Dublin is Ireland's oldest known settlement. It is also the largest and most populous urban centre in the country, a position it has held continuously since first rising to prominence in the 10th century (with the exception of a brief period in the late 19th and early 20th centuries when it was temporarily eclipsed by Belfast). The historic town grew up on the southern bank of the River Liffey, a few kilometres upstream from the river's outfall into Dublin Bay. The original settlement was situated on a ridge overlooking a shallow ford in the river, which had probably been a regular crossing-point since earliest times. The bedrock underlying the city is calp limestone, a dark shaly sedimentary rock which was once quarried locally and whose mottled grey appearance can still be seen today in