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/no. 23,

State of Connecticut

State Geological and Natural History Survey BULLETIN No. 23

CENTRAL CONNECTICUT

IN THE

GEOLOGIC PAST2

By

JOSEPH BARRELL, E.M., Ph.D. Professor of Structural Geology in Yale University

EXCHANGE

EARTH

SCIENCES LIBRARY

BULLETINS

OF THE

State Geological and Natural History Survey of Connecticut.

1. First Biennial Report of the Commissioners of the State Geological and Natural History Survey 1903-1904.

2. A Preliminary Report on the Protozoa of the Fresh Waters of Connecticut: by Herbert William Conn. (Out of print. To be obtained only in Vol. i, including Bulletins 1-5.)

3. A Preliminary Report on the Hymeniales of Connecticut: by Edward Albert White.

4. The Clays and Clay Industries of Connecticut : by Gerald Francis Loughlin.

5. The Ustilaginese, or Smuts, of Connecticut: by George Perkins Clinton.

6. Manual of the Geology of Connecticut : by William North Rice and Herbert Ernest Gregory.

7. Preliminary Geological Map of Connecticut : by Herbert Ernest Gregory and Henry Hollister Robinson.

8. Bibliography of Connecticut Geology : by Herbert Ernest Gregory.

9. Second Biennial Report of the Commissioners of the State Geological and Natural History Survey, 1905-1906.

10. A Preliminary Report on the Algse of the Fresh Waters of Connecticut : by Herbert William Conn and Lucia Washburn (Hazen) Webster.

11. The Bryophytes of Connecticut: by Alexander William Evans and George Elwood Nichols.

12. Third Biennial Report of the Commissioners of the State Geological and Natural History Survey, 1907-1908.

13. The Lithology of Connecticut: by Joseph Barrell and Gerald Francis Loughlin.

331852

14- Catalogue of the Flowering Plants and Ferns of Con- necticut growing without cultivation: by a Committee of the Connecticut Botanical Society.

15. Second Report on the Hymeniales of Connecticut: by Edward Albert White.

1 6. Guide to the Insects of Connecticut: prepared under the direction of Wilton Everett Britton. Part I. General Introduc- tion : by Wilton Everett Britton. Part II. The Euplexoptera and Orthoptera of Connecticut: by Benjamin Hovey Walden.

17. Fourth Biennial Report of the Commissioners of the State Geological and Natural History Survey, 1909-1910.

18. Triassic Fishes of Connecticut: by Charles Rochester Eastman.

19. Echinoderms of Connecticut: by Wesley Roscoe Coe.

20. The Birds of Connecticut : by John Hall Sage and Louis Bennett Bishop, assisted by Walter Parks Bliss.

21. Fifth Biennial Report of the Commissioners of the State Geological and Natural History Survey, 1911-1912.

22. Guide to the Insects of Connecticut : prepared under the direction of Wilton Everett Britton. Part III. The Hymen- optera, or Wasp-like Insects, of Connecticut: by Henry Lorenz Viereck, with the collaboration of Alexander Dyer MacGillivray. Charles Thomas Brues, William Morton Wheeler, and Sievert Allen Rohwer. (In press.)

23. Central Connecticut in the Geologic Past: by Joseph Barrell.

24. Triassic Life of the Connecticut Valley: by Richard Swann Lull.

25. Sixth Biennial Report of the Commissioners of the State Geological and Natural History Survey, 1913-1914.

Bulletins I, 9, 12, 17, 21, and 25 are merely administrative reports containing no scientific matter. The other bulletins may be classified as follows :

Geology: Bulletins 4, 6, 7, 8, 13, 18, 23, 24. Botany: Bulletins 3, 5, 10, n, 14, 15. Zooloe^v: Bulletins 2. 16. 10. 20. 22.

jouuuiy : x^uueims 3, 5, lu, 11, 14, j.

