[Taken from Valmiki Ramayan 5/1/76]

[5/2/3]
This verse, and others too, clearly mention that Hanumaan crossed an ocean of 100 Yojan distance. At the present time, the shore to shore distance between southern tip of India and Northern tip of Sri Lanka is around sixty miles. Even with a measure of 2.5 miles per Yojan, 100 Yojan distance translates to 250 miles.

To explain this anomaly, we need to consider that around the time of Raamaayan, the distance was lot more than the current sixty miles. This is in sync with the current theories by geologists that the ocean between India and the current Sri Lanka had been replaced by land mass over a period of time. Most of the land to the south of current Raghunaath Pur had been covered by landmass in antiquity. 

Also according to Raamaayan, the mountains Trikoot, Lamb and Suvail were to the north of Lankaa, but currently the mountains are found in the southern part of Lankaa. This suggests that the northern parts of current Lankaa were submerged in water during the Ramayana period.

[Taken from Valmiki Ramayan 5/2/3]