State Geologist Vermo4

OF THE STATE GEOLOGIST ON THE MINERAL INDUSTRIES AND GEOLOGY OF / VERMO4 1923-1924 FOURTEENTH OF THIS SERIES GEORGE H. PERKINS State Geologist and Professor of Geology University of Vermont BURLINGTON. VT.: FREE PRESS PRINTING Co. 1924. STAFF OF THE VERMONT GEOLOGICAL SURVEY, CONTENTS. 1923-1924. PAGE ç GEORGE H. PERxrrcs, State Geologist, Director. PERKINS .................... 1 ' LIST OF ALTITUDES IN VERMONT, G. H. Professor of Geology, University of Vermont. /GEOLOGY OF GRAND ISLE COUNTY, G. H. PERKTNS .................... 63 ELBRIDCE C. JACOBS, Assistant Geologist. ......................... 72 - Professor Mineralogy, University of Vermont. THE OLDEST CORAL REEF, P. E. -RAYMOND CITA1ILES H. RICHARDSON, Assistant Geologist. TERRANES OF BETHEL, VERMONT, C. H. RICHARDSON ................. 77 - Professor of Mineralogy, Syracuse University. CAMBRIAN SUCCESSION IN NORTHWESTERN VERMONT, ARTHUR KEITH. .105 CLARENCE E. GORDON, Assistant Geologist. P. E. RAYMOND ................... 137 Professor of Geology, Massachusetts Agricultural College. NEW TRILOBITES FROM VERMONT, E. J. Fovus, Field Assistant Geologist. G:oi.00y OF BRIDPOBT, SHOREIIAM AND FORT CASSIN, E. J. FOYLES .... 204 - Am. Museum Natural History, New York City. STUDIES IN THE GEOLOGY OF WESTERN VERMONT, C. E. GORDON ........ 218 Contributing Geologists: Gi-:or.00y OF SOUTH CENTRAL VERMONT, G. H. HUBBARD .............. 260 ARTHUR KEITH, U. S. Geological Survey. 344 PROFESSOR PERcy E. RAylloxu. Harvard University. MINERAl. RESOURCES, G. H. PERKINS ................................ PROFESSOR GEORGE D. Hcr.rARIJ, Oberlin College. .1 I PLATE XV. THE GEOLOGY OF SHOREHAM, BRIDPORT AND FORT CASSIN, VERMONT. EDWARD J. FOYLES TABLE OF CONTENTS. Introduction. Shoreham and Bridport. Physiography. Structural geology. Stratigraphic geology. Palae&ntology. Fort Cassin. Introduction. Palaeontology. Bibliography. INTRODUCTION. The present paper is the second of a series in which I am attempting to show the true ages of those rocks in the Champlain valley of Vermont which have been called Beekmantown [1, pp. 1-25] 1. Part of the area studied is pictured on the maps (Plates XV and XVI), which include the towns of Shorehani and Brid- port in western Vermont. The reader is referred to the Ticon- deroga, Brandon and Port Henry topographic maps published by the United States Geological Survey for a more detailed study of the localities listed in this paper. The district may be reached by automobile from the railroad at Middlebury, Vermont. Sec- tions were made and fossils collected within the one hundred square miles of the area. In the summer of 1921 three weeks were spent in field work, and in the summer of 1922 another three weeks were given to checking the previous work and adding to the data already QIARTZITE L,ES1O€ ARGILLAIRlE Liasroe CALCLATR1 acquired. The results of this work were published in the Thir- teenth Report of the State Geologist of Vermont [2, pp. 71-86]. In the summer of 1924 three weeks were spent in the field. The field work has been supplemented by laboratory studies of the fossils from Shoreham and Bridport, and more particularly the I The numbers within brackets refer to the oibliography attached to this paper. Oeol)gcal map of B1iOpaIt. REPORT OF THE VERMONT STATE GEOLOGIST. 205 fauna of Fort Cassin at the mouth of Otter Creek, where an attempt had been made to show that the rocks are not Beekman- town in age, but belong to higher horizons. I wish to acknowledge my indebtedness to Drs. Chester A. Reeds and the late Edmund 0. Hovey of the American Museum of Natural History for their assistance in providing part of the time for field work. I also wish to thank the State Geologist for his help in furthering this study. SHOREHAM AND BRIDPORT. PIIYSIOGRAFIIY. This area possesses the nature of a glaciated rolling plain which is known as the Champlain lowland. Aside from ground moraine, glaciation is evidenced by numerous undrained areas and marshes, by occasional waterfalls, and by a lack of systematic relation between the higher lands and the stream courses. It is probable that glaciation effaced many topographic features, per- haps filling and obliterating many small valleys and smoothing down many hills. Glaciation interrupted the cycle of erosion that was in progress before the ice period, and since the glacier with- drew, erosion has been renewed. A distinct north-south trend of the elevations is caused partly by the direction of the ice movement and partly by the attitude of the underlying rocks. The topography and drainage are youthful, a youth that was superimposed upon surfaces that were in various stages of their erosion history before the ice affected them. Only one cycle of erosion is apparent, that being the post-Tertiary trenching of the streams in the