Zoology: Bulletins 2, 16, 19, 20, 22

These bulletins are sold and otherwise distributed by the State Librarian. Postage, when bulletins are sent by mail, is as follows: No. i, $0.01 ; No. 3, .08; No. 4, .06; No. 5, .03; No. 6, .12; No. 7, .06; Na 8, .05; No. 9, .02; No. 10, .08; No. n, .07;

No. 12, .02; No. 13, .08; No. 14, .16; No. 15, .06: No. 16, .07; No. 17, .02 ; No. 18, .07 ; No. 19, .08 ; No. 20, .14 ; No. 21, .02 ; No. 23, .03; No. 24, .10; No. 25, .02. The prices when the bulletins are sold are as follows (including postage) : No. i, $0.05 ; No. 3, .40; No. 4, .30; No. 5, .15; No; 6, .50; No. 7, .60* ; No. 8, .20; No. 9, .05 ; No. 10, .35 ; No. n, .30; No. 12, .05 ; No. 13, .40; No. 14, .75 ; No. 15, .35 ; No. 16, .35 ; No. 17, .05 ; No. 18, .25 ; No. 19, .45 ; No. 20, .50; No. 21, .05 ; No. 23, .15 ; No. 24, .65 ; No. 25, .05.

Bulletins 1-5 are bound as Volume I. The price of this Volume' is $1.50. Bulletins 6-12 are bound as Volume II. The price is $2.45. Bulletins 13-15 are bound as Volume III. The price is $2.50. Bulletins 16-21 are bound as Volume IV. The price is $2.15.

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Applications or inquiries should be addressed to

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Hartford, Conn.

* If map is mounted as a wall map, and sent by express, $1.60.

CATALOGUE SLIPS.

Connecticut. State geological and natural history survey.

Bulletin no. 23. Central Connecticut in the geo- logic past. By J. Barrell. Hartford, 1915.

44 pp., 5 pis., 23cm.

Barrel^ Joseph.

Central Connecticut in the geologic past. By Joseph Barrell: Hartford, 1915.

44pp., 5 pis., 23cm.

(Bulletin no. 23, Connecticut geological and natural history survey.)

CATALOGUE SLIPS.

Geology.

Barrell, J. Central Connecticut in the geologic past. Hartford, 1915.

44pp., 5 pis., 23cra.

(Bulletin no. 23, Connecticut geological and natural history survey.)

Figure I. GEOLOGIC MAP of CENTRAL CONNECTICUT

with STRUCTURE SECTION on Lat. 41° 35' N.

of (Eatmwttrut

PUBLIC DOCUMENT No. 47

State Geological and Natural History Survey

COMMISSIONERS

MARCUS H. HOLCOMB, Governor of Connecticut (Chairman) ARTHUR TWINING HADLEY, President of Yale University WILLIAM ARNOLD SHANKLIN, President of Wesleyan University FLAVEL SWEETEN LUTHER, President of Trinity College (Secretary) CHARLES LEWIS BEACH, President of Connecticut Agricultural College

SUPERINTENDENT WILLIAM NORTH RICE

BULLETIN No. 23

HARTFORD

Printed for the. State Geological and Natural History Survey 1915

PUBLICATION

APPROVED BY

THE BOARD OF CONTROL

THE CASE. LOCKWOOD ft BRAINARD CO. HARTFORD. CONN.

CENTRAL CONNECTICUT

IN THE

GEOLOGIC PAST

By JOSEPH BARRELL, E.M., Ph.D. Professor of Structural Geology in Yale University

HARTFORD

Printed for the State Geological and Natural History Survey 1915

Central Connecticut in the Geologic Past.*

"The hills are shadows, and they flow From form to form and nothing stands;

They melt like mists, the solid lands,

Like clouds they shape themselves and go."

Tennyson.

OUTLINE.

Page

Introduction . . . ^ Plan of the paper . ...

Geologic history expressed by structure sections . . .8

The forces of geologic change . . . . 10

The measure of geologic time ... .12

Description of Central Connecticut . . . .14

A part of the Appalachian province , The surface features .... The rock structure . . .19

» Structure Sections of Successive Geologic Periods The present geologic time Connecticut during the Glacial period

The close of the Tertiary period . 23

In the Cretaceous period . 24

The block mountains of the early Jurassic . . 26

Close of the Triassic Sedimentation . .28

Beginning of the Triassic Sedimentation . . 32

Close of the Appalachian Revolution

The Panorama of Geologic Time . . 34

The Meaning of the Shifting Scenes . . . . .40

*This paper, in its original form, was read before the Wyoming Historical and Geo- logical Society, April 28, 1911, and was published in their Proceedings and Collections, Vol. xii, pp. 25-54. It is now reprinted, with alterations and additions, by permission of the Wyoming Historical and Geological Society.

INTRODUCTION.

