The Potowmack Chapter of the Virginia Native Plant Society invites you to the following field trip:
Holmes Run Gorge Field Trip
with Tony Fleming and Rod Simmons
Saturday, October 8
2 pm to 5 pm
Location: Holmes Run Gorge
(Meeting location will be disclosed upon registration.)
VNPS programs are free and open to the public,but registration for field trips is required due to limited space.
Join experts, Tony Fleming and Rod Simmons on a walk through Holmes Run Gorge, which offers a striking diversity of geologic settings and natural communities within a compact area. This trip showcases key geologic features and processes and how they influence the structure and distributions of natural communities. Several stops also provide ample opportunity for discussion of issues related to urban streams and water management. The terrain on this ~3-mile walk ranges from dead level to rugged and will mostly be traversed on trails and sidewalks with minor off-trail segments. Sturdy, water-resistant footware is advised.
Geologist Tony Fleming, is the author of the recently published Geologic Atlas of the City of Alexandria, Virginia and Vicinity (2016) and several other publications on local geology. Rod Simmons is a plant ecologist and Natural Resource Manager for the City of Alexandria.
From both a geologic and floristic perspective, the route provides a look at the entire catena of landscapes, starting in the topographically lowest and geologically oldest terrain and ascending stepwise through younger strata in progressively higher parts of the landscape, before returning to the bottom of the gorge.
Stop 1. Beauregard St granite flume. Primarily a scenic stop, but also to familiarize attendees with the Occoquon granite and Indian Run Formation in big, clear outcrops.
Stop 2. Impoundment Wetland and Environs. Wetland hydrology and flora. Point bar-natural levee and natural disturbance. Basic mesic and small stream floodplain forests.
Stop 3. Chambliss Ravine. Base of Potomac Formation and bedrock surface. Potomac Formation basal sand unit and lower aquifer. Chambliss St. fault. Hydrology. Seepage wetland. Chestnut oak forest variants and ecotones.
Stop 4. Scott St. ravine. Contact of terrace gravel and Lincolnia silty clay member of the Potomac Formation. Colluvium. Acidic oak heath terrace forest.
Stop 5. Rosser St. Nature and origin of the terrace surface. Dowden Terrace swamp.
***We may stop at Dowden Terrace Park en route between stops 5 and 6 to look for springs emerging at the base of the terrace gravel.
Stop 6. Lillian Carey Park. Colluvial slope, feather edge of Potomac Formation, deeply weathered bedrock saprolite, acidic oak hickory forest, acid loving plants.
Stop 7. South facing valley wall and tributary. Colluvium. Indian Run Formation. Contrast acidic oak heath forest on south facing colluvial bench with more mesic variants on other nearby aspects of tributary walls and at later stops.
Stop 8. Old granite quarry. Occoquon granite satellite pluton. Evidence for old quarry. Ecotones between mixed mesic and oak heath forests. Natural succession before invasives were rampant.
Stop 9. Active point bar-cut bank pair. Discussion of stream processes, sediment transport, and natural bank erosion. Plants of the active floodway.
Stop 10. Glasgow School ravine. Falls Church tonalite. Bedrock fault and quartz vein. Weathering and saprolite formation. Old basic mesic forest. Natural succession in the age of rampant invasives. Unnatural stream incision caused by stormwater loading.
Stop 11. North facing valley wall. Monzogranite outcrops and boulderfield. How favorable aspect can overcome thin acid soil to produce a more mesic community.
Stop 12. Mouth of Rynex ravine and low water crossing. Bedrock fault juxtaposing several rock types. Quaternary stream terraces. Stream processes. History of proposed stream crossing methods and their relative merits and demerits, impacts on hydrology of the major point bar community immediately downstream of the crossing site (seen at stop 2). Group discussion of streams and “stream restoration” methods.
End-return to starting point.
For questions, please email: firstname.lastname@example.org.
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Virginia Native Plant Society, Potowmack Chapter
The Virginia Native Plant Society (VNPS) was founded in 1982 as The Virginia Wildflower Preservation Society. It is a statewide organization with approximately 2000 members supported primarily by dues and contributions. Membership is open to anyone, amateur or professional. Its purpose is to further appreciation and conservation of Virginia's native plants and habitats. Incorporated in Virginia as a not-for-profit, publicly supported organization, it is tax-exempt under the U.S. Internal Revenue Code. The Society's work and activities are carried out by volunteers.
The Society's programs emphasize public education, protection of endangered species, habitat preservation, and encouragement of appropriate landscape use of native plants.
The Potowmack chapter is the largest VNPS chapter, representing over 400 members in the counties of Arlington and Fairfax; cities of Alexandria, Fairfax, and Falls Church.