There are no bore holes with cores to provide direct information about the rock units underlying the basalt flows in the Monument. It is assumed that rhyolite exists at depth beneath the surface of the Monument, as it does elsewhere in the ESRP. Xenoliths of pumice, Challis volcanics, and granulite have been found in some of the basalts of the COM lava field. The pumice may be related to the rhyolitic eruptions that occurred within the ESRP, Challis volcanics, Mississippian sedimentary rocks, or Tertiary intrusives. The granulite is a metamorphic rock generally found in the cratonic basement and may be the rock type underlying the rhyolite.

The north end of the Monument is unlike the rest of the Monument and contains six sedimentary, volcanic, and intrusive rock units. Two intrusive-rock map units are exposed in outcrop in the north end of the Monument. Hornblende quartz monzonite is highly weathered and altered, weakly foliated, medium grained and equigranular. Plagioclase is the most abundant mineral in the monzonite and is accompanied by orthoclase, quartz and chloritized hornblende. Biotite granite is also exposed along the base of the Pioneer Mountains within the Monument. The granite contains quartz, biotite (altered to chlorite) and orthoclase (altered to the sericite). It also is highly weathered and is medium-coarse grained and equigranular in texture.

Eocene age Challis volcanics are present in the north end of the Monument and consist of welded tuff, lava flows interbedded with tuff breccia, and tuff breccia. The Challis tuff is an ash-flow deposit that overlies the tuff-breccia unconformably. The tuff ranges in color from light brownish gray to moderate orange pink with silica veins and lenses. The tuff breccia consists of lithic fragments (some of which are pumice), crystals and devitrified glass. These fragments were apparently derived from previously deposited or interbedded rhyodacite lava flows. Both units probably originated from ash flows and breccia flows issuing out of eruptive centers north of the monument boundary during Eocene time.

There are also Mississippian age sandstone, siltstone, claystone, and minor conglomerate of the Copper Basin Formation in the north end of the Monument. The conglomerate is gray with clasts to cobble size. The sandstone is very fine to fine grained, olive gray to medium gray, and sole marks are common in places. The siltstone is locally laminated and medium to dark gray. The claystone is dark gray to black, locally laminated, and contains pebbles of chert and quartzite in places.

Surficial deposits are the youngest materials mapped inside the Monument. The thickness of the sediments in the north end of the Monument ranges up to about 100 feet (30m) along some of the stream drainages based on the well logs from the Monument water wells. Eolian silts and sands mantle some of the older lavas and continue to be eroded, transported, and deposited, particularly after such events as fire. Recent fires in the Kings Bowl area freeing sediments of their anchors clearly demonstrated eolian processes in action, i.e., deflation, active ripple migration and formation and migration of small sand dunes. Cinders also can often be observed saltating on cinder cones on windy days, thus the cones are a landform in flux and ever changing.