Plan of the Paper. The great lesson taught by the study of the outer crust is that the earth-mother, like her children, has attained her present form through ceaseless change change which marks the pulse of life change which will cease only when her internal forces slumber, and her outer envelopes, the cloudy air and surf-bound ocean, no more are moving garments. The flowing landscapes of geologic time may be likened to a kinetoscopic panorama. The scenes transform from age to age, as from act to act; seas and plains and mountains of different types follow and replace each other through time, as the traveler sees them succeed each other in space. At times the drama quickens, and such rapid geologic action has marked the epochs since man has been a spectator on the earth.

Science demonstrates that mountains are transitory forms, but the eye of man through all his lifetime sees no change, and his reason is appalled at the thought of duration so vast that the millenniums of written history have not recorded the shifting of even one of the fleeting views whose blendings make the moving picture. The reason becomes convinced by argument, but draw- ings assist the imagination to rebuild on the visible rock foundations and eroded structures the shadowy outlines of the former landscapes which they imply. For such graphic study, Central Connecticut is here chosen. Statements of the present surface forms and geologic structures are given as a basis for the reconstruction by drawings of the forms and structures of the past. Having followed the evidence backward through the geologic ages to that period in which obscurity darkens the farther past, our vision is then turned forward and, while taking homeward flight to the present age, we behold the panorama of geologic time. But science not only reconstructs the past. It also asks the questions why and whither. In order not wholly to omit an answer there is given therefore at the close of this study a brief conclusion on the meaning of the shifting scenes.

7

8 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

The limits of ibis J)aper and the great number of events which are reviewed prevent an extensive discussion of the local evi- dence, which may be found in large part in other publications.1 The conclusions, however, depart from those previously ex- pressed in a number of particulars, wherein studies of the field or theoretical considerations have led the present writer to other views. Since the subject deals with a graphic visualization of the past, it lends itself to popular treatment, and technical writ- ing has therefore been avoided as much as possible even at the cost of some expansion in length. It should be added that the structure sections here presented are wholly new, and it is hoped that they and parts of the discussion may be not without interest to geological specialists.

Geologic History expressed by Structure Sections. Geologic studies commonly center in a written description, and are illus- trated by maps and structure sections which show the rock formations as they exist at the present time. In this article the form of presentation is reversed, and the later geologic history of central Connecticut is made to center about a succession of graphic portrayals, with written descriptions to precede and ex- plain these views. A structure section passing east and west near Meriden and Middletown shows the rock formations as they would appear on the walls of a deep trench, and the surface outline shows the magnitude and relations of hills and valleys. Upon this structure section, as upon a wide canvas, the spectator may in imagination review the changes which have passed from age to age over this one portion of the earth.

The structure section of Present Geologic Time, as shown in Figures I and 2, is based upon the location of surface out- crops, and the information which these give to the geologist con- cerning the underground structure. But, except for the surface line, this, like other structure sections, is the product of the scientific imagination. The deeper the section is carried and the more complicated the geology, the more it must fail of accuracy,

1 See especially Davis, W. M., The Triassic Formation of Connecticut. 18th Ann. Rpt. U. S. Geol. Surv., Part ii, pp. 9-192, 1898. Also, Rice, W. N., and Greg- ory, H. E., Manual of the Geology of Connecticut, Bull. No. 6, Conn. Geological and Natural History Survey, 1906. For geologic studies of the same formations in Massachusetts see especially Emerson, B. K., Holyoke Folio, No. 50, U. S. Geol. Surv., 1898.

NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 9

though its value may still be great in graphically explaining the geologic history of the region.

The surface of the earth, which alone is open to observation, is, however, a changing surface, the product of erosion, separat- ing that portion of the rocks which is invisible because destroyed, from that other portion which is invisible because not yet brought to the scene of destruction. From the study of this soil-clad surface which intersects the original structure of the rocks, the vanished portion above our heads may be as legitimately por- trayed, by the same methods of reasoning and with the same degree of accuracy, as the invisible structures below our feet. The structure may then be progressively simplified by taking away the effects of successive crustal movements and thereby graphically show the structural evolution.

The corresponding landscape may be restored for each stage by invoking the principles which underlie erosion and deposi- tion and applying these to interpret the relations between the present and the past. It has been noted that the accuracy of details in the structure section becomes less the farther they are from the controlling surface of observation, and, in a similar manner, the accuracy of the delineation of the ancient surface of erosion becomes less the farther it is removed from relation- ship with the present landscape. Limits are therefore reached in geologic time as well as in hidden depth, beyond which inference weakens and portrayal cannot go.

