sentences: lichens can produce acid to break down rocks & acid can cause rocks to weather by chemical reaction & physical weathering time vomiting weathering because of physical dryness ->  lichens can cause rocks to break down by chemical weathering  chemical weathering is a kind of weathering & sand is formed by weathering -> soil can be formed by chemical weathering soil can be formed by chemical weathering  lichens can cause rocks to break down by chemical weathering & sand can be formed by physical weathering -> lichens breaking down rocks can form soil