But the "free-floating" hypothesis presented its own difficulties; it did not explain the ten bills missing from one packet, nor was there a logical reason that the three packets would have remained together after separating from the rest of the money. Physical evidence was incompatible with geologic evidence: Himmelsbach observed that free-floating bundles would have had to wash up on the bank "within a couple of years" of the hijacking; otherwise the rubber bands would have long since deteriorated, an observation confirmed experimentally by the Cooper Research Team (see Subsequent FBI disclosures above). Geological evidence suggested that the bills arrived at Tina Bar well after 1974, the year of a Corps of Engineers dredging operation on that stretch of the river. Geologist Leonard Palmer of Portland State University found two distinct layers of sand and sediment between the clay deposited on the riverbank by the dredge and the sand layer in which the bills were buried, indicating that the bills arrived long after dredging had been completed. The Cooper Research Team later challenged Palmer's conclusion, citing evidence that the clay layer was a natural deposit. That finding, if true, favors an arrival time of less than one year after the event (based on the rubber band experiment), but does not help to explain how the bundles got to Tina Bar, nor from where they came.