The method has its value on the one hand in overcoming the confusion of words and in visualizing impressively change follow- ing change in the protean earth. It shows with some degree of geologic precision the chronologic mile-posts of the flowing land- scape. But the limitation of scale of the drawings precludes the representation of details, such as met the eyes of. the changing denizens of each age. The restoration of these bygone forms of life and the scenes among which they lived requires the imagin- ation and the pencil of the geologic artist.1

1 See Bulletin No. 24 of the Connecticut Geological and Natural History Sur- vey, Triassic Life of the Connecticut Valley, by Richard Swann Lull, Professor of Vertebrate Paleontology in Yale University. This bulletin treats in detail the life of Triassic times as drawn in part from knowledge of the bones, but especially from the wonderfully rich and unique footprint record of the Triassic rocks of Connecticut arid Massachusetts.

IO CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

The graphic method has the disadvantage, on the other hand, of requiring the definite expression of detail, where in the nature of the problem a knowledge of detail is more or less absent ; but this defect, inherent in drawings, is seen to be unimportant if the reader follows the evidence on which they are based and uses them for the purpose of visualizing general conclusions.

The conventional structure sections show neither the land- scape of the background nor the clouds above, but for the present purpose these may effectively be added. The atmosphere and its clouds belong to the earth. In wind and rain they play their parts in the geologic drama. Climate is expressed in the present to some extent by cloud forms, and ancient climates are recorded in the crust by the character of the contemporaneous erosion and sedimentation, the work of former sun and frost, of rain and wind, of moving ice and water. Furthermore, each type of cloud has a tendency toward a certain size and elevation, and gives a rude gigantic scale against which may be measured the mountain heights. Observations at the Blue Hill Observatory, for example, showed that the cumulus, or summer day clouds, in summer have their flat bases at an average elevation of 4,900 feet above the land surface, in winter at an elevation of 4,600 feet. Their rounded, tumultuous summits rise to an average of 1,500 feet above their bases.1 The heights as found in other countries are not markedly different, but the average height increases about 1,400 feet from morning to noon, and from day to day may depart from the mean for the hour of the day within somewhat similar limits. The flat base of the cumulus may be regarded, therefore, as usually ranging from three-quarters of a mile to a mile and a quarter above the surface of the plains.

The Forces of Geologic Change. Sun and frost, air and rain, slowly cause the rocks to crumble, and with the aid of plants convert them into soil. But the soil creeps down the slopes ; it is partly dissolved by water and partly blown away by wind. Rivulets carry this land waste to rivers. Rivers grind their channels deeper by sweeping along the bottoms the pebbles and the sands. When the rivers have reached their lowest level they lay down their burden by spreading it over lowlands or giv-

: Clayton, II. H., and Ferguson, S. P., Measurements of Cloud Heights and Velocities. Annals of Astronomical Observatory of Harvard College, Vol. xxx, Part iii, 1892.

NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. II

ing it to the sea. Layer after layer is buried beneath ever younger layers, and, as sedimentary strata, with their record told by fossils, the whole in after ages is reconverted into rock.

But it follows that the uplands are being degraded always toward the level of the sea; the marginal seas and basins are likewise being filled. But in the lofty mountains the forces which destroy the rocks work with greatest power ; the cliffs are broken down to talus and the slopes subside more slowly into soil-mantled hills. The hills in the course of ages flatten down, so that the ultimate landscape has valleys which are broad and ill-defined, separated by low and gently sloping hills, the whole a surface of erosion which is known as a peneplain. Thus after crustal uplift the landscape passes through an erosion cycle from young and rugged mountains, to maturity marked by gentler mountain slopes, thence to prolonged old age. The work of erosion is then insignificant save for the ceaseless fretting of the ocean on its shores.

The length of the erosion cycle is, however, dependent upon the durability of the rocks. Shales are soft and limestones soluble. These will melt away in a fraction of the time needed) to destroy equal volumes of quartzite or granite-gneiss. An erosion cycle in such soft rocks as the shales of central Connecti- cut will in consequence pass into old age, while the erosion cycle begun at the same time on the harder masses of crystalline rocks which exist to the east and west is still in the stage of youth. Erosion cycles of different beginnings, different lengths, and different degrees of progress toward completion will therefore be coexistent.

But what are the causes which rejuvenate erosion and con- tinue geologic change? The crust, through the action of vertical or horizontal forces, periodically becomes broadly warped and bowed; or it breaks into blocks, or is folded into mountain ranges. Molten masses may invade the crust from the depths below or pour out upon its surface. Thus the renewal of erosion is dependent upon diastrophism and vulcanism, under which terms are included all processes emanating from the inner earth.

