West Meade Ecology

Where did West Meade's Hills come from? Below, geologist Molly Miller explains our local geology:

When the rates of sea floor spreading are high, the mid ocean ridges are high and wide with a lot of hot material coming out fast. Under these circumstances, the ocean basins can't hold all the water and it sloshes out onto the continents. The high sea level can also reduce the amount of sand and mud delivered to the continental shelf from the continents -- so the sediment that is deposited is from the sediment produced on the continental shelf, which is calcium carbonate generated by organisms (coral, foraminifera, calcareous algae, brachiopods, bryozoans, molluscs - today, etc). The result of this scenario is the Ordovician limestones of Nashville. Fundamentally, we have limestone because there was insufficient sand and mud being brought in from the continental highlands (where activity related to the Appalachians was occurring).

Sea level dropped, then rose again in the Late Devonian, resulting in deposition of the Chattanooga Shale which is poorly exposed on the hills of West Meade. The relatively shallow sea spread over much of the eastern US -- it was stratified and the bottom anoxic, resulting in deposition of the black, organic-rich Chattanooga Shale -- a thin (~10m or less) unit traceable all over the Nashville area and in the subsurface for miles to the east and west. Circulation in this sea was eventually restored in the Early Mississippian, and the (Mississippian) Fort Payne Formation was deposited under low oxygen conditions. Some (unknown amount of) time later the Mississippian sediments were infused with groundwater that precipitated silica cement between the grains -- resulting in a silica-cemented siltstone/dolostone that is very difficult to erode. These and the overlying Mississippian limestones are the youngest rocks around on the Highland Rim. However, because the limestones above the Fort Payne Formation are not as silica-rich as the Fort Payne Formation, they are not as resistant to chemical weathering.

The bottom line is that in our wet climate limestones weather by dissolving very quickly. The broad upwarp of the Nashville dome caused the upwarping of the Mississippian limestone and Fort Payne Formation. The Fort Payne Formation weathers slowly, resulting in "caps" of resistant rock that make high hills as in West Meade. The soil derived from these Mississippian rocks can form steep unstable slopes, which can fail -- as happened along Jocelyn Hollow and Cornwall Dr in the 2010 rainfall event. (Note that our house is built on Ordovician bedrock beneath the Chattanooga and Fort Payne formations - this is not a coincidence!) Once the "tough" silica-rich Fort Payne Formation is eroded, the underlying Ordovician limestones are rapidly chemically eroded (dissolved) and they disappear quickly -- making the Nashville basin.