende association.

percentages average S of Playa de las Sinas, toward Villanueva (locations

47 to 51), the sub-association becomes more mixed metamorphic minerals 56—88 78 with epidote (forming a transition to the epidote staurolite 4—64 28 association further S) or hornblende.Great differences kyanite 0—28 7 in heavy-mineral composition at different localities andalusite 7—44 30

on and the beach show that lateral of sillimanite 1—39 13 one same mixing

material is rare.

N ofPlaya de las Sinas too, the content ofmetamorphic minerals decreases and hornblende and/or epidote This is found in the El Vado assemblage cuspate increase. foreland (Plate III). There is a difference between the which dry part of the foreland, is no longer inundated

and the beach: the d. sub-association. — In this sub-association the even at storm tide, present stauro- Jidoiro which staurolite lite content in the most recent, wet part is higher than metamorphic minerals, among pre- beach: mixed with of other in the older dry part ofthe 31—64 (average 48) dominates, are a greater variety 4—34 whereas the andalusite minerals i.e. tourmaline, and horn- as against (19) per cent, garnet, epidote, percentages are 7—31 (19) and 22—44 (35) respecti- blende. and the sillimanite vely, (fibrolite variety) percentages 0—11 3—38 (5) as against (15). Roughly speaking, Composition range: staurolite takes over the leading position of andalusite and sillimanite. percentages average

The sub-association of the El Vado spit is comparable minerals 26—65 44 to Koldijk's El Vado sub-association for the sands metamorphic staurolite 19—50 29 occurring in the channel between the Isla de Arosa and kyanite 0— 6 1 the mainland. He also mentions the relatively high andalusite 3—21 11 amount of kyanite. sillimanite 0— 5 2

epidote 7—25 15

garnet 3—24 12 c. Playa de las Sinas sub-association. — As far as the meta- tourmaline 3—15 10 morphic minerals are concerned, this sub-association hornblende 0—21 8 is not unlike the preceding one but it is often mixed Coastal sediments of the southeastern shores of the Ma de Arosa (Galicia, NW Spain) 163

This sub-association is found on the islands of Jidoiro samples, which show the El Grove sub-association,

Pedregoso and Jidoiro Arenoso, and along the southern there are samples with hornblende as dominant the others which part of Isla de Arosa. As will be shown, sub- mineral, in epidote is important and a association be considered mixture of the M- few in which tourmaline zircon This may a + predominate. association with the hornblende and epidote associa- divergence may be due to the large variation in grain tions. It is identical to Koldijk's Jidoiro sub-association size found in the Ensenada del Grove. in the bottom-sediments.

Hornblende association

— The of the e. El Grove sub-association. Among the metamorphic heavy-mineral composition association is

minerals, the andalusite content is often only slightly characterized by a high hornblende content and in lower than that of staurolite, and in the S this mineral many cases a considerable amount of zircon. is often dominant over staurolite. Tourmaline is

generally an important admixture, and locally other Composition range: minerals and reach (zircon, garnet, epidote) may considerable In the northern of the percentages. part percentages average peninsula titanite is found in very high percentages hornblende 12—59 31

(up to 69%) in a limited area. Such a high content of zircon 6—52 32 one mineral reduces the mentioned average figures metamorphic minerals 4—24 14 for the other minerals. epidote 3—21 9

tourmaline 1—15 7

garnet 1—18 5 Composition range:

percentages average The heavy-mineral composition of sediments rich in

hornblendeand zircon is strongly dependent of grain metamorphic minerals 12—67 35 size. Fractionated heavy-mineral analyses demonstrate staurolite 2—39 20 that zircon, as expected, is by far the most important kyanite 0— 5 1 mineral in the finest fraction (50 —75 micron), whereas andalusite 3—33 12 hornblende takes share sillimanite 0— 4 2 a relatively large in the coarser tourmaline is found the zircon 1—67 21 fractions; among coarsest tourmaline 2—42 17 grains (Plate II, nr. 68, two samples at the LWM). epidote 1—29 14 Beach sands assigned to the hornblende association