But, if the uplifts have been so great that erosion finally cuts down to levels which were once miles below the surface, the rocks then exposed are found to be contorted and mashed, crystal-

12 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

lized and hardened from the pressure and heat of the depths. In this way limy sediments of the sea, after deep burial and sub- jection to mountain-making forces, become crystallized to marbles. Muds, first hardened into shales, are finally trans- formed into sparkling mica schists. Sandstones pass into lustrous quartzites. Granites are mashed into banded rocks known as gneisses. All these crystallized and hardened forms are classified as metamorphic rocks. They are the foundation whose broad exposure at the surface testifies to the destruction of ancient mountain ranges. Their resistance prolongs the erosion cycles which upon eacli renewed uplift begin their re-destruction. Let them be planed down to the level of the sea and then uplifted. The upwarped peneplain will endure for a time, largely undes- troyed, existing as a plateau trenched by narrow valleys even after the softer rocks have been again eroded to another low-lying plain. Or let submergence occur. The rivers and the sea will then lay down layers of sand and mud across the level floor of contorted and eroded mountain structures. The surface of the ancient land is now a surface of unconformity, and the latter as a record of an erosion cycle has become a part of the geologic story, but the record is concealed unless new forces warp or fold the rocks and again subject them to erosion.

The Measure of Geologic Time. Man measures his life by a few scores of years, but the years of the earth are measured by many millions, an abyss of time so vast in comparison that the mind cannot fathom it save by the use of analogy. Let a year be represented by a foot; the average length of human life is measured then by the breadth of a dwelling house, 'and human history is limited approximately to a mile; but the duration of geologic time is measured in terms of the circumference of the globe.

The length of geologic ages cannot be stated accurately in years, but the rather conservative estimates of J. D. Dana are given in the annexed table of geologic time. Certain lines of evidence suggest that the geologic ages may be many times longer, but no reliable estimate yields a lesser duration than that given long since by Dana. The ratios of the relative duration of the eras are presumably more reliable than the estimates of the lengths in years. It is seen that each preceding era of the last

NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 13

four is longer than the sum of all succeeding eras, but as to the duration of the first two eras not even their ratio to the later times is known. In the region selected for the present study the history can be well deciphered as far back as the beginning of the Mesozoic era, and it will be seen that many events which have transformed the face of nature have been crowded into that time. Yet it is probably not more than the last fourth of that geologic time which has elapsed since the beginning, in the Cambrian, of the fossil record of living forms ; nor more than a tenth of the entire history of the world. The length of the geologic periods is measured by the work of erosion and deposition; and the changes which have passed over central Connecticut from period to period, as expressed in the accompanying drawings, enable the reader to form some estimate for himself of their relative dura- tion. In most cases it is seen that each preceding change involves a greater transformation and implies a longer lapse of time than those which follow, corresponding thus to the estimates of the table. But knowledge becomes vague in proportion as the evidence has been obliterated in the recording of later events, and the student of geologic time looking over the illimitable past sees the vista recede like a mountainous landscape. Beyond the near-by foothills range after range breaks the view, each rising higher, the scale of magnitude continually increasing; but the eye gradually loses all detail of form. Beyond the blue horizon's rim the reason knows still other mountains lie.

14 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

TABLE OF GEOLOGIC TIME

Minimum estimate of length in years

Eras

Periods

Ages

30,000

Psychozoic

Human

Age of Man

3,000,000

Cenozoic

Quaternary or Glacial Tertiary

Age of Mammals

9,000,000

Mesozoic

Cretaceous Comanche Jurassic Triassic

Age of Reptiles

36,000,000

Paleozoic «

Permian Pennsylvanian Mississippian

Age of Amphibians or Carboniferous Age

Devonian Sihirian

Age of Fishes

Ordovician Cambrian

Age of Higher Invertebrates

>

Proterozoic

Keweenawan Animikian or Upper Huronian Middle Huronian Lower Huronian

Age of Primitive Invertebrates (Fossils almost unknown)

Age of Protozoa (Fossils wholly unknown)

Archeozoic l

Keewatin Coutchiching

, DESCRIPTION OF CENTRAL CONNECTICUT.

