How was the direction of Mecca indicated in early hypostyle mosques?

In early hypostyle mosques, the direction of Mecca—known as the qiblah—was primarily indicated by the greater depth of the colonnade on the side of the structure facing Mecca.1

In these early congregational buildings, such as those constructed in Kūfah, Basra, and Al-Fusṭāṭ, the hypostyle design featured a covered colonnade, or ẓullah, placed specifically on the qiblah side. While this architectural feature served as the initial indicator of the direction of prayer, the later development of the mihrab—a prayer niche set into the interior wall facing Mecca—became the standard method for marking the qiblah in mosques. The mihrab, which originated during the reign of the Umayyad prince al-Walīd I, eventually became a central, often ornately decorated element of the qiblah wall.1234