1 — 18 7 garnet are concentrated along the coast of the mainland S of hornblende 1—19 3 the Umia River. Some samples from other beaches also this is the may belong to association, as case for samples from the beaches around Villagarcia and The distribution of the El Grove sub-association area Villajuan. Koldijk's Ulla, Rianjo, and Caldas asso- comprises the whole of the El Grove Peninsula. ciations in the bottom sediments are also characterized

from the Atlantic beaches S ofLa hornblende Proceeding Lanzada, by high percentages. The relatively high northward the the mineral hornblende in northwestern beaches along peninsula, heavy percentages some with than andalusite and assemblages start more 60% of the Isla de Arosa (Plate I, samples 151 and 153) less than whereas in the northward reflect the hornblende 20% staurolite, relatively high percentages on andalusite decreases and direction staurolite increases, the bottom ofthe ria W of the Isla de Arosa (Koldijk, that the northern shores andalusite and so along 1968: Plate 2, stations 528, 650, and 651). staurolite become and in equal many cases, as already In part of the area of the hornblende association, the mentioned, staurolite is dominant the even among Ensenada del Grove, samples showing a composition minerals. with low hornblende and metamorphic relatively high epidote or The southwestern of El Grove is free of part virtually staurolite percentages were also collected. These and which is also the S of epidote hornblende, case derive from eroded cliffs or samples may soils rather La Lanzada. The Lanzada the S coast of El beach, than from a direct supply by the Umia River. Grove and the northwestern of the Peninsula, part From Koldijk's work (1968) it is known that the

Peninsula show 30 cent combined some per epidote H-association (his Umia association) extends from the hornblende. and mouth of the Umia far to the W on the bottom of the

The E coasts of El well the Ensenada del Grove, as as ria. High hornblende contents also occur in the show the in which andalusite and Grove, picture northern part of the ria (Rianjo association) and, staurolite reach in the with in the Ulla high percentages, especially together epidote, estuary. salt-marsh and cliff samples and in those collected on the small islets. Andalusite and staurolite alternatively Tourmaline association dominate.

The heavy-mineral composition in the Ensenada del This heavy-mineral association, found only in three

Grove is very variable. In addition to the majority of samples, is characterized by the occurrence of high 164 J. D. de Jong & H. H. Poortman

of tourmaline these taken percentages ; samples were Provenance of the heavy minerals on Cortegada Island.

For some important minerals in three out of the four

Composition range: heavy-mineral associations, i.e. the E, H, and T

the be established associations, provenance can with a percentages reasonable probability. The first its two are epidote (with microcrystalline tourmaline 41—59 variety alterite) and hornblende. Since almost the metamorphic minerals 18—41 whole southeastern coast of the ria is situated in the (either staurolite or andalusite dominant) Caldas coarse-grained postkinematic granite, we shall epidote 2—12 have to consider which minerals are supplied by this hornblende 0—18 granite. According to Floor (1968), the most important

are epidote (pistacite) and hornblende, with zircon in The results of fractionated shown in analyses are the fine-grained size grade. Accordingly, the epidote

Plate II, Locality 1 at the LWM and HWM. association should be considered the association natural from the bottom of the ria Samples near Cortegada to the Caldas granite coasts. The appreciable zircon

(Koldijk, 1968) have a very different composition: content of this association is consistent with this view, of hornblende low hornblende relatively high percentages (17 —50) but the generally percentages should and epidote (10 —23), and low values of tourmaline be attributed to the low resistance of this mineral to

(2 —7). This clearly shows that the source of the weathering.

Cortegada beach samples should be sought not in the The epidote association, however, is found in only

Ulla River sediments but in the rocks of the island of the coast the section from Villanueva some parts : to itself. NW Cambados, the of the Isla de Arosa, and some

isolated points scattered along the coasts. In the

coastal section this association Epidote association remaining gives way

to other associations. This association is characterized by relatively high The hornblende association is, according to Koldijk's percentages of epidote plus alterite (the latter com- (1968) findings, typical of the Umia River, which for considered ponent to be as a microcrystalline aggregate considerable ofits lower the a part course runs through of epidote), metamorphic minerals with much stauro- Caldas granite. On the beaches the hornblende asso- lite, and occasionally much hornblende. ciation is found directly S of the mouth of the Umia

River. Recent supply must be the reason why the Composition range: hornblende is weathered here. Isolated not yet away

of hornblende contents at other localities percentages average cases high also be due local offresh hornblende. may to a supply 10—44 24 In the inner of the ria there epidote northern, part is, however, 17 áltente 2—33 another source of hornblende: the Ulla River also and alterite (epidote brings this mineral into the ria (Koldijk, 1968), together at least 25%) possibly acquiring it from basic metamorphic rocks zircon 6—50 17 Hornblende farther upstream (Floor, 1968). accumu- staurolite 3—37 14 lated in the of the ria when the level upper part sea other metamorphic minerals 3—17 9

was lower than it is and would have been washed hornblende 1—26 8 now, when the level ashore, e.g. near Villagarcia, sea rose.