A Part of the Appalachian Province. The geologic province of the Appalachian mountain system stretches from Newfound- land to Georgia and in width it reaches from the Atlantic coastal plains to the plains of the Central States. It is divided into many belts, which form sub-provinces, each with its own geologic record, each telling better than another some particular geologic story. The history of each region is in part local, in part general. A description, therefore, of the geologic past of central Connecti- cut since the Paleozoic gives a general view of events similar to those which have passed over all that belt of the Appalachian

1 The Archeozoic and lower Proterozoic make up together the complex of basal rocks often called the Archean.

NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 15

system which stretches through Massachusetts, Connecticut, New Jersey, and southeastern Pennsylvania to central North Caro- lina. To a lesser degree the history has corresponding stages in those belts of the Appalachians to the east and west. The local description, besides giving details of local interest, serves, by concentrating the attention, to bring out sharply the magnitude of the changes which mark the passage of geologic time. It is thought, therefore, that such a discussion may serve for more than local interests.

The Surface Features. The surface of the land is the prod- uct of erosion. The erosion of the portions above sea level during each period has furthermore been carried to varying degrees of completion. The result has been to divide Connecticut into three geographic provinces, the Central Lowland, and the Eastern and Western Highlands. The Central Lowland trends nearly north and south across the central part of the State and extends north- ward across Massachusetts. On the northern boundary x of Con- necticut it has a breadth of twenty miles, but narrows southward to about eight miles at the latitude of New Haven. It constitutes throughout most of its length the broad valley of the Connecticut River, but the latter abandons the Lowland at Middletown and has carved from that point a gorge diagonally across the Eastern Highland to Long Island Sound. The southern end of the Cen- tral Lowland is consequently drained by several small rivers which flow into New Haven harbor. With the exception of the narrow belts of marble which occur in the western part of the State, the Triassic shales and sandstones which underlie the Lowland are the rocks least resistant to decay and erosion, and have, therefore, been worn low, rapidly from the geologic standpoint. The Eastern and Western Highlands are, on the contrary, with the exception of the small Pomperaug Valley lying west of the map, Figure I, underlain wholly by metamorphic rocks; these are crystallized sediments or mashed and recrystallized igneous rocks. With the exception of the marble belts the metamorphic rocks are hard and insoluble and therefore slow to decay into soil. But this means slow erosion, as discussed under a previous heading, save where the stream currents, carving with the sand and gravel of their beds, wear out narrow valleys. Thus it is perceived that

l6 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

the geologic structure is the fundamental factor which controls the nature of the surface.

The Central Lowland is in its larger aspect a plain,- but in detail it is seen to consist largely of low hills with flowing out- line. The rivers meander through the Lowland in broad valleys but with well-defined channels. 'Prominent but interrupted ridges of trap rock run the length of the Lowland and rise several hun- dred feet above the general level. The principal streams are less than a hundred feet above the sea, but the rolling surface of the Lowland lies mostly from 100 to 400 feet higher, the northern parts in Connecticut averaging about 100 feet higher than the southern. The gentle slopes and deep soil are suited to agricul- ture; numerous small cities and several larger ones have developed and communication is easy in all directions.

The Lowland plain bevels the strata of the rocks beneath and is therefore a plain of erosion. But, even if the present narrow river valleys be in imagination refilled with the rock which the streams have excavated, the Lowland surface will be seen to be not level, but diversified by low hills 100 to 200 feet in height. It is therefore not a plain but a peneplain ; that is, almost a plain. The general uniformity of level at an elevation which in central Connecticut averages about 200 feet, indicates, furthermore, that the peneplain was developed by subaerial erosion when the land stood about 200 feet lower than at present. A more recent uplift has permitted the streams to cut to a lower level, and erosion has begun to destroy the peneplain which formerly it brought into existence, by beginning to create a new one at the present level of the rivers.

The Eastern and Western Highlands are in their larger aspects plateaus, and in regions removed from the principal rivers, as at Litchfield, this relative flatness of the upper surface is con- spicuous, the local relief being no greater than in the Central Lowland, though the average elevation may be more than a thousand feet above the sea. Over most of the highland area, however, the rivers and their tributaries have sunk into the upland, eroding narrow valleys of considerable grade, dissecting the plateau into a greater or less ruggedness, and making com- munication across the drainage systems more difficult than in the Lowland. If the valleys be filled in imagination with the rock which the rivers have removed from them, the plateau character

NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. I/

of the Highlands becomes apparent. But it is not a level plateau ; on the northern boundary line of Connecticut it attains an eleva- tion of about 1700 feet above the sea in the west and descends to an elevation of about 600 feet at the eastern limit of the State. From this elevation on the north the plateau slopes southward, and the place where it reaches sea level determines the Connect- icut shore line of Long Island Sound. At the southern limit the dissected Highlands therefore grade into an undissected lowland, albeit one of rocky character. The result is that along the shore Lowland and Highlands lose their distinction in elevation; and the only railroad which runs across the State independently of both rock structure and river valleys is the line of the New York, New Haven and Hartford Railroad which runs along the shore from the New York to the Rhode Island boundary. On the Highlands the soil is in general thinner and more stony than on the Lowland, and agriculture meets with less reward.

