The coastline of the shire is dominated by Kockatea Shale, formed in the Triassic period from volcanic and sedimentary rocks and consisting of shale, minor siltstone and sandstone. Leeman town lies on the Woodada formation, formed during the Mesozoic period from sedimentary and volcanic rocks and consisting of fine to medium grained sandstone interbedded with laminated siltstone. Part of the eastern portion of the Shires lies on the Lesueur Sandstone formation, formed during the Triassic period from volcanic and sedimentary rocks and made up of quartz sandstone with subordinate granule conglomerate and minor siltstone. The remainder lies on the Eneabba formation and the Cattamarra Coal Measure, both formed during the Jurassic period from volcanic and sedimentary rocks. The Eneabba formation consists of interbedded red-bed sandstone and siltstone and the Cattamarra Coal Measure consists of siltstone, shale, claystone, coal and sandstone.
Further prominent features are the Coomberdale subgroup,the South West Terrain (part of the Yilgarn Craton) ad the Billeranga subgroup. The Coomberdale subgroup consists of sandstone, siltstone, dolomite rock and silica minerals and the Yilgarn craton consists of granite. The Billeranga sub-group was formed during the Mesoproterozoic era from sedimentary and volcanic rock and consists of sandstone, siltstone, basalt and volcanic sandstone.
Coastal soils are sandy, forming a complex dune system, and include some gravels over limestone outcrops. Saline soils are common in the salt lake systems of the coastal flats and the eastern portion of the shire. The terrain around the salt lake systems of the shire is typically characterised by low dunes and plains. Seasonally wet plains inland are characterised by red calcareous clays. The soils of the middle f the shire are deep, siliceous sands over undulating low hills, gravelly ridges and the occasional remnant dune.
The shire is located within the Lesueur Sandplains (GES02) and Merredin (AVW01) IBRA sub-regions and is characterised by sandy proteaceous heath and York Gum and Jam Wattle woodlands. Patches of Subtropical and Temperate Coastal Saltmarsh can be found in the shire. This is a nationally protected Threatened Ecological Community (TEC) listed as vulnerable under Australia’s national environment law. The plant, animal and micro-organism community is found in the intertidal zone, often associated with estuaries. The plant community consists mainly of salt-tolerant vegetation (halophytes) including grasses, herbs, reeds, sedges and shrubs. The region is also home to patches of the Ferricrete Floristic Community and Plant Assemblages of the Inering System TECs.
The main land uses in the Shire of Carnamah are agriculture and nature conservation. Of the total land area, 49% is arable. The 162, 334 ha total area of agricultural holdings is operated by 39 agricultural businesses and produced agricultural commodities worth $45, 807, 182 gross value in 2016 (ABS 2018).