My trench wall caved in on Tuesday and a $12 tarp saved the whole dig

Honestly I thought the season was done. We were about 40 cm down at a site outside Flagstaff when the west wall of our 2x2 unit just slid in after two days of rain. Half a bucket of pottery sherds got buried and my trowel was somewhere under it. I sat there for a bit feeling dumb because I had seen the cracks on Monday and did nothing. A volunteer ran to the truck and grabbed a cheap blue tarp we use for shade, and we laid it over the exposed profile and weighted the edges with rocks. Next morning the wall held and we recovered every sherd except one tiny rim piece. We ended up getting three more levels done that week, which never happens for us. Ngl it felt like a real win after a rough start. Has anyone else had a wall collapse mid dig, and did a tarp actually work for you or was it luck?