The tourmaline association, as already mentioned before,

to Fractionated analysis shows some preference of is certainly oflocal origin. It is confined Cortegada staurolite for intermediate grain sizes; of andalusite, Island, which consists of two-mica granite. The that tourmaline could have been derived either from this sillimanite, and alterite for coarse grain sizes, and in fine well in sizes rock from its dikes epidote occurs as as coarse grain type or pegmatite (Floor, 1968). LWM the (Plate II, Locality 54, and HWM). The greatest problem is posed by metamorphic since this has the widest distribution The samples of the epidote association were taken on association, along the coastal section between Villanueva and Cambados the southeastern ria coast, although the geological and the northwestern of Isla de Arosa. shows no of rocks part According map outcrops metamorphic along to Koldijk (1968), the ria bottom around the latter the coasts from Carril in the N to La Lanzada beach de also contains sediments rich in in the S. Yet the hinterland of this area (Con Navio) coast may con- epidote, but along the coast S of Villanueva the tribute to the supply of metamorphic minerals to the bottom sediments are characterized by a metamorphic beaches. association. Apparently, the beach deposits of this (a) In the first place, the intrusive Caldas granite xenoliths of rocks coastal section are locally derived from the weathered contains numerous metamorphic granite and the soils exposed in the low cliffs, whereas (Bisdom, 1967). The granite is widely covered by which weathered that in theriametamorphic mineralshave been transported, colluvium, is so strongly only also have been in it. It is as is shown by the El Vado spit. quartz pebbles preserved likely Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 165

MINERALOGY: LIGHT MINERALS that also among the heavy minerals the least weather-

able components have been preserved, especially the zircon from the granite and the metamorphic minerals Quartz/feldspar ratio (Fig. 13) from the xenoliths. This colluvium, partially reworked

by waves during the interglacials, is exposed in many The light minerals were also determined in the places in the cliffs and could thus have supplied 50—500 micron size grade in 24 samples, according metamorphic minerals to the beaches. to the method described by Vogel (1965). To avoid (b) Another source ofsupply, also from the hinterland, attack or solution of the minerals, especially of the the bedded are (torrential) slope deposits S of the the feldspars, samples were washed with water only. Ensenada del Grove, which also supplied the kaolinite Bisdom (1967, p. 43) has shown that in a weathering to the Ensenada and the adjoining land. These profile in granitic rocks the feldspar content is still which deposits, originated from the metamorphic area much than the higher quartz content (Fig. 13, locality S of the Ensenada and contain high of percentages B). In the river sands the relative amounts of these staurolite and andalusite mainly (Amstelveen, 1965), minerals have already been reversed (Koldijk, 1968): continue below the and the sand fraction sea, may about 15 km from its mouth the Ulla River upstream have been washed ashore during the rise in sea shows 64% Qu and 36% Fsp, and at its mouth 71% level. Qu and 29% Fsp. At the mouth of the Umia River (c) A more important supply, however, have may these values are 61% Qu and 39% Fsp. from come the opposite shore of the ria, which consists Quartz/feldspar ratios in the coastal sediments are partially of crystalline schists. Continuous seismic highly variable although two values occur with a profiling ofthe ria bottom 1970) has shown that (Hinz, much greater frequency. This is clearly illustrated by of the ria bottom is filled part by sedimentary deposits the quartz/total feldspar ratios in the beach sands of with thickness of 30 and a up to m locally even the El Grove Peninsula :

more.

