186 Sub-formation of Marls and Calcareous Sandstones 



cliaracters between ashwoods and oakwoods. Quercus 

 sessiliflora is the oak oharacteristic of some of these woods, 

 and the wych elm (TJlmus montana) and the white birch 

 (Betula alba) may be conspicuous features. 



Scrub and Grassland associations. The scrub asso- 

 ciation of this sub-formation has not been studied. 

 Comparatively little semi-natural grassland belonging to 

 this sub-formation appears to exist, because the type of 

 soil is valuable for arable land or for permanent pasture. 

 It has scarcely been studied, but appears, as might be 

 expected, to contain a mixture of species belonging to 

 neutral grassland and of those belonging to calcareous 

 soils. Derelict pasture on the chalky boulder clay of 

 Cambridgeshire appears to be very readily invaded by a 

 loose scrub consisting largely of the hawthorn (Cratsegus 

 monogyna). 



