The Second Crossing refers to the relocation of Pern's colonists from the Southern Continent to the Northern Continent during their 10th year on the planet. The term "Second Crossing" was used since it was the second time that the colonists relocated their base of civilization, the "First Crossing" was the 15 year voyage from earth.
When the settlers first landed on Pern, the location they chose was at the Northeastern coast of the Southern Continent. This original settlement was referred to as "Landing" and served as the central colony for the first 9 years. The most prominent geological feature of this location was Mount Garben that towered above the settlement. Mount Garben, along with the other smaller mountains in the area were formed by ancient volcanic activity and it was thought that they were dormant. However, when smoke and minor eruptions occured, the colonists realized that they had to relocate to a safer environment.
The most suitable site for their relocated colonial headquarters was located on the Northern Continent which was less susceptable to volcanic activity and contained large number of caves that would offer natural shelter from threadfall.