The Elements of Geology

he material which the streams have worn from their beds and that dissolved by underground waters. In arid regions the winds sweep waste either into bordering oceans or into more humid

eds, dune sands, and sheets of glacial drift, mark but pauses in the process which is

estruction, and we must first take up its work in erosion before

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s there lead down valleys and ridges, carved by running water, which, if extended, would meet the water surface some way

gle. At low tide its inner margin is laid bare, but at high tide it is covered wholly, and the sea washes the base

ws that it has been cut by some agent which acts like a horizontal saw set at about sea l

ter. On the coast of Scotland the force of the blows struck by the waves of the heaviest storms has sometimes exceeded three tons to the square foot. But even a calm sea co

cliffs of solid rock. Storm waves arm themselves with the sand and gravel, the cobbles, and even th

not break and strike effective blows. They therefore erode but little until the talus fallen from the c

along its channels, holding them with a loose hand. Glacial ice grinds the stones of its ground moraine against the underlying rock with the pressure of its enormous wei

k ledges inland. It is abundantly wet with spray. Along its base the ground water of the neighboring land finds its natural outlet in springs which under mine it. Moreover, it is unprotected by any shield of talus. Fragment

that the upper portion has retreated at a more rapid rate than has the base. Whic

cks whose joints dip toward the land. Draw a diagram

vily barred doors have been burst outward by the explosive force of the air within, as it was released from pressure when a partial vacuum was formed by the refluence of the wave. Where a crevice is filled with water the entire force of the blow of the wave is transmitted by hydraulic pressure to the sides of the fissure. Thus

one may find on the breezy upland, perhaps a hundred yards and more back from the cliff's edge. In quiet weather the blowhole is a deep well; in storm it plays a fountain as the waves drive through the long tunne

, much as monuments are detached from inland escarpments by the weather; and as the sea cliff retreats, these remnant masses may be left behind as rocky islets. Thus the rock

s; while the more resistant rocks on either hand are left projecting as headlands (Fig. 139). After coves are cut back a short distance by the waves, the headlands come to protect them, as with breakw

shire, England, whose cliffs are cut in glacial drift, loses seven feet a year on the average, and since the Norman conquest a strip a mile wide, with farmsteads and villages and historic seaports, has

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nd some-what worn on their corners and edges. From this BOWLDER BEACH the smaller fragments of waste from the cliff and the fragments into which the bowlders are at last broken drift on to more sheltered places and there accumulate in a PEBBLE BEACH, made of pebbles well rounded by the wear which they have suffered. Such beaches form a mill whose raw material is constantly supplied by the cliff. The breakers of storms set it in motion to a depth of several feet, grinding the

slow one, and if we study these sand grains under a lens we may be surprised to see that, though their corners and edges have been blunted, they are yet far from the spherical form of the pebbles from which they were derived. The grains are

e sand of many beaches, derived from the rocks of adjacent cliffs or brought in by torrential streams from neighboring highlands, is dark with grains of a number of minerals softer than quartz. The white sand of othe

eavage in quartz affect th

Where waves strike a coast obliquely they drive the waste before them little by little along the shore. Thus on a north

made. But while the wave stirs the grains of sand and gravel, and for a moment lifts them from the bottom, the current carries them a step forward on their way. The curr

varying conditions, a number of distinct forms. When swept into the head of a sheltered bay it constitutes the bay-head beach. By the hi

and sinks to the bottom. The dump is gradually built to the surface as a stubby spur, pointing across the bay, and as it reaches the zone of wave action current and wave can now combine to carry shore drift along it, depositing their load continually at the point of the spur. An embankment is thus constructed

lso are able to keep open channels scoured by their ebb and flow. In such cases the most that land waste can do is to build spits and shoals, narrowing and shoaling the channel as mu

idge, termed the sand reef, separated from the land by a narrow stretch of shallow water called the LAGOON. At intervals the r

fs are but seldom interrupted by inlets as far north as Galveston Harbor. On this coast the tides are variable and exceptionally weak, being less than one foot in height, while the amount of waste swept along the shore is large. The l

m land. Where storm waves first drag bottom they erode and deepen the sea floor, and sweep in sediment as far as the line where they break

LEVATION AN

e low, sandy coast where the waves break usually upon the sand reef. To understand the origin of these two types we must know that the meeting place of sea and land is dete

elving from the land. Since the new shore line is drawn across this even surface it is simple and regular, and is bordered on the one side by shallow water gradually deepening seaward, and on the other by low land composed of material which has not yet thoroughly consolidated to firm rock. A sand reef is soon beaten up by the waves,

and reeds which have learned to grow in salt and brackish water; the marsh, laid bare only at low tide, is built a

They now wear it back, and, driving the shore line across the lagoon or meadow, cut a line of low cliffs on the mainland. Such a shore is that of Gasc

the maturity of such a coast may be long delayed. The waste from the land keeps the sea shallow offshore and constantly renews the sand reef. The energy of the waves is consumed in handling shore drift, and no energy is left for an effective attack upon the land. Indeed, with an excessive

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rregular and indented in proportion to the relief of the land and the amount of the submergence which the land has suffered. It follows up partially submerged valleys, forming bays, and bends round the

lleys penetrate the land in long, narrow bays, and rugged divides project in long, narrow land arms prolonged seaward by islands representing the high portions of their extremities. Of this exceedingly ragged shore there are said to be two thousand miles from the New Brunswick boundary as far west as Portland,-a straight-line dis

nd narrow islands, all parallel to the trend of the coast. A region of parallel mountain ranges has

from the sea, was channeled with broad, shallow valleys. The sea has invaded the valley of the trunk stream and

e land has brought the sea in over low plains of large extent, thus deeply indenting the contin

ous relief of the land, and but little on erosion by the sea. Sea cliffs and narrow benches appear where headlands and outlying i

ore minutely ragged. The irregularity of the coast, due to depression, is for a while increased by differential wave wear on harder and softer rocks. The rock bench is still narrow. Shore waste, though

e heads of the smaller bays with beaches, building spits and hooks, and tying islands with sand bars to the mainland. It bridges the larger bays with bay bars, while their length is being reduced as their inclosing promontori

nds formed by subsidence have been planed away, and when the shore line has been driven back behind the former bay heads. The sea now attacks the land most effectively along a continuous and fairly straight line of cliffs. Although the first

thither and in abrading the bench when they drag bottom upon it. Little by little the bench is deepened by tidal currents and the drag of waves; but this process is so slow that meanwhile

SE, i.e. the lowest limit of the wave's effective wear. The widened rock bench forms when uplifted a plain of marine abrasi

on be expected to differ from a

outhful rocky, coast of Britain-it would require more than ten million years to pare a strip one hundred miles wide from the margin of a continent, a time suf

orm waves can therefore always ride in to the base of the cliffs and attack them with full force; shore wa

dened indefinitely by the retreat of the cliffs at whose base it was formed, and preserved by the finer deposits laid upon it in the constantly deepening water as the land subsides. Such basal conglomerates ar