The Surficial Geology and Hydrogeology of Dorset, Vermont
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THE SURFICIAL GEOLOGY AND HYDROGEOLOGY OF DORSET, VERMONT By David J. De Simone and Marjorie Gale June 2009 Vermont Geological Survey 103 South Main Street Logue Cottage Waterbury, VT 05671-2420 Laurence R. Becker Vermont State Geologist Introduction Field work for this project was carried out during the 2006 and 2007 field seasons. Data analysis and map revisions were done in 2008 and completed in 2009. Maps discussed in this report are: 1. Surficial Geology 2. Stratigraphic Cross Sections 3. Field Data Sites 4. Springs, Seeps & Water Well Locations 5. Deglacial History 6. Depth to Bedrock 7. Bedrock Topography 8. Discharge Areas and Possible Shallow Overburden Aquifers 9. Hydrogeologic Units 10.Recharge Potential to Bedrock Aquifer 11.Potentiometric Surface Map and Groundwater Flow Lines Location and Geologic Setting The town of Dorset encompasses portions of the West Rupert, Manchester, Peru, Dorset and Danby 1:24000 topographic quadrangles and covers approximately one quadrangle of area. Two major valleys separate three upland regions. The Dorset Valley trends northwest and contains the West Branch Batten Kill that flows southeast into Manchester. In the vicinity of Dorset Village, there is a broad drainage divide consisting of wetlands. North of this divide, the Mettawee River enters the valley from head waters in the central upland region and flows northwest into New York. The upper Mettawee River represents a notable east-west tributary valley referred to as Mettawee Hollow. In the east, the East Dorset Valley trends north-northeast and is a continuation of the Vermont Valley. The Batten Kill flows southward from a divide just north of the village of East Dorset. Otter Creek flows northward from this drainage divide toward Wallingford. Both major valleys are underlain predominantly by Cambrian through Ordovician carbonate rocks including limestone, marble and dolomitic marble that comprise the Champlain Valley Sequence (CVS). There are some quartzite interbeds in the carbonate valley rocks, especially the Monkton formation. The forthcoming new State bedrock geology map (Ratcliffe et. al., in review) best displays the current interpretation of the distribution of major rock types, their ages and the structural relationships in the rock. The southwest corner of town rises moderately up the flanks of Mother Myrick Mountain, part of the Taconic Mountains, whose 3,361 ft summit is just south of the town boundary. Goodman, Gilbert and Daley Brooks drain the mountain flank and cut through Taconic Sequence (TS) lithologies composed of schist and phyllite above approximately 2,400 ft elevation with some interbedded carbonates. Below this elevation, the mountain flank is underlain by Cambro-Ordovician carbonate lithologies of the CVS with a major Taconian thrust fault separating the distinct lithologies of these 2 sequences. The eastern edge of the town rises steeply up the flank of the Green Mountains toward the summit of Mount Tabor and Bromley Mountain, both beyond the town boundary. Elevation exceeds 2,400 ft and the steep slopes are underlain predominantly by the Cambrian Cheshire quartzite, considered part of the CVS. Little Mad Tom, Mad Tom and Mt. Tabor Brooks descend steeply along the Green Mountains flank into the East Dorset Valley. In the extreme eastern portion of the town, the oldest Pre-Cambrian gneiss of the Green Mountain Sequence (GMS) is exposed above the Cheshire quartzite. A large central upland region of the town separates the Dorset and East Dorset valleys and can be informally called the Dorset Mountains. This includes the summits of Owl’s Head (2,481 ft), Green Peak (3,230 ft), Dorset Hill (2,782 ft), Netop Mountain (2,875 ft) and the south summit of Dorset Peak at approximately 3,730 ft. This upland region is underlain by Taconic Sequence lithologies composed of phyllite and schist above a major Taconian thrust fault. Kirby Hollow, Dorset Hollow and several unnamed tributaries form the head waters of the Mettawee River. Surficial Geology The Surficial Geology map shows the observed and inferred contacts between sediment types based upon field data sites and interpretation of aerial photographs and soils maps. The general distribution of surficial materials enabled the interpretation of the deglacial history of the town. This interpretation was guided by the recent completion of mapping in Manchester (De Simone, 2004), Arlington (De Simone, 2001) and Wallingford (De Simone, 2005) which establishes the new regional framework for understanding the glacial geology of southwestern Vermont. The Green Mountains, Dorset Mountain and Taconic Mountain summits and ridge lines are underlain by a very thin veneer of till with frequent outcrops. The steep mountain flanks are similarly underlain by till veneer with less frequent outcrops. The carbonate rocks on the Taconic and Dorset mountain flanks tend to outcrop in a series