New England plateau. The area represents only surface drainage. STRUCTURAL GEOLOGY. During the Cambrian and Ordovician periods the valley between the Green Mountains and the Adirondacks was partly filled with sediments which became consolidated by induration and pressure. At some unknown time after the cessation of deposition the Green Mountains were disturbed and moved westward. This movement caused the sediments in the valley to bow upward, forming a low dip to the west and a sharper dip to the east. As the westward movement continued the anticline broke along its axis and the east rocks rode up onto the west limb of the anticline. Continued westward movement of the Green 'Mountains pushed the east limb of the anticline farther onto the west limb. The rocks of the west limb were not greatly disturbed thereafter, but those of the east limb were forced on end, bent into drag folds and variously faulted. This movement ceased at some unknown time and the processes of erosion smoothed the 206 REPORT OF THE VERMONT STATE GEOLOGIST. broken landscape. The observer now sees the gently sloping and little disturbed west limb of the anticline projecting from J1ATI XVI. beneath the tilted fragments of the east limb. This in general was the course of events in this area after the close of the Ordovician. The following illustrations (Plates XV and XVI) may serve to indicate the structural conditions which now obtain in Shoreham and Bridport. The rocks in southern Shoreham dip i-Ast and west on an anticline or drag fold at angles varying from 300 along the axis to 80° at places one-half mile to the east. The strike of the strata is generally N. 18° W. The field evidence indicates that the rocks have been subjected to great pres- / I S sure from the east. In the great westward movement of the I H strata as a whole, many sections were dislocated and dragged ( behind. On the Whiting-Shoreham road sheared rock exposures and revoluted sections of strata indicate a long fault running east-west. One-half mile south of Shoreham Center a large section of strata has dragged behind in the westward movement of the rocks and is bounded on either side by transverse normal faults. These rocks lie upon Trenton beds which slope gently to the west. An interesting exposure of the Beekmantown may be seen $000 HAM A- H in the Bascom Ledge, which is situated two hundred feet south H B of the Laura Bascom farm at East Shoreham, Vermont. It con- sists of a high, steeply sloping bank facing the west. The strata L - dip 15° to the east and the strike is north-south. The rocks / T-I - exposed consist of division D-3 and D-4 of the Beekmantown as -- defined by Brainerd and Seely or the Chazy of the writer. At the top of the bank the thin, tough shaly layers are plainly evident / in division D-4. The intraformational conglomerate is disposed in patches throughout the rock. In one place I saw the coils of H a gastropod one inch in diameter outlined in the conglomerate. The rock is distorted in some places. Half way down the slope I Nile the one-inch ridges of division D-3 are seen. At least two horizons in this stratum are very fossiliferous, but evidence of early life may be found in nearly all levels of the deposit. Twenty- IiiHiJ1H L jj1 H five feet below the contact of the two beds the outlines of several • coiled gastropods are etched upon the surface of the rock. One QunRrzITE FORMATION LIMESTONE AhILL000IC LIMESTONE hundred feet below the contact the Chazy fossil, Isotelus platy- 0- -A marginatus Raymond, was found in abundance. In Bridport the structure is not so complicated as it is in Shoreham. In the southern part of the town, along the Lemon A Fair, the rocks have been bent into anticlines and synclines. The valley through which the Letpon Fair flows in going from Shore- ham into Bridport is caused/by a fault. Along the sides of this Geological map of Slioiehani. valley the strata have been greatly twisted and broken up. This fault may continue northeastward along the western margin of the river valley. At least one sheared outcrop of Chazy rocl, REPORT OF THE VERMONT STATE GEOLOGIST. 207 on the east side of Hemingway Hill tends to support this theory. In the east central part of the town the Chazy strata have been shoved up on end over the Trenton. The Cambrian quart- zite of Grand View Mountain projects onto the Trenton in north- eastern Bridport. In a traverse east from the southern end of Bridport village shale was encountered as far east as the east side of the road on the west side of the 500' hill. The original bedding planes can be seen in the shale with the obscure cleavage planes of slate crossing them at an angle of 45°. Concretions were found in the shale, which in some places was crossed by thin layers of lime.

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