These four postcards present scenes from Colorado in three different styles. These styles can help identify the era in which each card was issued.
First, a card showing a lily pond in Rocky Mountain National Park has a white border and uses the UP system logo on the back, both of which date it to around 1915-1930.
The second card shows Echo Lake, which is due west of Denver and south of what is now Interstate 70. This has a perforation on the bottom, which means it was one of a paired postcard. Most of the pairs postcards I’ve presented before seem to be from the 1930s. This one, however, uses more advanced printing techniques and has a post-1942 UP logo on the back, which suggests it was from the late 1940s.
The third and fourth cards both have linen finished, which were supposedly used from about 1930 to 1945. The logo on the back of the Hallett Peak card, however, dates from the post-1950 era. The logo on the back of the LochVale card is a little more ambiguous but probably is from the 1942-1950 period.