One area of the county, generally from Leesburg northward, has a different type of bedrock known a limestone conglomerate. This rock which contains the mineral carbonate, is dissolved by rain water and groundwater which enlarges fractures in the rock. The areas of dissolved rock may form underground conduits and caverns which affects the infiltration of rain into the ground, the flow of groundwater, and may create surface features like sinkholes. The more about the geology in this part of the county:
Groundwater and surface water are inextricably linked. Groundwater in aquifers is recharged by the normally slow infiltration of water from the land surface downward through layers of soil and sediment. A significant portion of the water flowing in streams comes from the seepage of groundwater into the stream channel. Most of the water you see in a stream when it hasn't rained for a while, came from groundwater seepage.
Compared to streams, water in the ground moves very slowly - often only a few" or feet per day under natural conditions, although it can be much slower or faster in certain areas and situations. If groundwater and the matrix of soil and rock in which it occurs become polluted, it can be quite difficult, expensive, and time consuming to clean. Therefore, protecting groundwater resources from pollution is extremely important.