of cliffs separated by steps or ledges. The Green Mountain flanks are underlain chiefly by quartzite with less frequent outcrops and few or subtle ledges. A thick till blanket occurs along the lower flanks of the mountains. The topography typically reveals the contact between thick and thin till as a small but distinctive break in slope along a mountain flank. Below this slope break, outcrops are less frequent due to the thicker till blanket. The thick till-thin till contact also roughly coincides in places with the trace of the major Taconian thrust fault along the Taconic and Dorset mountain flanks. This thick till blanket may not be present everywhere as both mapping and well logs indicate. In those areas where significant flow of glacial meltwater away from the lateral margin of the glacier occurred, there are kame terraces composed of gravel and sand sediment. In these areas of ice contact deposits, there is little or no till visible at the surface. Till may exist under the kame terrace deposits or these gravel and sand sediments may persist for many tens of feet all the way to rock. Ice contact and glaciofluvial sediment predominate in the valleys. Two small areas of moraine and kame moraine at the southern margin of town represent still stands or minor surge margins of the Dorset valley glacier and East Dorset valley glacier. For reference purposes, the latter valley glacier was called the Danby glacier in the 2004 Manchester report. These sediments occur in the vicinity of Morse Hill Road. Ice contact stratified sands and gravels occur along the lower valley flanks in both the Dorset and East Dorset valleys. Mettawee Hollow contains a series of stepped kame terraces along the northern valley bottom that are composed of glaciofluvial sand with gravel. Thick ice marginal deposits occur just north of Dorset Village and at North Dorset. Both of these sites represent still stands of the respective retreating valley glaciers. The head of outwash at Dorset is beautifully kettled and links to a lateral margin kame terrace. The North Dorset ice margin is a site of extremely thick ice contact sediment with esker deposits and a notable former stagnation zone that heads at Emerald Lake, a large kettle depression with several other large kettles to the south. Much of the Dorset and East Dorset valley bottoms consist of outwash sands and gravels that form a prominent sandur or valley train – both terms used to describe the gently sloping landform - of outwash. Mettawee Hollow contains a similar valley train of sands and gravels that descends along the southern margin of the hollow and merges with the Dorset valley outwash. This deposit represents inwash as the distal portion of this inwash was deposited against or along the Dorset valley glacier. Inwash differs from outwash in that the waters transporting and depositing the sediments flow back toward and/or against the ice for inwash but flow away from the glacier for outwash. Modern stream dissection has resulted in alluvium along the present flood plain with some slightly higher fluvial terraces. Alluvial fans occur where a few of the tributaries enter the major valleys. Minor colluvium from landslides was recorded in a few places but was not a mappable unit. However, better exposure might reveal that landslide deposits are more frequent along the lower mountain flanks as the terrain frequently exhibits the appearance of old slides with boulders fallen from higher and steeper slopes and a “mixed” texture of the sediments. Wetlands with muck and peat occur in the Dorset swamp area and in the large kettles south of Emerald Lake. Deglacial History A sequence of 2 ice margins is recognized through town that records brief still stands of the retreating valley ice. Most geologists believe the ice retreated quite rapidly from Vermont and adjacent parts of New York and New England. Where dates from organic materials are available, these data confirm the ice retreated at rates in the Champlain Lowland approaching 0.4-0.5km/yr (De Simone et al, 2008, Franzi et al, 2007). If similar rates of retreat held true for the ice in the Dorset valleys, then ice retreat may have been accomplished in as little as 15-20 years. The 2 ice margins are: 1. South Dorset ice margin (position A) 2. Dorset ice margin (position C and C1-C2) Positions B, B1 and B2 represent intermediate “steps” in the retreat of the ice from the South Dorset to the Dorset ice margins. Position B in the Dorset valley represents a step in the retreat which resulted in a lower set of kame terraces in Mettawee Hollow. Positions B, B1 and B2 in the East Dorset valley represent steps in the retreat which enabled the esker, kame moraine and kame terrace sediments mapped to accumulate in a sequential fashion. The moraine and kame moraine deposits in the Morse Hill Road area define the first ice marginal positions recognized on the Deglacial History map. These were previously identified during mapping in Manchester and named the South Dorset Ice Margin.