| Clue | Source | Date |
|---|---|---|
| Dense buildup in makeup powder containers | New York Times | 17 Mar 2023 |
| Tricky to wash gravel through subsoil layer | ||
| Drainage hinderer | Wall Street Journal | 03 Jun 2020 |
| Under-soil layer of clay | Premier Sunday | 09 Jun 2019 |
| Clay layer under soil | Premier Sunday | 15 Feb 2015 |
| Clay layer under soil | ||
| Drainage impairer | ||
| Subsoil layer | ||
| Compact soil layer | ||
| Firm, dry ground | ||
| Firm soil | ||
| Subsoil layer | ||
| Bedrock | ||
| Bedrock | ||
| Type of subsoil. | ||
| Solid base. | ||
| Solid, unplowed land. | ||
| Bedrock. | ||
| Solid foundation. | ||
| Rock bottom. | ||
| Firm layer of clay underlying soft soil. |