The open-air Tolbor-21 site in the Selenga River basin provides important evidence for the settlement of northern Mongolia by Paleolithic populations. The Initial Upper Paleolithic (IUP) is represented by Archaeological Horizon 4 (AH4) and dates between 41,000-42,000 years ago. AH4 is present in all four pits excavated in different locations of the site, with Pit 2 having the highest concentration of finds and combustion features. During new excavations in Pit 2, an elongated depression was found directly under AH4. This feature was about 0.3 m deep and contained a concentration of bones and artifacts. Our excavation uncovered only a portion of this depression measuring 1.6 x 0.6 m. This is the first depression of this size known in the IUP of Mongolia. In order to determine whether this depression is of anthropogenic origin, we studied its main features and compared it with overlying AH4. Vertical cross sections constructed using the three-dimensional coordinates of each find revealed an almost horizontal concentration of finds in the depression, in contrast to AH4, which slopes to the south. To study the orientation of elongated artifacts, we conducted a fabric analysis. The results showed that the content of the depression, in contrast to AH4, was not affected by solifluction. The Mann-Whitney test confirms that the finds in the depression are significantly smaller and contain a higher percentage of faunal remains than AH4. According to our preliminary results, the feature underlying AH4 was human-made and intrusive from the surface of AH4. Its position under the main archaeological horizon protected the finds from natural disturbances that followed the burial of AH4 and contributed to the preservation of the original composition of the finds represented mainly by small spalls and organic debris. This research was supported by the Russian Scientific Foundation, Project #19-18-00198.