These rocks are found exposed in a number of mountain ranges in Utah. See the obvious large chunks surrounded by a finer-grained groundmass? This rock is a classic diamictite, made of two distinctly different grain sizes.
A diamictite (pictured) is a sedimentary rock filled with large chunks of rock down to mud. This contrast in the size and variety of the particles in the rock is called “very-poorly sorted” by geologists. Most sedimentary rocks have one or two grain sizes, which makes Diamictites an odd ball. They form in a wide variety of environments, which makes interpretation of them difficult. One explanation of the rock is a glacial deposit over a body of water. Also known as a dropstone, it forms when glacial ice brings large boulders to water then drops them in the middle of the water, thereby placing big rocks in mud. You can form the same rocks with debris flows or other mass wasting events like landslides. Even volcanos and impacts from space can form them.