December 30 , 2016:

The Morrison Formation contains a number of large quarries that have yielded dinosaurs and other vertebrates, and many of these occur in sandstone beds representing ancient river channels. However, a number of very productive sites occur in mudstone beds representing other environments such as ephemeral ponds, and some of these yield both large dinosaurs and microvertebrates; these localities in mudstone beds represent different taphonomic modes of preservation and often preserve vertebrate taxa in different relative abundances from the channel sandstone sites. Among these important and very productive mudstone localities are the Cleveland-Lloyd Quarry, the Mygatt-Moore Quarry, and the microvertebrate sites of the Fruita Paleontological Area, and each of these preserves distinct vertebrate paleofaunas, different from sandstone sites and from each other, suggesting that mudstone localities had a very different mode of sampling the local biotas than did sites in sandstone.

John R. Foster, Julia B. McHugh, Joseph E. Peterson & Michael F. Leschin (2016)
Major bonebeds in mudrocks of the Morrison Formation (Upper Jurassic), northern Colorado Plateau of Utah and Colorado.
Geology of the Intermountain West, [S.l.] 3: 33-66