Geology About Concord, North Carolina

Concord is located in southwestern Cabarrus County, North Carolina, approximately 25 miles northeast of Charlotte. The city sits at an elevation of 745 feet above sea level on the rolling Piedmont plateau between the Blue Ridge Mountains and the Atlantic coastal plain. Concord has a population of approximately 94,000 residents.

The geology of Concord consists largely of igneous and metamorphic bedrock overlain by sandy and clay soils. The topography is characterized by gentle hills with some areas of nearly level to gently rolling terrain. No major rivers run through Concord, but numerous creeks flow through the area.

Bedrock Geology

Igneous & Metamorphic Basement Rock

The bedrock underneath Concord consists primarily of igneous and metamorphic basement rock of the Charlotte and Milton belts. The Charlotte Belt contains volcanic and sedimentary rock about 620-550 million years old that was metamorphosed during continental collisions. The Milton Belt is made up of metamorphic and igneous rock, such as metagraywacke, metasiltstone, gneiss, and granite.

Some of the specific rock types include:

• Biotite gneiss: This metamorphic rock contains visible minerals biotite mica and quartz.
• Amphibolite: A metamorphic rock composed mainly of amphibole and plagioclase minerals.
• Granite: An intrusive igneous rock composed primarily of quartz, feldspar, mica, and amphibole minerals.
• Metagraywacke: A highly metamorphosed sandstone comprised of quartz and feldspar grains in a fine clay matrix.

Faults & Seismicity

There are no active faults running directly through Concord. However, minor ancient faults have been mapped in the Piedmont near the city. Overall, the seismic hazard risk is low for most of North Carolina. Only one or two minor earthquakes have been recorded within 50 miles of Concord since the 1900s.

Surficial Geology & Soils

Glacial Sediments

Concord lies just south of the maximum southern extent of glaciation during past ice ages. As a result, the city does not have any glacial till or outwash deposits at surface. Early researchers mapped some areas of Cabarrus County as being covered by Carolina Bays, which are hypothesized to be ancient sandy depressions created by winds during the last glacial period. However, more recent research suggests Carolina Bays do not extend as far inland as Concord.

Alluvium

Narrow bands of alluvial sediment have been deposited along streams and rivers passing through Concord. These finer grained sands, silts, and clays were eroded from adjacent uplands and transported by flowing water. The largest area of alluvium sits along Irish Buffalo Creek in the northwest part of the city.

Saprolite

Below the soil layer across much of Concord lies saprolite, a chemically weathered bedrock that formed in place over time. Saprolite consists of clay minerals and quartz grains resulting from extensive chemical weathering of crystalline rocks. In some areas, the saprolite grades directly into partially weathered bedrock at depth without a clearly defined boundary.

Soils

According to the Web Soil Survey published by USDA Natural Resources Conservation Service, Concord is dominated by sandy and clayey soils derived from igneous and metamorphic bedrock. The majority of the soil series consist of fine sandy loams, sandy clay loams, and clay loams.

The most common soil associations mapped within the city limits are:

• Wilkes-Davidson-Pacolet: Very deep, well drained soils formed in residuum primarily from igneous and high-grade metamorphic rock. Occurs on ridges and side slopes on the landscape.
• Wedowee-Badin-Iredell: Deep to very deep, well drained soils formed in residuum from mafic rocks. Found on ridges and side slopes.
• Chewacla-Wehadkee-Rion: Deep, well drained, moderately permeable soils formed in residuum from igneous and metamorphic rock high in mica content. Located on ridges, side slopes, and foot slopes.

Topography & Landforms

Concord has an average elevation around 750 feet. The tallest natural point is McGill Bluff at 860 feet, located northwest of the city center. From this high point, the terrain gradually slopes downward to about 680 feet along Cabarrus County’s southern border with Mecklenburg County.

Hills & Valleys

The topography of Concord consists mainly of rounded hills bisected by small stream valleys. Some of the prominent rounded hills include Blakeney Hill, Irish Buffalo Hills, Loves Hill, and Assembly Hill. McGill Creek, Irish Buffalo Creek, and several other streams flow through valleys between the eroded hills. The maximum local relief is about 200 feet between the hill crests and adjacent valleys.

Nearly Level Areas

Several large areas characterized by more gently rolling terrain exist in Concord and the surrounding countryside where stream downcutting is less pronounced. These nearly level regions have very low relief, with adjacent hilltops and valleys varying less than 30 feet in elevation. Examples include the airport area in west Concord and regions south of Stough Creek.

Drainage & Hydrology

Concord lies within the upper Catawba River Basin. All surface water flows north and east out of the city limits towards the Catawba River, eventually reaching the Atlantic Ocean. Named streams passing through Concord include Cold Water Creek, Irish Buffalo Creek, McAlpine Creek, and several branches of McGill Creek. Most creeks are slow moving, winding waterways flowing through broad valleys with adjacent wetlands.

The creeks passing through Concord have incised into the saprolite and bedrock below the surface soils. Stream downcutting has formed the rolling topography of eroded hills and valleys characteristic of the Piedmont region. During heavy rain events, rapid overland flow off the hill slopes causes small streams to quickly rise out of their banks.

Concord has a humid subtropical climate with average annual precipitation around 45 inches, fairly evenly distributed throughout the year. Drought periods may occur any time of year but tend to be more common in late summer and fall. The risk of flooding is highest during the winter and early spring due to frontal storms producing heavy rainfall for extended time periods. Late summer storms tend to be more localized and intense.

Economic Geology

Gold Mining History

The early economy of Concord can be traced to small scale gold mining activities in the 1830s. Placer gold deposits were exploited from streams draining the area’s crystalline bedrock highlands. In the Reed Gold Mine area 6 miles east of Concord, underground mining of gold-bearing quartz veins occurred on a relatively large scale, with an estimated production of over 1 million troy ounces between 1831 and 1912.

Dimension Stone

Local outcrops of gneiss and granite have been quarried near Concord to produce dimension stone, crushed stone, and aggregate. In particular, the black and white banded gneiss has been cut and polished for use as an architectural and landscaping stone. The North Carolina Granite Corporation operates several active quarries and stone finishing facilities within 5 miles of Concord. Marketed under the “Winnsboro Granite” trade name, this stone has been used in numerous public and private projects throughout the country.

Geologic Hazards

Overall, Concord has relatively low geologic hazard risk compared to other regions of North Carolina:

• Earthquakes: Low hazard – Only a 2% chance of minor damage in the next 50 years
• Landslides: Minimal hazard due to gently sloping topography
• Sinkholes/Karst: Low hazard – No significant carbonate bedrock
• Erosion: Moderate sheet & rill erosion potential on steeper slopes during construction or loss of vegetative cover
• Expansive Soils: Moderate potential – Clayey soils may undergo shrink/swell behavior and damage structures without proper moisture controls
• Floods: Moderate flood risk exists along streams during extreme rainfall events

Proper site development, drainage design, and foundation engineering can help mitigate any issues stemming from marginal expansive soils or erosion. Maintaining vegetative cover on steep slopes is also important. Very heavy rainfall may produce localized flooding along small drainages, but risk can be reduced by controlling housing density within floodplain areas.

Conclusions

In summary, the underlying bedrock geology and past stream erosion patterns have shaped the Piedmont landscape around Concord, creating a pleasant area characterized by moderately rolling topography. Soils derived from the saprolite support a variety of urban and agricultural land uses. Gold mining and production of dimension stone are notable past economic activities tied to local geological resources. Major geologic hazards are relatively low for the city, with the possible exception of expansive soils and flood risk. Careful planning and design is needed in areas underlain by clayey soil or along stream floodplains.