i finally figured out what the weird cone-shaped snail shells i kept finding were - they’re called decollate snails and they are a type of snail that is used in southern california for pest control. they started importing them initially to eat snails that were threatening the citrus crops, but kept them around when brown snails (regular garden snails) became pests to home gardeners. they only come out when the soil is wet, and when i picked up a brick i found a bunch of their shells.