| SANANDREASFAULT | Tectonic boundary between the North American and Pacific plates in California |
| MASONDIXONLINE | Historical boundary between the North and South in the U.S. (5,5,4) |
| THEARCTICCIRCLE | Boundary between the North Temperate and North Frigid zones |
| EURASIAN | Iceland is being slowly split by the drift apart of two continental plates, the North American and . |
| RINGOFFIRE | Linear zone of seismic and volcanic activity that coincides with the Pacific Plate: 3 wds. |
| ALOHASTATE | Pacific plate phrase |
| SANANDREAS | Californian boundary between the Pacific and North American plates, the ... Fault (3,7) |
| DATELINE | Imaginary division extending between the North and South Poles, the boundary between one calendar day and the next (4,4) |
| STYX | In Greek myths, the river boundary between the Earth and the Underworld (4) |
| METRE | One ten-millionth of the distance between the North Pole and the equator (as defined by the French Academy of Sciences in 1791) |
| STRATOPAUSE | Level of the atmosphere forming the boundary between the stratosphere and the mesosphere, around 30 miles above sea level (11) |
| RHYMNEY | River in Wales forming the boundary between the historic counties of Glamorgan and Monmouthshire (7) |
| TERMINATOR | Boundary between the illuminated and darkened part of the moon, in astronomy |
| MOHO | The boundary between the crust and the mantle of planet Earth (4) |
| HENHARRIER | North American and European hawk which nests in marshes and open land (3,7) |
| MASON | Who, along with Jeremiah Dixon, surveyed a line in the US which was considered the dividing line between the North and South? (5) |
| WEALD | The ?, region of SE England between the North Downs and the South Downs (5) |
| THEWEALD | Area in South East England between the North and the South Downs |
| TEES | River that forms the boundary between the historic counties of Durham and Yorkshire (4) |
| WAVENEY | River that forms part of the boundary between the counties of Norfolk and Suffolk (7) |
