Earth features and their meaning; an introduction to geology for the student and the general reader

Front matter

CORNELL

UNIVERSITY

LIBRARY

Preface

The series of readings contained in the present volume give in somewhat expanded form the substance of a course of illustrated lectures which has now for several years been delivered each semester. at the University of Michigan. The keynote of the course may be found in the dominant characteristics of the different earth features and the geological processes which have been betrayed in the shaping of them. Such a geological examination of landscape is replete with fascinating revelations, and it lends to the study of Nature a deep meaning which cannot but enhance the enjoyment of her varied aspects.

That there is a real place for such a cultural study of geology within the University is believed to be shown by the increasing number of students who have elected the work. Even more than in former years the American travels afar by car or steamship, and the earth's surface features in all their manifold diversity are thus one after the other unrolled before him. The thousands who each year cross the Atlantic to roam over European countries may by historical, literary, or artistic studies prepare themselves to derive an exquisite pleasure as they visit places identified with past achievement of one form or another. Yet the Channel coast, the gorge of the Ehine, the glaciers of Switzerland, and the wild scenery of Norway or Scotland have each their fascinating story to tell of a history far more remote and varied. To read this history, the runic characters in which it is written must first of all be mastered ; for in every landscape there are strong individual lines of character such as the pen artist would skillfully extract for an outline sketch. Such character profiles are often many times repeated in each landscape, and in them we have a key to the historical record.

An object of the present readings has thus been to enable the student to himself pick out in each landscape these more significant lines and so read directly from Nature. In the landscapes which viii PREFACE

have been represented, the aim has been to draw as far as possible upon localities well known to travelers and likely to be visited, either because of their historical interest or their purely scenic attractions. It should thus be possible for a tourist in America or Europe to pursue his landscape studies whenever he sets out upon his travels. The better to aid him in this endeavor, some suggestions concerning the itinerary of journeys have been supplied in an appendix.

Eegarded as a textbook of geology, the present work offers some departures from existing examples. Though it has been Customary to combine in a single text historical with dynamical and structural geology, a tendency has already become apparent to treat the historical division apart from the others. Again, a desire to treat the science of geology comprehensively has led some authors into including so many subjects as to render their texts unnecessarily encyclopedic and correspondingly uninteresting to the general reader. It is the author's belief that there is a real need for a book which may be read intelligently by the general public, and it must be recognized that the beginner in the subject cannot cover the entire field by a single course of readings. The present work has, therefore, been prepared with a view to selecting for study those dominant geological processes which are best illustrated by features in northern North America and Europe. It is this desire to illustrate the readings by travels afield, which accounts for the prominence given to the subject of glaciation ; for the larger number of colleges and universities in both America and Europe are surrounded by the heavy accumulations that have resulted from former glaciations.

Emphasis has also been placed upon the dependence of the dominant geological processes of any region upon existing climatic conditions, a fact to which too little attention has generally been given. This explains the rather full treatment of desert regions, of which, in our own country particularly, much may be illustrated upon the transcontinental railway journeys.

More than in most texts the attempt has here been made to teach directly through the eye with the efl&cient aid of apt illustrations intimately interwoven with the text. For such success as has been reached in this endeavor, the author is greatly indebted to two students of the University of Michigan, — Mr. James H. Meier, who has prepared the line drawings of landscapes, and Mr. Hugh M.

PREFACE ix

Pierce, who has draughted the diagrams. Though credit has in most cases been given where illustrations have been made from another's photographs, yet especial mention should here be made of the debt to Dr. H. W. Eairbanks of Berkeley, California, whose beautiful and instructive photographs are reproduced upon many a page.

As given at the University of Michigan, the lectures reflected in the present volume are supplemented by excursions and by so much laboratory practice as is necessary to become familiar with the more common minerals and rocks, and to read intelligently the usual topographical and geological maps. In the appendices the means for carrying out such studies, in part with newly devised apparatus, have been indicated. .

The scope of the book precludes the possibility of furnishing the reader with the sources for the body of fact and theory which is presented, although much may be inferred from the names which appear beneath the illustrations, and more definite knowledge will be found in the references to literature supplied at the ends of chapters. A large amount of original and unpublished material is for a similar reason unlabeled, and it has been left for the professional geologist to detect these new strands which have been

drawn into the web.

WILLIAM HERBERT HOBBS.

Ann Arbor, Michigan, October 25, 1911.

