The Gemara has been discussing various details of the parah adumah procedure. This process was a rare occurrence, for it only happened nine times in the history of the world. Even so, a great deal of preparation and construction was put into place so that the process could be carried out properly when the need arose.
We learned on 16a that the parah adumah was shechted on the Mount of Olives (Har Hamishchah), east of the Beis Hamikdash. Mount Moriah and the Mount of Olives were separated by the Kidron Valley, and to allow the Kohanim to easily travel between them a walkway was constructed starting from the eastern gate of Har Habayis. This walkway was supported by two levels of arches, with the columns of the upper level located over the airspaces of the lower level. This arrangement ensured that anyone standing on the walkway would be completely protected against the tumah of a grave that might be present in the ground below.
The walkway itself was [presumably] built at the same level as the floor of the Har Habayis. When it reached the Mount of Olives it was still 20 amos or so below the top of the eastern Har Habayis wall. To bring the Kohen up to the needed elevation to peer over this wall, a large structure was built there that housed a mikveh and a large pit for burning the parah adumah (see illustration).
The ashes of each parah adumah were divided into three parts. One-third of the ashes remained on the Mount of Olives itself, another third was placed in a stone jug stored in the Cheil just outside the Ezras Nashim, and the remaining ashes were distributed among the 24 mishmaros of Kohanim. The leaders of the mishmaros would take the ashes to their home town in order to provide purification from corpse-tumah to people in their region of the country, thereby sparing them the need to travel all the way to Yerushalayim.
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