THE GREEN MOUNTAIN ANTICLINORIUM IN THE VICINITY OF WILMINGTON AND WOODFORD VERMONT By JAMES WILLIAM SKEHAN, S. J. VERMONT GEOLOGICAL SURVEY CHARLES G. DOLL, Stale Geologist Published by VERMONT DEVELOPMENT DEPARTMENT MONTPELIER, VERMONT BULLETIN NO. 17 1961 = 0 0. Looking northwest from centra' \Vhitingham, from a point near C in WHITINCHAM IPlate 1 Looking across Sadawga Pond Dome to Haystack Mountain-Searsburg Ridge in the background; Stratton and Glastenburv Mountains in the far distance. Davidson Cemetery in center foreground on Route 8 serves as point of reference. TABLE OF CONTENTS PAGE ABSTRACT 9 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . 10 Location ........................ 10 Regional Geologic Setting . . . . . . . . . . . . . . . 13 Previous Geologic Work ................. 15 The Problem ...................... 16 Present Investigation ................... 18 Acknowledgments .................... 19 Topography . . . . . . . . . . . . . . . . . . . . . 19 Rock Exposure ..................... 20 Culture and Accessibility ................. 20 STRATIGRAPHY AND LITHOLOGY ............... 23 General Statement . . . . . . . . . . . . . . . . . . 23 Stratigraphic Nomenclature . . . . . . . . . . . . . . 25 Lithologic Nomenclature ................. 26 Pre-Cambrian Rocks . . . . . . . . . . . . . . . . . 27 General Statement . . . . . . . . . . . . . . . . . 27 Mount Holly Complex ................. 28 Stamford Granite Gneiss ................ 35 Wilmington Gneiss .................. 37 Readsboro Formation ................. 45 Searshurg Conglomerate Member ........... 45 Rcadsboro Schist .................. 49 Sherman Marble Member . . . . . . . . . . . . . 53 Heartwellville Schist ...................57 Paleozoic Rocks ..................... 62 General Statement .................. 62 Cambrian and/or Ordovician Rocks ............ 65 Tyson Formation ................... 65 Hoosac Formation ................... 66 Turkey Mountain Member of Hoosac Formation 72 Mendon Formation and Cheshire Quartzite ........ 75 Pinney Hollow Formation ............... 76 Chester Amphiholite .................. 80 Ottauquechee Formation ................ 82 Ordovician Rocks .................... 85 Stowe Formation ................... 85 More town Formation . . . . . . . . . . . . . . . . 88 Cram Hill Formation . . . . . . . . . . . . . . . . 91 Siluro-Devonian Rocks 96 Northheld Slate .................... 96 Unstratified Rocks ................... 99 Ultramafic Rocks . . ................ 99 General Statement .................. 99 East Dover Ultramafic ................ 99 Other Ultramafic Rocks ............... 102 ['egmatites and Veins ................. 102 Mafic and Felsic I)ikes ................. 103 STRUCTURAL GEOLOGY ................... 103 Major Structures .................... 103 General Statement .................. 103 Simple Anticlines and Synclines ............. 105 General Statement ................. 105 Readshoro Anticline ................. 105 Sherman Anticline .................. 106 Whitingham Anticline ................ 107 Folds of the Central Green Mountains ......... 107 Heartwellville Syncline ................ 107 Sumner Brook Syncline . . . . . . . . . . . . . . . 108 Flaystack Mountain Anticlinc ............. 108 Mount Snow Anticline ................ 108 Somerset Anticline ................. 109 Wilmington Anticline . . . . . . . . . . . . . . . . 109 Flogback Mountain Syncline ............. 110 Other Anticlinal and Synclinal Folds .......... 110 Domes of the Wilmington-Woodford Area ......... 110 Sadawga Pond Dome . . . . . . . . . . . . . . . . 110 Lake Rayponda Dome ................ 111 Minor Structures and their Relation to the Major Structures 112 Structural Features Associated with Folds ....... 112 Lineation ...................... 118 General Statement .................. 118 Orientation and Lineation of Quartz Rods ....... 119 Pebble Orientation and Lineation ........... 121 Mineral Streaming ................. 122 Boudinage ..................... 122 Cleavage ....................... 124 joints and Quartz Veins ................ 127 Summary of Relationships of Minor to Major Structures 128 Origin of the Simple Anticlines and Domes ........ 130 METAMORPHISM 130 General Statement 130 Metamorphic Zones ................... 131 Method of Study .................... 133 Metamorphism of Argillaceous Rocks . . . . . . . . . . 137 Metamorphism of Carbonate Rocks ............ 138 Metamorphism of Mafic Rocks .............. 139 Metamorphism of Ultramafic Rocks ............ 139 Retrograde Metamorphism ................ 143 PRE-METAMORPHIC NATURE OF THE ROCKS .......... 