Part of this material have been the may deposited by Ulla River but itself, a large part was probably La Lanzada 3.0 supplied by tributaries from the NW (Pannekoek in S coast 6.3 Hinz, 1970). This is corroborated by the heavy-mineral W coast 3.8 composition. Part of the northwestern coastal zone NW coast 1.0 consists of andalusite-rich and schists, consequently E coast 1.0 andalusite is far the mineral in the by best-represented La Toja 1.1 NW beaches of the The outer ria. same holds for the La Toja 0.8

beaches along the SE shore of the outer ria, i.e. the western end of the Peninsula del Grove. It is therefore assumed that the andalusite-rich sands were supplied In this distributionthe values —6.3 form 3.0 one group from the NW into the ria bottom when this area was and the values around 1 another. Similar values are and when the level dry, then, sea rose were found Vado offers the again, in the ria. The cuspate spit ofEl washed ashore on both coasts of the outer ria. best of sands with values: 11 —14. examples very high More difficult to for is the dominance of account Cortegada Island and Playa de las Sinas also show staurolite the southeastern of the inner along coast ria, moderately high values (about 2.5). In other localities

since staurolite is mineral on especially usually a rare sampled for light-mineral determinations, values of the opposite coast. Staurolite, with horn- about show together 1 were found frequently. Some, however, blende and is the Ulla River epidote, supplied by valuesof¿, so that feldspar is strongly dominant; such a

(Koldijk, 1968) and thus will occur in the bedded sample was taken from beach sands apparently directly sediments the ria in the inner on bottom, especially derived from weathered basement rocks. The high From there it have been when the parts. may supplied, quartz/feldspar ratios along the La Lanzada beach, level the with less sea rose, to present beaches, albeit the S of El and the El Vado coast Grove, in cuspate hornblende due to weathering. spit, are in agreement with Diez Taboada's finding Since to the current is that the is where staurolite, according literature, (1967) quartz content higher moderately weatherable, its dominance not only in the influence sediment is „marine" on greater. beaches but also in the strongly weathered colluvium, is One of the authors perhaps surprising. present

is inclined to that staurolite is (H.H.P.) suppose Carbonate content actually less susceptible to weathering than is generally assumed. There instances where staurolite is found to the results obtained for 60 of are According samples beach carbonate in association with the least weatherable minerals: sands, percentages vary widely : along rutile, zircon, and tourmaline, and also Grimm (1957, the LWM from 0.02—28.2, along HWM from 1.3— and from p. 171) indicates that staurolite is much less weather- 15.9, in samples tombolos and the El Vado able than andalusite. This assumption would also spit from 0.3—52.0 The highest carbonate contents explain the wide distribution of staurolite in the ria were found in samples from beaches with the greatest ria the NW of the Isla area. exposure to rough water : point 166 J. D. de Jong & H. H. Poortman

Fig. 13. Quartz/feldspar ratios of the Ría de Arosa (including data from Koldijk, 1968, and Bisdom, 1967). Approximate scale 1:125,000. Coastal sediments of the southeastern shores of the Ria de Arosa (Galicia, NW Spain) 167

de Arosa, the S point of the Isla de Arosa, and the actuales de la Ría de Arosa. Bol. R. Soc. Española Hist. Nat. 361—364. Jidoiro Islands. (Geol.) 62, p. 1965. Procesos de acumulación evolutivo de las fases —, y

arenosas actuales de la Ría de Arosa. Bol. R. Soc. Es-

Hist. Nat. 129—135. pañola (Geol.) 63, p.

1967. de las arenas CONCLUSIONS —, Composición mineralógica super- ficialesactualesdelaRía deArosa-I. Bol. R. Soc. Española

317—329. Hist. Nat. (Geol.) 65, p. The beaches along the southeastern shore ofthe Ría de Galicia Floor, P., 1968. Basement rocks of western as sources differences in size and Arosa show great grain miner- for the minerals in the Ría de Arosa. Leidse Geol. Meded. from alogical composition. They vary high-energy 69—76. 37, p. beaches the ocean and some along open exposed Grimm, W. D., 1957. Stratigraphische und sediment- beaches spots, through moderately exposed to wave petrographische Untersuchungen in der Oberen Süss-

action, to protected beaches and, in embayments, even wassermolasse zwischen Inn und Rott (Niederbayern).

Beihefte Geol. 97—199. to tidal-flat areas. Jahrb. 26, p.

