Read the ground: five zones across the Fall Line at Macon
Macon sits where hilly Piedmont rock meets the Coastal Plain's sand and clay, with the Ocmulgee floodplain in between. Pick a zone below and the guide shows what the state's geologic survey says about it and what that usually means for a septic design.
How to use this guide on your own lot
- Find the zone that sounds like your lot. If it fits two, you may be near a boundary.
- Read what the evaluator checks first, and tell them about anything that matches.
- Get the lot evaluated. See perc test and soil evaluation.
- If the zone points to water, rock, or slope, read alternative systems before assuming the lot is lost.
Where these facts come from
- Geology and Ground-Water Resources of the Macon Area, Georgia (Bulletin 72, 1962), Georgia Environmental Protection Division.
- Bibb County, Georgia Encyclopedia.
- Georgia Department of Public Health, Manual for On-Site Sewage Management Systems, Section B, and rule 511-3-1-.09 for alternative systems.
The survey is from 1962 and describes regional geology. It does not describe individual lots, and it is not a substitute for a soil evaluation.
The five zones in more detail
Piedmont upland
North of the Fall Line, the state survey describes a hilly upland over crystalline rock, with the highest parts near 800 feet. Water mostly comes from dug wells in weathered material and from fractured rock. For septic, the questions are how deep the weathered soil runs and whether the slope leaves room for a trench. Georgia's manual wants 2 feet of soil between the bottom of the trench and bedrock, and treats slopes over 25 percent as unsuitable without a special investigation.
The Fall Line terrace
The survey notes that downtown Macon is built on a terrace about 75 feet above the river. This is where the two provinces meet, and a single lot near the boundary can show ground from both. If so, the evaluator may need more than one pit to understand it.
The Ocmulgee floodplain
The river's alluvium is the youngest sediment in the area: unsorted clay, sand, and gravel, usually under 40 feet thick, and reaching one or two miles on each side of the river. In flat areas with slowly draining material, the survey says the water table is within a few feet of the surface. The manual wants 2 feet of separation from seasonal high groundwater, or 1 foot with Class I aerobic pretreatment.
Coastal Plain sand hills
South of the line, the survey describes loose surface sand over layers of pink or red hardpan, and soils that are so well drained and coarse that they have low fertility. Water moves quickly. The hardpan layers below are what the evaluator maps.
Coastal Plain clayey uplands
On the Tertiary uplands the survey says clayey subsoil or parent material lies within a few feet of the surface in most places. Clay holds water and releases it slowly, so fields there are often larger or laid out differently.
What a 1962 survey can and cannot tell you
The survey is a regional description of geology and ground water. Geology changes on a scale of millions of years, so its description of the layers still holds. What it cannot do is describe any one lot. Roads, grading, fill, and drainage changes since 1962 may have altered the top few feet of ground, which is exactly the part a septic design cares about. That is the reason for the soil evaluation.