Contents

CHAPTEE I

The Compilation of Earth History

PAea The sources of the history — Subdivisions of geology — The study of earth features and their significance — Tabular recapitulation — Geological processes not universal — Change, and not stability, the order of nature

Chapter II

The lithosphere and its envelopes — The evolution of ideas concerning the earth's figure — The oblateness of the earth — The arrangement of oceans and continents — The figure toward which the earth is tending

— Astronomical versus geodetic observations — Changes of figure during contraction of a spherical body — The earlier figures of the earth

— The continents and oceans at the close of the Paleozoic era — The flooded portions of the present continents — The floors of the hydrosphere and atmosphere — Reading references 8

CHAPTER III The Nature of the Materials in the Lithosphere

The rigid quality of our planet — Probable composition of the earth's core

— The earth a magnet —The chemical constitution of the earth's surface shell — The essential nature of crystals — The lithosphere a complex of interlocking crystals — Some properties of natural crystals, minerals — The alterations of minerals — Reading references . . 20

Chapter IV

The processes by which rocks are formed — The marks of origin — The metamorphic rocks — Characteristic textures of the igneous rooks — • The classification of rocks — Subdivisions of the sedimentary rocks

Xll CONTENTS

PAOB

— The different deposits of ocean, lake, and river — Special marks of littoral deposits — The order of deposition during a transgression of the sea — The basins of deposition of earlier ages — The deposits of the deep sea — Reading references .... ... 30

CHAPTER V Contortions of the Strata within the Zone of Flow

The zones of fracture and flow — Experiments which illustrate the fracture and flow of solid bodies — The arches and troughs of the folded strata — The elements of folds — The shapes of rock folds — The overthrust fold — Restoration of mutilated folds — The geological map and section — Measurement of the thickness of formations — The detection of plunging folds — The meaning of an unconformity — Reading references 40

CHAPTER VI The Akchitectdee of the Pkactuked SuPERSTRucrnKE

The system of the fractures — The space intervals of joints — The displacements upon joints: faults — Methods of detecting faults — The base of the geological map — The field map and the areal geological map — Laboratory models for study of geological maps — The method of preparing the map — Fold vs. fault topography — Reading references 55

Chapter VII

The Interrupted Character of Earth Movements : Earthquakes AND Seaquakes

Nature of earthquake shocks — Seaquakes and seismic sea waves — The grander and the lesser earth movements — Changes in the earth's surface during earthquakes: faults and fissures — The measure of displacement — Contraction of the earth's surface during earthquakes

— The plan of an earthquake fault — The block movements of the disturbed district — The earth blocks adjusted during the Alaskan earthquake of 1899 67

Chapter VIII

The Interrupted Character op Earth Movements : Earthquakes AND Seaquakes {concluded)

Experimental demonstration of earth movements — Derangement of water flow by earth movement — Sand or mud cones and craterlets — The earth's zones of heavy earthquake — The special lines of heavy shock

— Seismotectonio lines — The heavy shocks above loose foundations

Construction in earthquake regions — Reading references ... 81

CONTENTS xiii

CHAPTER IX The Rise op Molten Rock to the Eakth's Surface ; Volcanic

Mountains of Exudation

PAGE

Prevalent misconceptions about volcanoes — Early views concerning volcanic mountains — The birth of volcanoes — Active and extinct volcanoes— The earth's volcano belts — Arrangement of volcanic vents along fissures, and especially at their intersections — The so-called fissure eruptions — The composition and the properties of lava — The three main types of volcanic mountain — The lava dome — The basaltic lava domes of Hawaii — Lava movements within the caldron of Kilauea

— The draining of the lava caldrons — The outflow of the lava floods . 94

Chapter X

The Rise op Molten Rock to the Earth's Surface ; Volcanic Mountains of Ejected Materials

The mechanics of crater explosions — Grander volcanic eruptions of cinder cones — The eruption of Volcano in 1888 — The eruption of Taal volcano on January 30, 1911 — The materials and the structure of cinder cones — The profile lines of cinder cones — The composite cone — The caldera of composite cones — The eruption of Vesuvius in 1906

— The sequence of events within the chimney — The spine of Pel6

— The aftermath of mud flows — The dissection of volcanoes — The formation of lava reservoirs — Character profiles — Reading references 115

Chapter XI

The two contrasted processes of weathering — The r61e of the percolating water — Mechanical results of decomposition: spheroidal weathering

— Exfoliation or scaling — Dome structure in granite masses — The prying work of frost — Talus — Soil flow in the continued presence of thaw water — The splitting wedges of roots and trees — The rock mantle and its shield in the mat of vegetation — Reading references . . 149

