Here’s an 1855 map from the Library of Congress, American Memory, Map Collections. This was surveyed a year before the Mellors traveled to Utah with the Martin Handcart Company.
From the valley of Green River to the Great Salt Lake. From explorations and surveys made under the direction of Hon. Jefferson Davis, Secretary of War by Capt. E. G. P. Beckwith, 3d. Artillery, F. W. Egloffstein, Topographer for the route, 1855.