For the most part the Riddermark is a land of wide, rolling, grassy plains situated atop rich alluvial deposits, carried from the Ered Nimrais and Misty Mountains, which cover a layer of porous strata (limestone and chalk.)

Along the edge of the Ered Nimrais this topography gives way to that of a higher, flatter plain (situated atop a harder layer of igneous rock) which is dotted with the outlying foothills of the larger mountain range. 

The mountains around Helm's Deep appear to be formed by igneous rock and, in places such as the Caves of Aglarond, soluble limestone and more resistant marble.

The Ered Nimrais would appear to be formed by a seismic shift, or upward thrusting of tectonic plates. It is assumed for the most part that the range is formed by igneous rock, such as basalt,  but, if the range's name does not derive alone from its snow-capped peaks, in places a visible layer of limestone or marble might exist. 
The Ered Nimrais is scoured with many dales and glens formed by rivers and tributaries which flow towards the Entwash Vale. There is some evidence of glacial activity, primarily in the description of Harrowdale where features such as the "hanging valley" near Dunharrow occur. The black mountain of the Dwimorberg seems to have been formed by volcanic activity of some sort.

The wide lowlands of the Entwash valley bisect the country from the northern border, at the southern edge of Fangorn Forest, to the border with Gondor in the south east. It seems likely that the valley's strata is of a type less resistant to erosion than that which exists upon the great plains. Thus the Entwash valley cuts down deeply into the land mass, until it reaches harder strata. The decrease in inclination slows the speed of the river, resulting in large sedimentary and alluvial deposits, which in turn form the sprawling inland delta known as the Mouths of the Entwash.

Along the Mark's eastern border a series of uplands, probably formed by folds in sedimentary strata (anticlines & synclines), are found in the forms of gently eroded chalk downs which lie to the north and south of the taller, broken highlands of the Wold.