As you move forward a Mark of Set appears. You can use the Staff of Osiris to close it...

...or just destroy the scarabs that swarm out of it until the Mark disappears on its own. Obviously you'll get more points by killing the beetles.

In the single-player game there's a time bomb here. Raise the Staff to slow the countdown as you roll the bomb onto the square pressure pad near the edge.

Doing so raises 2 wooden ledges between the spinning gears above the pit lined with icicles.

The ledges only remain raised as long as the bomb stays on the pad, so jump quickly but carefully across the ledges, past the gaps that appear as the gears turn, to the other side of the pit. Note that the gears also bear the Mark of Osiris, so you can lower the staff to affect the speed at which they spin, but this is not necessary, and of course it means speeding up the countdown on the time bomb.

In the co-op game there is no time bomb. Instead there are 2 pressure pads, one on each side of the icicle pit. Have one character stand on the first pad to raise the wooden ledges so the other character can jump past the spinning gears. (If you like you can use the Staff to change the gears' speed, but this is not necessary.)

Once the second character reaches the other side of the pit, she should stand on the pressure pad there to keep the ledges up so the other character(s) can cross.

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