Hinz, K., 1970. Seismic reflection measurements with a These beaches are generally small and separated by pneumatic sound source in the Ria de Arosa (NW Spain). low often almost sub- capes, granite tors; entirely Leidse Meded. 169-184. Geol. 37, p. merged tors are also found on the beaches and in the Ingle Jr., James C, 1966. The movement of beach sand; an the is than water. Actually, present coast nothing more analysis using fluorescent grains. Developments in accidental line a subaerial from a an separating Sedimentology 5, Elsevier Publishing Company, Amster- submerged part of a uniform granite topography dam-London-New York.

(Pannekoek, 1966). Klein, G. De Vries, 1963. Bay of Fundy intertidal zone

from sediments. Sed. Petr. 844—854. A large part of the beach sand is locally derived Journ. 33, p. — the low cliffs along the beaches. Where weathered & Sanders, J. E., 1964. Comparison of sediments from Bay of Fundy and Dutch Wadden Sea tidal flats. Journ. granite is exposed on the shore it supplies granules to Sed. Petr. 34, 18—24. for the and p. the beaches, which accounts coarseness Koldijk, W. S., 1968. Bottom sediments of the Ría de Arosa angularity ofmany ofthebeach deposits. But colluvium (Galicia, NW Spain). Leidse Geol. Meded.37,p. 77—134. and older marine terrace deposits also contribute Nonn, H., 1958. Contribución al estudio de las playas material to the the terrace beaches; locally, deposits de Galicia Notas Común. Inst. Geol. antiguas (España). y have supplied well-rounded pebbles. Min. 175—193. España 50, p. The minerals show that the hinterland is not 1964. Los sedimentos de la Ría de Arosa: heavy —, antiguos the of the beach sands. conclusiones Notas Común. only source Epidote, zircon, Algunas geomorfológicas. y the hornblende derived Inst. Geol. Min. 143—156. tourmaline, and part of are España 74, p. 1966. Les régions cotièrcs de la Galice (Espagne). directly from the local bedrock, but the widely —, Étude géomorphologique. Publ. Fac. Lettres Strasb., distributed metamorphic minerals are more difficult Fondation Baulig III. 591 from hinter- pp. to account for. They may derive partially Pannekoek, A.J., 1966a . The Ria problem. Tijdschr. Kon. land colluvium, which took these minerals from the Aardr. 289—297. Ned. Genootsch., 83, p. xenoliths in the and became enriched in granite b 1966 The geomorphology of the surroundings of the —, . minerals of metamorphic through weathering epidote Ría de Arosa (Galicia, NW Spain). Leidse Geol. Meded. and hornblende. However, minerals 7—32. metamorphic 37, p. also the Ulla and its ed. were probably supplied by Shepard, Francis P., 1963. Submarine geology, sec. York-Evanston-London. northern tributaries to the bedded sediments on the Harper & Row Publishers, New

Vogel, D. E., 1965. Thin sections for determining the com- bottom of the ria (Hinz, 1970), from which they were position of the light-mineral fraction of unconsolidated gradually washed ashore during the interglacial and Geol. & 64—65. sediments. Mijnb. 44, p. postglacial rises of the sea level. beaches thus reflect of the The clearly parts compli- Internal reports, Department of Geology, Leiden State Uni- cated history of the ria. versity :

Amstelveen A. L. E., 1965. Morfologisch ensedimentologisch

de Ponte- onderzoek in het westelijk deel van provincie

vedra REFERENCES (Galicië, NW-Spanje).

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het schiereiland El Grove het eiland La Arps, C. E. S. & Kluyver, H. M., 1969. Sedimentology of petrografie van en the northwestern shores of the Ria de Arosa (NW Spain). Toja (Galicië, NW-Spanje).

Geol. Meded. H. 1969. Strandsedimenten de linker- Leidse 37, p. 135—145. Poortman, H., van eilanden Bisdom, E. B. A., 1967. Micromorphology of a weathered oever van de Ría de Arosa en de er voor liggende

granite near the Ría de Arosa (NW Spain). Leidse Geol. (Galicië, NW-Spanje).

Meded. 33—67. Wiersma, D. 1967. onderzoek van de 37, p. J., Sedimentologisch

Diez Taboada, J. M., 1964. Sedimentos costeros arenosos Ensenada del Grove (Ría de Arosa, Galicië, NW-Spanje').