Chapter XII

The intricate pattern of river etchings — The motive power of rivers — Old land and new land — The earlier aspects of rivers — The meshes of the river network — The upper and lower reaches of a river contrasted— The balance between degradation and aggradation — The accordance of tributary valleys — The grading of the flood plain — xiv CONTENTS

PAGE

The cycles of stream meanders — The out-ofi of the meander — Meander scars — River terraces — The delta of the river— The levee — The sections of delta deposits '■"°

CHAPTER Xni

Earth Features shaped et Rukning Water

The newly incised upland and its sharp salients —The stage of adolescence — The maturely dissected upland — The Hogarthian line of beauty — The final product of river sculpture : the peneplain — The river cross sections of successive stages — The entrenchment of meanders with renewed uplift — The valley of the rejuvenated river — The arrest of stream erosion by the more resistant rocks — The capture of one river by another — Water and wind gaps — Character profiles — Reading references

Chapter XIV

The descent within the unsaturated zone — The trunk channels of descending water — The caverns of limestones — Swallow holes and limestone sinks — The sinter deposits — The growth of stalactites — Formation of stalagmites — The Karst and its features — A desert from the destruction of forests — The ponore and the polje — The return of the water to the surface — Artesian wells — Hot springs and geysers — The deposition of siliceous sinter by plant growth — Reading references 180

CHAPTER XV Sun and Wind in the Lands of Infrequent Rains

The law of the desert — The self -registering gauge of past climates — Some characteristics of the desert waste — ■ Dry weathering : the red and brown desert varnish — The mechanical breakdown of the desert rocks — The natural sand blast — The dust carried out of the desert . . 197

Chapter XVI

The wandering dunes — The forms of dunes — The. cloudburst in the desert — The zone of the dwindling river — Erosion in and about the desert — Characteristic features of the arid lands — The war of dune and oasis — The origin of the high plains which front the Rocky Mountains — Character profiles — Reading references .... 209

CONTENTS XV

CHAPTER XVII Eepeating Pattekns in the Earth Reliep

FAQB

The ■weathering processes under control of the fracture system — The fracture control of the drainage lines — The repeating pattern in drainage networks — The dividing lines of the relief patterns : lineaments

— The composite repeating patterns of the higher orders — Reading references 223

CHAPTER XVm

The Porms carved axd molded by Waves

The motion of a water wave — Free waves and breakers — Effect of the breaking wave upon a steep, rocky shore : the notched cliff — Coves, sea arches, and stacks — The cut rock terrace — The cut and built terrace on a steep shore of loose materials — The work of the shore current — The sand beach — The shingle beach — Bar, spit, and barrier— The land-tied island — A barrier series — Character profiles — Reading references . . 231

CHAPTER XIX Coast Records of the Rise ok Fall of the Land

The characters in which the record has been preserved — Even coast line the mark of uplift — A ragged coast line the mark of subsidence — Slow uplift of the coasts ; the coastal plain and cuesta — The sudden uplifts of the coast — The upraised cliff — The uplifted barrier beach — Coast terraces — The sunk or embayed coast — Submerged river channels — Records of an oscillation of movement — Simultaneous contrary movements upon a coast — The contrasted islands of San Clemente and Santa Catalina — The Blue Grotto of Capri — Character profiles — Reading references 245

CHAPTER XX The Glaciers op Mountain and Continent

Conditions essential to glaciation — The snow-line — Importance of mountain barriers in initiating glaciers — Sensitiveness of glaciers to temperature changes — The cycle of glaciation — The advancing hemicycle

— Continental and mountain glaciers contrasted — The nourishment

of glaciers — The upper and lower cloud zones of the atmosphere . 261

Chapter XXI

The inland ice of Greenland — The mountain rampart and its portals — The marginal rock islands — Rock fragments which travel with the

Xvi CONTENTS ice — The grinding mill beneath the ice — The lifting of the grinding tools and their incorporation within the ice — Melting upon the glacier margins in Greenland — The marginal moraines — The outwash plain or apron — The continental glacier of Antarctica — Nourishment of continental glaciers — The glacier broom — Field and pack ice — The drift of the pack — The Antarctic shelf ice — Icebergs and snowbergs and the manner of their birth — Beading references .... 271

CHAPTER XXII The Continental Glaciees of the "Ice Age"

