Geology

Page updated on Apr 19, 2017 at 3:12 PM

Geologic Atlas of the City of Alexandria, Virginia and Vicinity


Welcome to the Geologic Atlas of the City of Alexandria. You can navigate directly to each of the maps, expanded explanations, and databases that make up the Atlas by clicking on the corresponding links below. If you are a first-time user, or new to either the city or the science of geology, we recommend proceeding first to the brief overview of the Atlas, where you can learn about the various components of the atlas and how to use them. 

For best resolution, you may want to open the plates in a different viewer, such as Adobe Acrobat, once they have loaded into your browser.


Geologic Maps and Expanded Explanations

Plate 1. Map Showing the Distribution and Sources of Geologic Data

Plate 2. Geologic Cross Sections

Read the Expanded Explanation of Plates 1 and 2 to learn about the kinds of geological data available in the city and how the maps and other parts of the atlas were constructed. This document also explains what geologic cross sections are and how they illustrate the “third dimension” of Alexandria’s geologic architecture

Plate 3. Map Showing Bedrock Geology, Topography of the Bedrock Surface, and Areas of Bedrock Exposure

Read the Expanded Explanation of Plate 3 to learn about the bedrock under the city, its history, where it is exposed, and why its buried topographic surface is so important  

Plate 4. Geologic Map of the Potomac Formation

Read the Expanded Explanation of Plate 4 to learn about this formation of river deposits, which dates from the “Age of Dinosaurs” and makes up the great bulk of the geologic section beneath the city 

Plate 5. Geologic Map Showing Surficial Geology, Landforms, and Major Areas of Artificially Modified Land

TheExpanded Explanation of Plate 5discusses the types of rocks and unconsolidated sediments found at the surface in different parts of the city: their history, properties, and relationships to the major landforms

Plate 6. Map of the Piezometric Surface of the Cameron Valley Sand Member (lower aquifer) of the Potomac Formation and Other Aspects of Urban Hydrogeology

Read the Expanded Explanation of Plate 6 to learn about water in the City of Alexandria: streams, aquifers, wetlands, stormwater runoff, and more

Plate 7. Slope Stability Map

This map shows which parts of the city are the most and least susceptible to landslides and expansive soils, and explains why

Part 8. Overview of Tectonic Setting, Fault Systems, and Seismic Hazards

Many Alexandria residents felt the shaking from the August 23, 2011 Mineral, Virginia earthquake, which caused significant damage to several iconic structures in and around DC. This section of the atlas provides an overview of the potential seismic hazards in the city, how certain kinds of soils can amplify seismic shaking while others attenuate it, and the locations of known and suspected faults


Geologic Data

Alexandria Exposures:
MS-Excel database of natural outcrops and excavations

Alexandria Geotechnical Borings:
MS-Excel database of geotechnical boring sites

Alexandria USGS Wells:
MS-Excel database of historical wells catalogued by the USGS

Drillers Logs for 5 Deep Wells (PDF):
Formation descriptions for five deep public water supply wells from the USGS historical collection

Sample Locations and Field Descriptions:
MS Excel database of the particle size and clay mineralogy samples analyzed and presented in the atlas

VDOT Woodrow Wilson Bridge-Capital Beltway Cross Sections (PDF):
Series of geologic fence diagrams generated from VDOT’s geotechnical boring database along the route of this major construction project

Schematic Interpretive Diagrams of Geotechnical Boring Sites (PDF):
Simplified cross-section diagrams for most of the geotechnical boring sites in the database

Glossary of Geological Terms
Opens a glossary containing geological terms used in the atlas 

Bibliography
Full list of references, with links to downloadable publications

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