Here's why:

* Geological maps depict the subsurface geology: They show the distribution of different rock types, formations, and geological structures, which are essential for understanding the potential for water-bearing aquifers and potential hazards like faults or unstable formations.

* Identifying aquifers: Geological maps help identify the location and depth of aquifers, which are underground layers of rock or sediment that can hold and transmit groundwater. This is crucial for determining the optimal drilling location and depth.

* Assessing groundwater quality: Geological maps provide information on the types of rocks and sediments, which can influence groundwater quality.

* Identifying potential risks: They can reveal potential risks associated with drilling, such as the presence of faults, karst features (sinkholes), or areas prone to groundwater contamination.

* Understanding hydrogeology: Geologic maps are the foundation for understanding the movement of groundwater (hydrogeology), which is critical for designing and managing a well.

While topographic maps are important for understanding the surface features and elevations, they do not provide the crucial information about the subsurface geology needed for well drilling. They are helpful for determining accessibility, potential land obstacles, and surface water features, but they are not a substitute for a geologic map.

In summary:

* Geologic map: Essential for identifying aquifers, assessing groundwater quality, and understanding geological risks related to well drilling.

* Topographic map: Helpful for surface planning and accessibility, but not sufficient for well drilling decisions.