Earlier cycles of glaoiation — Contrast of the glaciated and nonglaoiated regions — The "driftless area" — Characteristics of the glaciated regions — The glacier gravings — Younger records over older : the glacier palimpsest — The dispersion of the drift — The diamonds of the drift — Tabulated comparison of the glaciated and nonglaciated regions — Unassorted and assorted drift — Features into which the drift is molded — Marginal or ' ' kettle ' ' moraines — Outwash plains — Pitted plains and interlobate moraines — Eskers — Drumlins — The shelf ice of the ice age — Character profiles 297

CHAPTER XXIII Glacial Lakes which marked the Decline op the Last Ice Aqe

Interference of glaciers with drainage — Temporary lakes due to ice blocking— The "parallel roads" of the Scottish glens — The glacial Lake Agassiz — Episodes of the glacial lake history within the St. Lawrence Valley — The cresoentic lakes of the earlier stages — The early Lake Maumee — The later Lake Maumee — Lakes Arkona and Whittlesey

— Lake Warren — Lakes Iroquois and Algonquin — The Nipissing Great Lakes — Summary of lake stages — Permanent changes of drainage effected by the glacier — Glacial Lake Ojibway in the Hudson's Bay drainage basin — Reading references 320

CHAPTER XXIV The Uptilt or the Land at the Close op the Ice Aqe

The response of the earth's shell to its ice mantle — The abandoned strands as they appear to-day — The records of uplift about Mackinac Island

— The present inclinations of the uplifted strands — The hinge lines of uptilt — Future consequences of the continued uptilt within the lake region — Gilbert's prophecy of a future outlet of the Great Lakes to the Mississippi — Geological evidences of continued uplift — Drowning of southwestern shores of Lakes Superior and Erie — Reading references 340

CONTENTS XVii

CHAPTER XXV NiA&AKA Falls a Clock of Recent Geological Time ^_, PAGE

Features in and about the Niagara gorge — Tlie drilling of the gorge — The present rate of recession — Future extinction of the American Fall

— The captured Canadian Fall at Wintergreen Flats — The Whirlpool Basin excavated from the St. David's gorge — The shaping of the Lewiston Escarpment — Episodes of Niagara's history and their correlation with those of the glacial lakes — Time measures of the Niagara clock — The horologe of late glacial time in Scandinavia — Reading references 352

CHAPTER XXVI Land Sculpture by Mountain Glacibks

Contrasted sculpturing of continental and mountain glaciers — Wind distribution of the snow which falls in mountains — The niches which form on snowdrift sites — The augmented snowdrift moves down the valley : birth of the glacier — The excavation of the glacial amphitheater or cirque — Life history of the cirque — Grooved and fretted uplands — The features carved above the glacier — The features shaped beneath the glacier — The cascade stairway in glacier-carved valleys — The character profiles which result from sculpture by mountain glaciers

— The sculpture accomplished by ice caps — The Norwegian tind or beehive mountain — Reading references 367

CHAPTER XXVII Successive Glacier Types of a Waning Glaciation

Transition from the ice cap to the mountain glacier — The piedmont glacier — The expanded-f oot glacier — The dendritic glacier — The radiating glacier — The horseshoe glacier — The inherited -basin glacier

— Summary of types of mountain glacier — Reading references . . 883

CHAPTER XXVIII The Glacier's Surface Features and the Deposits upon its Bed

The glacier flow — Crevasses and s&acs — Bodies given up by the Glacier des Bossons — The moraines — Selective melting upon the glacier surface — Glacier drainage — Deposits within the vacated valley — Marks of the earlier occupation of mountains by glaciers — Reading references 390 xviii CONTENTS

CHAPTER XXIX A Stcdt of Lake Basins PAOZ

Fresh water and saline lakes — Newland lakes — Basin-range lakes — Rift-valley lakes — Earthquake lakes — Crater lakes — Coulee lakes — Morainal lakes — Pit lakes — Glint or colk lakes — Ice-dam lakes — Glacier-lobe lakes — Rock-basin lakes — Valley moraine lakes — Landslide lakes — Border lakes — Ox-bow lakes — Saucer lakes — Crescentic levee lakes — Raft lakes — Side-delta lakes — Delta lakes — Barrier lakes — Dune lakes — Sink lakes — Karst lakes : poljen — Playa lakes — Salines — Alluvial-dam lakes — R&um^ — Reading references . 401

Chapter XXX

Lakes as settling basins — Drawing off of water by erosion of outlet -r- The pulling in of headlands and the cutting off of bays — Lake extinction by peat growth — Extinction of lakes in desert regions — The role of lakes in the economy of nature — Ice ramparts on lake shores — Reading references 426