The phrase "are mostly formed" is correct and usable in written English.
It can be used when discussing the composition or structure of something, indicating that the majority of it is made up of certain elements.
Example: "The clouds in the sky are mostly formed from water vapor and tiny ice crystals."
Alternatives: "are primarily composed" or "are mainly created".
What's not to like Owen John Gaston Teacherr of government and politics, Hove, East Sussex One point that Marina Hyde (Coalitions should stay where they belong – in Borgen, 2 May) and others miss about possible UK post-election alliances is that in Danish elections, coalition partnerships are mostly formed before the election takes place.
New wood-pastures are mostly formed from open habitats, often in former military areas.
Both of these regions are mostly formed from sand and have shallow groundwater tables.
Pyrazines are mostly formed through the Maillard reaction between saccharides and amino residues [28, 29].
Since midwinter topographic convergences are mostly formed by northwesterly winds blowing over the Sea of Japan, wind convergence becomes the strongest in the coastal areas of the Hokuriku district (since the wind blows perpendicular to the coastline there).
Angiogenic vessels are mostly formed by sprouting of endothelial cells from the existing vasculature.
The template was mostly formed, according to "Texas Tough," by slavery and its aftermath.
There haven't been any polls here and my views have been mostly formed by my conversations with the main candidates and their aides.
It is mostly formed of basalt and geologically recent sedimentary rocks, with the cones of numerous extinct volcanoes about 825 feet (250 metres) high in the northern and southern sections of the peninsula.
The GF scheme, for instance, shows that community 1 (where most users communicate about Occupy) is mostly formed by coordinators and gatekeepers, whereas community 3 (with a majority of 'indignados' users) is mostly formed by representatives and itinerants.