The Highland surface, like that of the Lowland, truncates the rock structure. It is, therefore, like the latter, the product of erosion, but during an earlier geologic period, when this plateau surface lay near the level of the sea, and erosion continued to sap the slopes of all hills which rose above its surface, but could not carve the rocks below. The hills gradually melted down until they possessed but a remnant of their former height. The valleys became broad and open. A peneplain extended far and wide, interrupted by a few remaining mountain knots. Then after a long interval a broad swelling uplift of the land created a lower sea level a lower base-level toward which the rivers began to etch their channels and the Highlands began to be destroyed.

The plateau surface has commonly been considered as entirely the product of one cycle of river erosion, but upwarped and tilted in several stages until it reached its present altitude. The opinion is held by many geologists that before the uplift the sea had planed the surface as far north as Meriden and Middletown, laying down a thin mantle of coastal plain deposits which since the uplift have been eroded from the surface as far south as Long Island.

Studies by the writer have led him, however, to a somewhat different view,1 the detailed evidence for which has not as yet

> Barrell, J., Piedmont Terraces of the Northern Appalachians and their Mode of Origin: Post-Jurassic History of the Northern Appalachians. Bull. Geological Soc. Am., vol. 24, pp. 688-691, 1913. 2

l8 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

been published. Careful study shows the Highland surface not to consist of one irregular sloping plain, with low hills rising above and deep valleys cut below. On the contrary, if the valleys be filled in imagination and the ravages of erosion repaired until the country is level with the higher hill tops, it will be found that the upland level resembles an irregular flight of giant stairs. The rises are commonly about 200 feet in height, but weathered down to very gentle slopes. The treads average from five to ten miles in breadth and are more nearly level than is the general slope of the upland surface.

Over much of the country the entire original surface has been destroyed and leaves no clue to its original nature, but on the Western Highland it may still be restored. Near Naugatuck many level-topped ridges rise to elevations of from 700 to 740 feet. Above them on the north is a belt of scattered higher hills which reach most extensive development in the town of Prospect at elevations of about 920 feet. In the region of Litchfield the next level shows in many flat hill-tops at iioo to 1140 feet. Rising above these on the north is a wide belt of rolling hill-tops in the town of Goshen which reach from 1340 to 1380 feet in height, and farther to the north are scattered higher hills which represent still older and higher levels.

The interpretation which this stair-like or terrace character of the restored Highland surface seems to demand is, that, after a peneplain had been developed, the sea planed inland along the entire Atlantic shore, completely across the state of Connecticut and over most of Massachusetts. During this invasion the sea may have partly cut the benches, but most of the terraces were doubtless cut as sea cliffs during oscillations of the shore line which accompanied emergence of the land. The cliffs cut by the successive inroads 9f the sea resulted in a surface partly of terrestrial, partly of marine erosion, a sea-benched peneplain, of which the lower and seaward terraces are much younger than the higher and landward ones. But the lower terraces, those below 700 feet in elvation, were imperfectly developed because of more rapid oscillations and a greater dominance of river trenching. The higher terraces have been so largely destroyed that the details of the landscape are due wholly to later subaerial erosion. It is in the original control of the higher levels that the

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ancient sea terraces become of importance. Little suggestion of them, however, can be seen by a casual study of the landscape. It is rather a comprehensive study of topographic maps which supplies the evidence, but a final conclusion on this subject must await a detailed publication.

The Rock Structure. The erosion surface gives the data for deciphering one side of geologic history, that of the surface activities; the rock structure gives another side of this history, that connected with the forming and transforming of the rocks.

The structure section shown in Figure I shows the attitude and nature of the rock formations, the oldest being united in one group the pre- Paleozoic complex gneisses. Back of the Paleo- zoic ages lies a tangled record, which speaks, however, of eras of mountain-making, erosion, and sedimentation, followed at last by a manifestation of mountain-making forces on a prodigious scale. The sediments were crystallized, mashed, and injected with sheets and masses of molten rock, thus developing the pre- Paleozoic complex gneisses, the result of internal forces so vast as to remake the crust and everywhere hide in obscurity the earli- est history of the earth. This " Basement Complex " does not rise to the surface on the line of the structure section, its nearest outcrops being in the northwestern portion of the State.

