Title: Stone Tools Can Drive You Crazy: Early Humans in Southern California For a century, reports of stone tool assemblages from desert regions of southern California and northern Mexico have been used to claim the presence of humans in the region before the last interglacial. To assign these assemblages to a chronostratigraphic order is difficult because they are found on surface sites in the hyper-arid environment of the Colorado and Mojave deserts rather than in stratified deposits that can be readily dated. The publication of the Cerutti Mastodon Site in San Diego dated to 130,000 years ago prompted a re-study of surface assemblages of stone tools in the Anza-Borrego Desert State Park in San Diego County. Research since 2015 employing new geomorphic models and surface age dating techniques is attempting to resolve the age and cultural associations of the Anza-Borrego assemblages with intriguing preliminary results.