144 Geologic History . . . . . . . . . . . . . . . . . . . 144 ECONOMIC GEOLOGY .................... 147 Iron Mines ....................... 147 Other Mines ...................... 147 Marble Quarries . . . . . . . . . . . . . . . . . . . . 148 Ultramafic Rocks .................... 148 Sand and Gravel . . . . . . . . . . . . . . . . . . . 148 Crushed Rock and Concrete Aggregate ........... 149 MAJOR PROBLEMS ..................... 149 Origin of I)ornes .................... 149 Roots of the Taconic Klippe ............... 151 SUGGESTED FUTURE INVESTIGATIONS IN THE WILMINGTON-WOODFORD AREA .............. 153 BIBLIOGRAPHY ...................... 155 Illustrations PA GE Frontispiece . . . . . . . . . . . . . . . . . . . . . . . 2 FIGURE 1. Index to Geologic Mapping in Southern Vermont and adjacent areas ......................... 11 2. Map showing Major Structures of Southern Vermont and their relationship to the Wilmington-Woodford area ....... 14 3. Haystack Mountain as seen from South Road, north of Bond Brook ........................ 21 4A. View looking NNW from crest of Mount Snow (Pisgah) toward Stratton Mountain ............... 22 4B. View of Haystack Mountain looking NNW across Harriman Reservoir from 3z mile NE of Reservoir Spillway ..... 22 5. Columnar section of the Wilmington-Woodford Area . . . 24 6. Highly contorted plagioclase gneisses, Molly Stark Trail, Dunville Hollow, Woodford ............... 30 7. Faulted plagioclase gneiss, Molly Stark Trail, I)unville Hollow, Woodford ................... 32 8. Gneiss- and microcline-pebble conglomerate, Castle Brook, Somerset ....................... 36 9. Recumbent Spruce-Tree fold in Wilmington gneiss in Sadawga Pond dome ...................... 42 10. Recumbent folds on southeast limb of the Sadawga Pond dome 43 11. Spruce-Tree folds in Wilmington gneiss .......... 44 12. Overturned folds in plagioclase gneiss beds of the Wilmington gneisses ........................ 44 13. Searshurg conglomerate outcrop ............. 46 14. Closeup of Searshurg conglomerate outcrop ........ 47 15. Green chlorite-garnet-quartz of lower part of Neartwellville schist ........................ 58 16. Typical appearance of dark graphite-muscovite-chlorite-garnet lensed Heartwellville schist at type locality ........ 60 17. \Tiew from North of Molly Stark Trail, Wilmington, to Freezing Knolls .................... 62 18. Closeup view of Tyson conglomerate, western Wilmington 67 19. Chester amphiholite northerly sloping surface, Hoslev Hill, Jacksonville; looking southwesterly ........... 80 20. Northeasterly plunging fold in the Chester amphibolite near the axis of Mt. Olga syncline, Sky View, Route 9, Floghack Mountain in Marlboro township ............. 81 21. Well -handed epidote-ankerite-actinol ite aniphiholite of Chester imphiholite, Route 9, Hogback Mountain ........ 82 22. Closeup of bedding of highly contorted Chester amphiholite, Sky View, Hoghack Mountain, Route 9, Marlboro ..... 83 23. I hotomicrograph of Chester amphiholite, Sky \'iew, Iloghack Mountain, Route 9, Marlboro . . . . . . . . . . . . . . 84 24. Highly contorted chlorite-garnet schist and amphibolite of the Stowe formation, Route 9, Hogback Mountain, Marlboro. 85 25. Amphiholite at top of the Stowe formation showing small fault and relative movement ................. 86 26. Outcrop area of Moretown formation looking N NE from Molly Stark Trail, one mile vest of Wilmington-Brattleboro quadrangle boundary .................. 89 27. Boudinage structure in amphiholite beds of the Cram I fill formation ....................... 95 28. 1 )iagrammatic cross-section of Sadawga I ond dome showing minor folds and inferred directions of movement ...... 104 29. Diagrammatic cross-section of Sadawga Pond dome showing Spruce-Tree folds and inferred directions of movement . . . 105 30. Major structures of Wilmington-Woodford area ...... 106 31. Block diagram of minor structural features of Sadawga Pond dome associated with gently plunging Spruce-Tree folds 111 32. Block diagram of minor structural features of Sadawga Pond dome associated with steeply plunging Spruce-Tree folds 111 33. Doubly plunging anticline in plan view .......... 112 34. Spruce-Tree dome in plan view ............. 113 35. Structural symbol for strike and dip of bedding, pattern of fold, and attitude of fold axis .............. 114 36. Structural symbol showing combined strike and dip of bedding, pattern of fold, and attitude of axial plane and fold axis. . 115 37. Diagrammatic cross-section through an overturned anticline shows relation of dip of axial plane to that of beds . . . . . 116 38. Block diagram