Task: Question Answering
Input (English): Passage: After Quiriguá's pivotal victory over Copán in 738, K'ak' Tiliw Chan Yopaat rebuilt the main group in the image of Copán itself. Thus, the acropolis, palace, and ballcourt all lie at the southern end of the Great Plaza. The ceremonial centre is laid out around three plazas, the northernmost is the Great Plaza. This plaza measures 325 m (1,066 ft) from north to south and is the largest plaza in the whole Maya region. At the southern end of the Great Plaza is the Ballcourt Plaza, surrounded on three sides by structures associated with the acropolis. The Acropolis Plaza is a fully enclosed plaza within the acropolis itself. The area to the west of the Ballcourt Plaza was probably the riverside docking area and there is evidence that the southern part of the Great Plaza was a marketplace. A number of ceramic-lined wells have been excavated close to the site core, these were all built in the 8th century and although some continued in use into the 9th century, none are known to have been built that late.
1A-1 is an enormous platform forming the northern part of the Great Plaza. It measures 100 by 85 metres (328 by 279 ft) and rises 0.5 metres (20 in) above the level of the southern part of the plaza. It forms the northern portion of the Great Plaza, being built by K'ak' Tiliw Chan Yopaat when he extended the plaza northward. The platform was built from river cobbles and was paved with stone slabs. Platform 1A-1 supported the stelae A, C, D, E and F and Zoomorph B. The platform was built in two phases over about 20 years.
1A-3 is a large mound marking the northern edge of the Great Plaza. It originally measured 82.5 by 20 metres (271 by 66 ft) and was 7 metres (23 ft) high. A 63-metre (207 ft) wide stairway climbed the southern face of the structure from the plaza. The structure was later extended to the north but this second phase of construction was never finished.
The acropolis is the largest architectural complex at Quiriguá, it lies at the southern limit of the ceremonial centre of the city. It is a complex construction, with new buildings and features being added over time. Construction of the acropolis began in 550 and continued through to 810 when the site was abandoned. The acropolis was a palace complex used primarily as an elite residence and for administrative purposes. The acropolis complex includes structures 1B-1, 1B-2, 1B-3, 1B-4, 1B-5 and 1B-6. Excavations of the acropolis encountered the fallen remains of corbel arches, but none are still standing. 
Question: What was built from river cobbles and paved with stone slabs?
Output (English): The platform.