The second group of rocks shown in the drawings comprises the Paleozoic sediments. During the greater part of that era most of the area of Connecticut was, as now, a part of the land, but then, in marked contrast with present conditions, it stood on the eastern side of an inland sea. Long Island Sound was not yet in existence, and the Appalachian continent, now in large part sub- merged, extended to the south. The mountain system was fur- thermore subjected more than once to movements of folding and uplift. The Paleozoic sediments therefofe represent only certain periods when the land stood lowest and the sea held widest sway. But not all of the sediments are positively marine, some of them may have been formed as delta deposits skirting uplands and built out against a western shallow sea. Only portions of the Paleozoic sediments have been preserved, that is, the parts which were folded down rather than thrust up. The folding, mashing, and crystallization to which these sediments were subjected in mountain-making movements of the Paleozoic, especially near

2O CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull.

its close, were so great in the Connecticut area as to transform them completely into crystalline schists and gneisses. All fossils which they once may have contained have been obliterated, and the age of the sediments, further than that they belong to the Paleozoic, is not positively known.

The third group of formations comprises the intrusive igne- ous rocks of Paleozoic age. They are mostly granite-gneisses, forced at repeated intervals into the older rocks as molten masses of great volume, solidifying into granites, later crushed into banded rocks known as gneisses. Their invasions record times of revolution, of uplift and mountain-making, even as the sediments into which they were forced record times of quiet and local sub- sidence. The intrusive rocks probably belong mostly to the clos- ing periods of the Paleozoic, when the ancient order of lands and seas and the life inhabiting them was being broken up, and the world stage was being reset for the drama of the Age of Reptiles. But, since the sediments are not precisely dated, neither can the age of the granite-gneisses be definitely known. Farther west, in New York State, seas prevailed much of the time until near the close of the Paleozoic, and the unmetamorphosed strata record with fulness the progress of life and the sequence of the ages. But near the western border of New England -many formations disappear, others change their sedimentary character, metamorphism masks their original nature, and before the Central Lowland is reached they pass into a tangle of metamorphic and igneous rocks, a second Basement Complex, only less profoundly changed than the pre-Paleozoic Complex below. Indeed, until within recent years no separation was made between them, and the greater part of Connecticut, with the rest of New England, was regarded as made of rocks of Archean age. Although the original nature of the sediments is so greatly blurred, the metamorphism and igneous intrusion clearly record a history still more impressive to the imagination; for they are the basement structures of an ancient range of mountains, the Paleozoic Alps of New England, a generation of mountains long since vanished, but whose rugged slopes and majestic heights the mind of man has learned to build anew.

The fourth group of rocks shown on the structure sections is that of the Triassic sediments and lavas. The sediments are

NO. 23.] CENTRAL CONNECTICUT IN THE GEOLOGIC PAST. 21

mostly red or brown shales and sandstones with, in certain local- ities, many conglomerate beds. Intrusions of tra.p were forced into these sediments as molten sheets, and at three separate times great floods of lava spread far and wide over the surface. These were poured out while the Triassic muds and sands were accumu- lating and subsiding, and each in turn became buried beneath the later beds of the formation. Uplift of the neighboring regions and subsidence over the region of accumulation permitted erosion and sedimentation to proceed until a maximum thickness of certainly more than two miles, very possibly as much as three miles, had accumulated. The sediments and the lavas were laid down in approximately horizontal sheets, but they now exhibit a regional dip to the east which averages from fifteen to twenty degrees. Erosion has planed across these inclined strata, ex- posing them to view from top to bottom. The trap flows consist of harder rock and have not been worn so low as the soft rocks which underlie the valley floor. The outcrops of the lavas, how- ever, are broken and offset and repeated, indicating that the Triassic formation has been shattered into great crust blocks which have slipped on fault planes hundreds or even thousands of feet with respect to each other. The original position of the sediments has therefore been modified by both tilting and fault- ing as shown on the structure section. The floor upon which the Triassic land waste began to be laid down has again become exposed as the eastern slope of the Western Highland. It is a fairly plane surface eroded across various metamorphic rocks, and indicates a great lapse of time following the elevation of the